Coated Steel Sheet
Coated Steel Sheet
Steel sheet is frequently coated in coil form before fabrication either at the steel plants or by the specialists coating units. This coated sheet is ready for fabrication and use without further surface coating. Coated products yield lower production costs, improved product quality, shorter processing cycles, elimination of production hazards, conservation of energy, minimized ecological problems, and production expansion without a capital expenditure for new buildings and equipment. Steel sheets are used throughout a broad spectrum of areas such as automobiles, home appliances, building materials, housing, beverage cans, and transformers. Coated steel sheets such as hot-dip galvanized steel sheets and pre-painted steel sheets are widely used in the construction industry as exterior, interior, and structural materials.
The definition of the coil coating process as given in the European Standard EN 10169-1: 2003 is ‘a method in which an organic coating material is applied on rolled metal strip in a continuous process. This process includes cleaning if necessary and chemical pre-treatment of the metal surface and either one side or two side, single or multiple application of (liquid) paints or coating powders which are subsequently cured or laminated with plastic films’.
Pre-painted steel sheets are produced by forming polymer films 10 micrometers to 30 micrometers thick on the surfaces of galvanized steel sheets in order to provide specific colours and designs. They are mainly used in applications such as roofing, siding, and shutters on various buildings. On the other hand, organic composite coated steel sheets are produced by forming thin organic composite films 1 micrometer to 2 micrometers thick on the surfaces of galvanized steel sheets. They are used in applications which do not need such aesthetic designs as pre-painted steel sheets; for example roofing, siding, and structural materials of non-residential, industrial and commercial buildings. Basic properties required of organic composite coated steel sheets are formability (the property which suppresses the peeling and scoring of metal coatings during roll forming) and corrosion resistance. In addition to these properties, pre-painted steel sheets have to have an excellent surface appearance, free from cracks and other damage. This has to be maintained even after forming.
Coated steel sheets, in particular are required to possess not only corrosion resistance but press formability, weldability, paintability, and various other properties as well. Coated steel sheets for meeting these properties need use of several other technologies in addition to metallurgy. These technologies are electro-chemistry, thin film engineering, paint engineering, inter-face engineering, corrosion science, thermal technology, and alloying control (diffusion) technology. If any of these technologies are lacking, then the user industry needs cannot be fulfilled.
Some precautions are necessary with the coated sheet. The product is to be handled with more care to prevent scratches and damage to its surface. Metal finishing of damaged areas is more difficult than on uncoated sheet. Fabrication methods are more restrictive, bend radii is to be more generous, and welding practices are to be carefully chosen.
The basic types of coating include metallic, treated, primed, and painted finishing. Metallic coating can be made up of zinc, aluminum, zinc-aluminum alloys, tin, and terne metal. Pre-treatment coatings are normally phosphates, and primed finishes can be applied as a variety of organic-type coatings. These can be used as a primed-only coating, or a suitable paint top-coat can be applied. Painting consists of applying an organic paint system to steel sheet on a coil coating line.
Coil coating makes use of a simple, but effective principle consisting of to clean, pre-treat and coat flat coils or sheets of steel in a continuous operation, before other stages of industrial manufacture. The slogan for this concept is ‘finish first – fabricate later’. A conventional process sequence in coil coating consists of (i) cleaning, (ii) conversion treatment (including optional post-rinse), (iii) drying, (iv) primer coating, (v) top coating, and (vi) foil lamination (optional). An overview of a large coil coating line is shown in Fig 1.
Fig 1 Schematic of a steel coil coating line
Galvanizing is a process for rust proofing steel by the application of a metallic zinc coating. It is applicable to products of nearly all shapes and sizes, ranging from nail, nuts, and bolts to large structural assemblies and steel sheet in coils and cut lengths. Other applications include roofing and siding sheets for buildings, silos, grain bins, heat exchangers, hot water tanks, pipe, culverts, conduits, air conditioner housings, outdoor furniture, and mail boxes. On all steel parts, galvanizing provides long-lasting, economical protection against a wide variety of corrosive elements in the air, water, or soil.
A large quantity amount of zinc coated sheets is used by the automotive industry for both unexposed and exposed panels, from frames and floor pans to doors, fenders, and hoods.
Metallic zinc is applied to the steel by three processes namely (i) hot dip galvanizing, (ii) electro-galvanizing, and (iii) zinc spraying. The majority of the galvanized steel sheet is coated by the hot dip process, although there has been strong growth in electro-galvanizing capacity.
Corrosion resistance – The use of zinc is unique among methods for the corrosion protection of steel. The zinc coating serves a two-fold purpose namely (i) it protects the steel from corrosive attack in most of the environments, acting as a continuous barrier shield between the steel and the environment, (ii) it acts as a galvanic protector, slowly sacrificing itself in the presence of corrosive elements by continuing to protect the steel even when moderate-sized areas of bare metal have been exposed. This latter ability is possible since zinc is more electro-chemically active than steel. This dual nature of zinc coatings is also available with some zinc / aluminum alloy coatings, but zinc coatings clearly offer the most galvanic protection.
With most protective coatings which act only as a barrier, rapid attack commences when exposure of the base steel occurs. The distance over which the galvanic protection of zinc is effective depends on the environment. When completely and continuously wetted, especially by a strong electrolyte (for example, seawater), relatively large areas of exposed steel is protected as long as any zinc remains. In air, where the electrolyte is only superficially or discontinuously present (such as from dew or rain), smaller areas of bare steel are protected. The order of magnitude of this throwing power is nominally around 3 mm, although this can vary significantly with the type of the environment. However, galvanized parts exposed outdoors have remained rust free for many years, and the two basic reasons are the sacrificial protection provided by the zinc and the relatively stable zinc carbonate film which forms on the zinc surface to reduce the overall corrosion rate of the zinc coating. The service life of zinc-coated steel is dependent on the conditions of exposure and on the coating thickness (Fig 2). Although the coating process used to apply the zinc coating generally does not affect the service life, experience has shown that the corrosion resistance of the zinc coatings in the field cannot be accurately predicted from accelerated laboratory tests. Environmental factors such as environmental contaminants (sulphates, chlorides, and so on) and time of wetness have a large influence on the service life of galvanized steel. In polluted areas, such as severe industrial areas, the normally protective zinc carbonate film which forms on the surface of the zinc coating tends to be converted to soluble sulphates that are washed away by rain, thus exposing the zinc to further attack and accelerating the corrosion. Service life as measured in years to the appearance of first significant rusting is shown in Fig 2.
Fig 2 Service life of zinc-coated steel sheet
Coating test and designations – The thickness (or weight), adhesion, and ductility of a zinc coating can have important effects on its service life and effectiveness against corrosion. Practical tests for these characteristics are described in relevant specifications of various standards. Tests for coating thickness normally include microscopic measurement of the cross-section, stripping tests in which the coating is removed from a given area, electro-chemical stripping from a given area, and magnetic and electro-magnetic methods of measurement. Adhesion can be tested and rated by bend test methods. Other adhesion test methods include reverse impact and draw bend test.
Since the service life of a zinc-coated part in a given environment is directly proportional to the thickness of zinc in the coating (Fig 2), measurement of this quantity is very important. The amount of coating is normally measured in terms of weight rather than thickness with the samples cut from one or three spots. These are weighed and the zinc is stripped (dissolved) in an acid solution, and the samples are reweighed. The weight loss is reported in grams per square meter. When samples from three spots are checked (triple-spot test), the value of weight loss is the average of the three samples. When the weight-loss method is used, the amount of coating measured is the total amount on both sides of the sheet. Normally, the zinc coating is applied to both sides of the sheet.
Chromate passivation – Several types of finishes can be applied to zinc-coated surfaces to provide additional corrosion resistance. The simplest type of finish applicable to fresh zinc surfaces is a chromate passivation treatment. This process is equally suitable for use on hot dip galvanized, electro-galvanized, zinc-sprayed, and zinc-plated products. Normally, the treatment consists of simply cleaning and dipping the products in a chromic acid or sodium dichromate solution at around 20 deg C to 30 deg C, followed by rinsing in cold fresh water and drying in warm air. The adherent chromate film can have a greenish or greenish-yellow indescent appearance. The standards give details of tests for measuring the adequacy and effectiveness of the chromate film. Chromate passivation helps prevent staining when galvanized sheet is stored under wet or humid conditions. Hence, a thin, almost clear chromate or chromate / phosphate passivation film is frequently applied to the coating on hot dip coating lines.
Painting – The selection of zinc coated steel as a material for sheds, buildings, roofs, sidings, appliances, and many hardware items is based on the sacrificial protection and the barrier coating afforded the base metal by zinc coating. For additional protection and aesthetic appearance, paint coatings are frequently applied to the zinc coated steel. The performance of the coatings is an important economic factor in the durability of this material.
Zinc coated steel, both new and weathered, can be painted with a minimum of preparation and with very good adherence. On hot dip galvanized or zinc-plated steel, it is necessary to use special corrosion-inhibitive primers to prepare the surface before the paint is applied. This is partly because these types of zinc coating are too smooth to provide a mechanical key for the paint or lacquer and partly because the paint appears to react with the unprepared zinc surface in the presence of moisture to weaken the initially formed bond.
Several exposure tests have shown that zinc dust-zinc oxide paints (finely powdered zinc metallic and zinc oxide pigment in an oil or alkyd base) adhere best to zinc coated steel surfaces under most of the conditions. Zinc dust-zinc oxide primers can be used over new or weathered zinc coated steel and can be top coated with most oil or latex house paints or alkyd enamels. For the maintenance painting of the zinc coated steel, one or two coats of zinc dust-zinc oxide paint are frequently used alone. The paint can be applied by brushing, rolling, or spraying. Zinc sheets to be painted are not to be treated at the coating unit with a chromate treatment, although, they can be given a phosphate treatment to improve the adherence of the paint. Zinc-coated steel sheet is frequently pre-painted in coil form by coil coating.
Packaging and storage – Zinc coated steel products in bundles, coils, or stacks of sheets are to be protected from moisture, including condensation moisture, until openly exposed to the weather. They are to be properly packaged and stored. Otherwise, wet-storage stain, a bulky white deposit which frequently forms on zinc surfaces stored under wet or humid conditions, can develop. It is important to examine packages of galvanized products for damage and to take prompt action where cuts, tears, or other damage is evident. If the packaging is damaged or if moisture is present, the product is to be dried at once and drying continued until thoroughly dry. Erection of materials is to begin as soon as possible after the package arrives at the installation site.
If temporary storage of the zinc coated product is very necessary, it is to be indoors. Where indoor storage is not possible, intact waterproof bundles can be stored at the site. The package is to be slanted so that any condensation drains out, and it is to be stored sufficiently high to allow air circulation beneath and to prevent rising water from entering. The stacks are to be thoroughly covered with a waterproof canvas tarpaulin for protection from rain, snow, or condensation. The use of airtight plastic coverings is to be avoided. To deter the formation of wet-storage stain, zinc coated sheet can be purchased with a mill-applied chromate or chromate / phosphate film. Various proprietary mixtures are also available.
Hot dip galvanizing – It is a process in which an adherent, protective coating of zinc and iron-zinc alloys is developed on the surfaces of iron and steel products by immersing them in a bath of molten zinc. Majority of zinc coated steel is processed by hot dip galvanizing. One method of hot dip galvanizing is the batch process, which is used for fabricated steel items such as structurals or pipes. This method involves cleaning the steel articles, applying a flux to the surfaces, and immersing them in a molten bath of zinc for different time periods to develop a thick alloyed zinc coating.
The most common form of hot dip galvanizing for steel sheet is done on a continuous galvanizing line. Coiled sheet is fed from pay-off reels through flatteners. It is then cleaned, bright annealed, and passed through the coating bath. After leaving the coating bath, the coating thickness is controlled by an air knife or steel rolls. The sheet is then cooled and recoiled or cut into lengths. The hot dip process normally coats both sides of the sheet. However, hot dip galvanized sheets can be coated on one side only for special uses, such as automotive exposed panels, by the use of special coating techniques. One-side coated sheet produced by the hot dip process is normally not available. Continuous coating lines are to be specially modified to make one-side coated product.
A typical hot dip galvanized coating produced by the batch process consists of a series of layers. The molten zinc is interlocked into the steel by the alloy reaction, which forms zinc-iron layers and creates a metallurgical bond. Starting from the base steel at the bottom of the coating, each successive layer contains a higher proportion of zinc until the outer layer, which is relatively pure zinc, is reached. There is, hence, no real line of demarcation between the iron and zinc, but a gradual transition through the series of iron-zinc alloys which provide a powerful bond between the base steel and the coating. The structure of the coating (the number and extent of the alloy layers) and its thickness depend on the composition and physical condition of the steel being treated as well as on a number of factors within the control of the operator of the galvanizing unit.
The ratio of the total thickness of the alloy layers to that of the outer zinc coating is affected by varying the time of immersion and the speed of withdrawal of the work from the molten zinc bath. The rate of cooling of the steel after withdrawal is another factor to be considered. Rapid cooling gives small spangle size. In the continuous strip processes normally the formation of alloy layers is suppressed by adding 0.1 % to 0.2 % Al to the bath. This increases the ductility of the coating, thus rendering the sheet more amenable to fabrication. Other elements can be added to galvanizing baths to improve the characteristics and appearance of the coating. Lead and antimony give rise to well-defined spangle effects. During batch galvanizing, the zinc-iron alloy portion of the coating represents 50 % to 60 % of the total coating thickness. However, certain combinations of elements can result in a coating which is either completely or almost completely alloyed.
Visually, the zinc-iron alloy coating has a gray, matte appearance because of the absence of the free-zinc layer. The free-zinc layer imparts the typical bright finish to a galvanized coating. Because of the greater percentage of the zinc-iron alloy present in the coating, the alloy-type coating can have a lower adherence than the regular galvanized coating.
The corrosion resistance of the zinc-iron and free zinc coating types is normally equal for all practical purposes. Steels containing carbon below 0.25 %, phosphorus below 0.05 %, and manganese below 1.35 % (either individually or in combination) normally develops regular galvanized coatings when conventional galvanizing techniques are used and when silicon is 0.05 % or less or ranges between 0.15 % and 0.3 %. Fabricators and consumers are to be aware that a gray matte appearance can occur in batch galvanizing if silicon content exceeds 0.06 %. This matte appearance does not reduce the long-term atmospheric corrosion protection of the galvanized coating.
Galvanized coatings on sheet products which are intended to be painted are often given treatments to make the spangle less obvious so that it does not show through the paint. A flat spangle without relief (suppressed spangle) can be achieved by small additions of antimony to the molten bath; smaller grain size (minimized spangle) can be produced by spraying the molten zinc with zinc dust, steam, air, or water just before it freezes. Finer grains are less visible through the paint and have narrower and smaller fractures on bending, often permitting the paint to bridge the gap and provide increased protection.
Galvanized steel sheet can be temper rolled to flatten surface irregularities such as dross and grain boundaries, thus providing an extra smooth surface more suitable for painting where critical surface requirements are there. At the galvanizing unit, galvanized steel sheet can be given a thermal treatment after coating, which converts the entire free zinc to zinc-iron alloy, thus providing a spangle-free surface which is more suitable for painting. It can be painted without pretreatment (but not with all paints). As an added benefit, there is no spangle to show through the paint. However, the zinc-iron alloy coating is somewhat brittle and tends to powder if severely bent in fabrication.
The general requirements for the products are described in the standards normally include the requirement of the bend test. However, the bend test requirements are not included for structural quality sheet.
Hot dip galvanized steel sheets are normally of commercial quality (CQ), drawing quality (DQ), and drawing quality special killed (DQSK). Commercial quality sheet is satisfactory for applications requiring bending and moderate drawing. DQ sheet has better ductility and uniformity than CQ and is good for ordinary drawing applications. DQSK sheet is superior to drawing quality and is good for applications requiring severe drawing. When higher strength is needed, structural quality (SQ) sheet, also called physical quality sheet, can be used, although at some sacrifice in ductility.
Electro-galvanizing – Very thin formable zinc coatings ideally suited for deep drawing or painting can be achieved on the steel products by electro-galvanizing. Zinc is electro-deposited on a variety of the products such as sheet, wire, and, in some cases, pipe. Electro-galvanizing the sheet and wire in coil form produces a thin, uniform coating of pure zinc with excellent adherence. The coating is smooth, readily prepared for painting by phosphatizing, and free of the characteristics spangles of hot dip zinc coatings. Electro-galvanizing can be used where a fine surface finish is needed. The appearance of the coating can be varied by additives and special treatments in the plating bath.
Electro-deposited zinc coatings are simpler in structure than hot dip galvanized coatings. They are composed of pure zinc, have a homogeneous structure, and they are highly adherent. These coatings are not normally as thick as those produced by hot dip galvanizing. Electro-galvanized coating weights as high as 100 grams per square meters have been applied to one or both sides of steel sheet. The normal ranges of coating weights are available in different standards. The coating thicknesses listed are typically less when the application does not subject the steel sheet to very corrosive environments or when the sheet is intended for painting. For more severe corrosion conditions, such as the need to protect cars from road salts and entrapped moisture, heavier coatings are used. These heavier coating weights are applied to the steel sheets used for most body panels.
Electro-deposited zinc is considered to adhere to steel as well as any metallic coating. Because of the good adhesion of electro-deposited zinc, electro-galvanized coils of steel sheet and wire have good working properties, and the coating remains intact after severe deformation. Good adhesion depends on very close physical conformity of the coating with the base steel. Hence, particular care is to be taken during initial cleaning. Electro-deposition also affords a means of applying zinc coatings to finished parts which cannot be dipped. It is especially useful where a high processing temperature can damage the part. One advantage of electro-deposition is that it can be done cold and hence does not change the mechanical properties of the steel.
Zincrometal – It is also used for outer body panels in automobiles. First introduced in 1972, Zincrometal is a coil coated product consisting of a mixed-oxide under layer containing metallic zinc particles and a zinc-rich organic (epoxy) top coat. It is weldable, formable, paintable, and compatible with normally used adhesives. Zincrometal is primarily used in one side applications to protect against inside-out corrosion. The corrosion resistance of Zincrometal is not as good as that of hot dip galvanized steels and its use is declining substantially as more electro-galvanized steels and other types of coatings are employed.
Zinc alloy coated steel – It has also been developed. Coatings include zinc-iron (15 % Fe to 80 % Fe) and zinc-nickel (10 % Ni to 14 % Ni) alloys. These coatings are applied by electro-deposition. Zinc-iron coatings offer good corrosion resistance and weldability. Zinc-nickel coatings are more corrosion resistant than pure zinc coatings, but problems include brittleness from residual stresses and the fact that the coating is not completely sacrificial, as is a pure zinc coating. This can lead to accelerated corrosion of the steel substrate if the coating is damaged.
Multi-layer coatings which take advantage of the properties of each layer have also been developed. An example of this is Zincrox, a zinc-chromium-chromium oxide coating. The CrOx top layer of this coating acts as a barrier to perforation and provides excellent paint adhesion and weldability. Another development in zinc alloy coatings is Galfan, a Zn-5Al-mischmetal alloy coating applied by hot dipping. Automakers have used Galfan in such applications as brake servo housings, headlight reflectors and frames, and universal joint shrouds. Galfan is also being considered for oil pans, and heavily formed unexposed body panels.
Zinc spray coating – It consists of projecting atomized particles of molten zinc onto a prepared surface. Three types of spraying pistols are in commercial use namely (i) the molten metal pistol, (ii) the powder pistol, and (iii) the wire pistol. The sprayed coating is slightly rough and slightly porous. The specific gravity of a typical coating is around 6.35, compared to 7.1 for the cast zinc. This slight porosity does not affect the protective value of the coating, since zinc is anodic to steel. The zinc corrosion products which are formed during service fill the pores of the coating, giving a solid appearance. The slight roughness of the surface makes it an ideal base for paint, when properly pre-treated. On-site spraying can be performed on finished parts of almost any shape or size. When applied to finished articles, welds, ends and rivets receive adequate coverage. Moreover, it is the only satisfactory method of depositing unusually thick zinc coatings (more than 0.25 mm)
Aluminized (aluminum-coated) steel sheet is used for applications where heat resistance, heat reflectivity, or barrier-layer resistance to corrosion is needed. Aluminum coating of steel sheet is done on continuous lines similar to those used for hot dip galvanizing of steel sheet. Cold-rolled steel sheet is hot dipped into molten aluminum or an aluminum alloy containing 5 % Si to 10 % Si. The coating consists of two layers, the outer layer of either pure aluminum or aluminum-silicon alloy and the base steel, with an aluminum-iron-silicon alloy layer in between. The thickness of this alloy can considerably affect the ductility, adhesion, uniformity, smoothness, and appearance of the surface and is controlled for optimum properties.
Aluminum-coated sheet steel combines the desirable properties of aluminum and steel. Steel has a greater load-bearing capacity, having a modulus of elasticity of about three times that of unalloyed aluminum. The thermal expansion of steel is around half as much as that of aluminum. The aluminum coating provides corrosion resistance, resistance to heat and oxidation, and thermal reflectivity. Typical applications include (i) automotive mufflers and related components, (ii) catalytic converter heat shields, (iii) drying and baking ovens, (iv) industrial heating equipment, (v) fire places, (vi) home incinerators and furnaces, (vi) fire and garage doors, (vii) kitchen and laundry appliances, (viii) metal buildings, (ix) agricultural equipment, (x) silo roofs, (xi) play-ground equipment, (xii) outdoor furniture, (xiii) signs, masts, and lighting fixtures, and (xiv), containers and wrappers.
Coating weight – Aluminum coatings on steel sheet are designated according to total coating weight on both surfaces in grams per square meter of the sheet. The coating categories are defined in the standards. Normally, a light coating, is recommended for drawing applications and when welding is a significant portion of the fabrication. Regular or commercial quality has around 25 micrometers thick coating on each surface. It is designated for applications requiring excellent heat resistance. Thicker coating around 50 micrometers on each side is often used for atmospheric corrosion resistance. Coating weight on samples from aluminum-coated sheet is determined by the test method defined in the standards. Typically a typical rear suspension of a front-wheel drive vehicle utilizes regular quality aluminized steel components having a coating of Al-9Si-3Fe in conjunction with galvanized front pivot hangers, mounting brackets, and braces.
Base metal and formability – Aluminum coating can be applied to CQ, DQ, or DQSK steel sheet, depending on the severity of the forming or drawing needed. Only moderate forming and drawing are suggested for aluminized steel sheet, but there are several intricate components for heating, combustion, and other equipment being produced.
Shallow crazing (hairline cracks) can occur in the coating if the bending and forming are too severe. To eliminate crazing, the radius of the bend is required to be increased. If the crazing is deep enough to expose the steel to the atmosphere during service, staining can occur. These stains normally have minimal effect on the serviceability of the product, since the corrosion stops at the crazed area after a relatively short exposure period. However, if water collects and does not drain off, corrosion products are dissolved and corrosion continues.
Mechanical properties – Mechanical properties of hot dip aluminized steel sheet are essentially the same as those of hot dip galvanized steel sheet. When high strength is needed, SQ aluminized steel sheet can be used, although at some sacrifice in ductility.
Corrosion resistance – The value of aluminum as a protective coating for steel sheet lies principally in its inherent corrosion resistance. In the majority of the environments, the long-term corrosion rate of aluminum is only around 15 % to 25 % that of zinc. Normally, the protective value of an aluminum coating on steel is a function of coating thickness. The coating tends to remain intact and hence provides long-term protection.
Aluminum coatings do not provide sacrificial protection in the majority of the environments, particularly in atmospheric exposure. This is since a protective oxide film forms on the coating, which tends to passivate the aluminum and retard sacrificial protection. If the oxide film is destroyed, the aluminum provides sacrificial protection to the base steel. In marine or salt-laden environments, the aluminum coating protects sacrificially wherever chlorides destroy the surface oxide film. Although staining or light rusting of the steel can occur at cut edges or crazing can occur where the aluminum does not protect, this action diminishes with further exposure time because of the self-sealing action of corrosion products. However, if insufficient slope or drainage permits water to pond or remain instead of running off freely, the corrosion products are dissolved and rusting continue.
Heat resistance – Aluminum coated sheet steel has excellent resistance to high temperature oxidation. At the surface temperatures below around 510 deg C, aluminum coating protects the base steel against oxidation without discoloration. Between 510 deg C and 675 deg C, the coating provides protection to the steel, but some darkening can result from the formation of aluminum-iron-silicon alloy. The alloy is extremely heat resistant, but upon long exposure at temperatures above 675 deg C, the coating can become brittle and spall because of a different coefficient of expansion from that of the steel.
Because of their good resistance to scaling, combined with the structural strength of the base steel, regular quality coatings are used in automotive exhaust systems, heat exchangers, ovens, furnaces, flues, and tubing. The higher strength of the base steel, which melts at around 1580 deg C, enables steel sheet coated with either regular quality or thicker coatings to perform for a longer time than aluminum alone in case of a fire.
Heat reflection – The thermal reflectivity of aluminum coated steel sheet is comparable to that of aluminum sheet. It is superior to galvanized steel sheet after a relatively short exposure time. All three sheet materials have thermal reflectivity of around 90 % before exposure. However, after a few years, the value for galvanized steel decreases more than that for aluminized steel. Aluminum and aluminum coated steel sheet retain 50 % to 60 % of their heat reflectivity. This is advantageous where heat is to be confined, diverted, or excluded, as in heat transfer applications. When used for roofing and siding, aluminum coated sheet keeps buildings cooler in summer and warmer in winter.
Weldability – Aluminum coated steel sheet can be joined by electric resistance welding (spot welding or seam welding). It can also be metal arc welded, flash welded, or oxy-acetylene welded. Thorough removal of grease, oil, paint, and dirt followed by wire brushing is recommended before joining. Special fluxes are required for metal arc and oxy-acetylene welding. During spot welding, electrodes tend to pick up aluminum, and the tips are to be dressed more frequently than during spot welding of uncoated steel. Also, higher current density is needed.
Painting – It is normally unnecessary, but aluminum coated sheet steel can be painted similarly to aluminum sheets. This includes removal of oil or grease and treatment with a phosphate, chromate, or proprietary wash type chemical before painting.
Handling and storage – The coating on aluminized steel sheet is soft, and care is to be taken to avoid scratching and abrasion of the soft coating, which spoils the appearance and allow staining if the coating is removed. Wet storage stains develop on aluminum-coated steel sheet which is continuously exposed to moisture. The sheet is to be inspected for the entrapped moisture when received and then stored indoors in a warm, dry place. Some added protection can be achieved if the sheet is oiled or chemically treated for resistance to wet storage stain.
Aluminum zinc alloy coatings
In recent years, the desire and need to improve the corrosion resistance of galvanized coatings while retaining sacrificial galvanic corrosion behaviour at the sheared edges, and so on, have led to the development of two types of hot dip aluminum-zinc alloy coatings. One type consists of around 55 % aluminum and 45 % zinc, while the other type consists of zinc plus 5 % aluminum. Both the coating types contain small amounts of other alloying elements to improve wettability and / or coating adhesion during forming. Descriptions of these products are contained in the standards. The specifications include the general requirements, the coating categories available, and the product types which are available.
The 55 % Al coating has been produced worldwide for more than 10 years. Its primary use is for pre-engineered metal buildings, especially roofing. In most environments, this coating has been found to have 2 to 4 times the corrosion resistance of galvanized coatings while retaining an adequate level of galvanic protection to minimize the tendency toward rust staining at edges and other breaks in the coating. Fig 3 shows the corrosion resistance of 55Al-Zn coated steel exposed to four atmospheres. The coated sheet is available in similar grades (CQ, DQ, high strength, and so on) as hot dip galvanized and can be subjected to similar types of forming. It can also be painted either by coil-line painting methods or post-painting after fabrication.
Fig 3 Coating thickness loss of 55Al-Zn-coated steel in four atmospheres
The coating microstructure consists of an aluminum-iron inter-metallic alloy bond between the steel and outer coating metal layer. This outer coating layer has a duplex microstructure, a matrix phase of an aluminum-rich composition, and a zinc-rich inter-dendritic phase. This zinc-rich phase corrodes preferentially to provide the galvanic corrosion protection. The coating contains around 2 % Si, which is present in the microstructure as an elemental silicon phase. The silicon is added only to inhibit growth of the alloy layer during the hot dip coating operation.
Although the 55 % Al coating is primarily used for metal-building applications, there are a variety of other applications, including appliances and automotive parts. It offers a level of heat oxidation resistance intermediate between galvanized and aluminized coatings. The Zn-5Al coating is also produced worldwide, but it is not as commonly available as the 55 % Al coating. Its primary characteristic is improved coating ductility compared to hot dip galvanized coatings. This feature, along with a somewhat improved corrosion resistance, makes this coated sheet steel attractive for deep drawn parts. Also, for pre-painted sheets such as roll-formed metal-building panels, the improved coating ductility minimizes the tendency toward cracking of the paint along tension bends. The Zn-5Al coated sheet is also available in similar grades (CQ, DQ, and so on) as hot dip galvanized. It is readily paintable, including coil-line pre-painting.
Both types of aluminum-zinc coating have features which make them more attractive than galvanized for certain applications. Selection of either one is to be based on consideration of the desired qualities and differences in fabricability, weldability, paintability, and so on, compared to the other coatings available.
Tin coatings are applied to steel sheet by electrolytic deposition or by immersion in a molten bath of tin (hot dip process). Hot dip tin coatings provide a non-toxic, protective, and decorative coating for food handling, packaging, and dairy equipment, and they facilitate the soldering of components used in electronic and electrical equipment. In many places, hot dip tin coating has been replaced by electrolytic tin coating.
Electrolytic tin coated steel sheet is used where solderability, appearance, or corrosion resistance under certain conditions is important, as in electronic equipment, food handling and processing equipment, and laboratory clamps. It is normally produced with a matte finish formed by applying the coating to base steel sheet called black plate, which has a dull surface texture, and by leaving the coating unmelted. It can also be produced with a bright finish by applying the coating to base steel having a smooth surface texture and then melting the coating. Electrolytic tin coated sheet is normally produced in nominal thicknesses from 0.38 mm to 0.84 mm and in widths from 305 mm to 915 mm.
Electrolytic tin coated steel sheet can be specified to one of the five coating weight designations (25, 50, 75, 100, and 125). The coating weight is the total amount of tin on both surfaces, expressed in grams per square meters of sheet area. Electrolytic coatings can be applied to CQ, DQ, or DQSK steel sheet, depending on the severity of the forming or drawing needed. They can also be applied to SQ steel sheet when higher strength is needed. Electrolytic tin coated steel sheet is covered in the standards. The mechanical properties of the steel sheet are unchanged by the electrolytic tin coating process.
Long terne steel sheet is carbon steel sheet continuously coated by the hot dip process with terne metal (lead with 3 % to 15 % tin). This coated sheet is duller in appearance than tin coated sheet, hence the name (terne) from the French, which means dull or tarnished. The smooth, dull coating gives the sheet corrosion resistance, formability, excellent solderability, and paintability. The term long terne is used to describe terne-coated sheet, while short terne is used for terne-coated plate. Short terne, also called terne plate, is no longer produced in many countries.
Because of its unusual properties, long terne sheet has been adapted to a wide variety of applications. Its maximum use is in automotive gasoline tanks. Its excellent lubricity during forming, solderability and special corrosion resistance make the product well suited for this application. Other typical applications include (i) automotive parts, such as air conditioners, air filters, cylinder head covers, distributor tubes, and oil filters, (ii) oil pans, radiator parts, and valve rocker arm covers, (iii) electronic chassis and parts for radios, tape recorders, and television sets, (iv) file drawer tracks, (v) fire doors and frames, (vi) furnace and heating equipment parts, (vii) railroad switch lamps, and (viii) small fuel tanks for lawn mowers, power saws, tractors, and outboard motors.
Long terne sheet is frequently produced to standard specifications. The coatings are designated according to total coating weight on both surfaces in grams per square meter of sheet area. For applications requiring good formability, the coating is applied over CQ, DQ, or DQSK low carbon steel sheet. The terne coating acts as a lubricant and facilities forming, and the strong bond of the terne metal allows it to be formed along with the base metal. When higher strength is needed, the coating can be applied over SQ low carbon steel sheet, although there is some sacrifice in ductility. In general, the mechanical properties of hot dip terne coated steel are similar to those for cold rolled steel. Terne coatings are applied by a flux-coating process which does not include in-line annealing. Hence, the mechanical properties are achieved by pre-annealing using cycles comparable to those used for cold rolled sheet.
Lead is well known for its excellent corrosion resistance, and terne metal is principally lead with some tin added to form a tight, inter-metallic bond with steel. The good corrosion resistance of terne sheet accounts for its wide acceptance as a material for gasoline tanks. However, because lead does not offer galvanic protection to the base steel sheet, care is to be taken to avoid scratches and pores in the coating. Small openings can be sealed by corrosion products of iron, lead, and oxygen, but larger ones can corrode in an environment unfavourable to the base steel.
Long terne sheet can be readily soldered with non-corrosive fluxes using normal procedures since the sreel sheet is already pre-soldered. This makes it a good choice for television and radio chassis and gasoline tanks, for which ease of solderability is important. It can also be readily welded by either resistance seam or spot welding methods. However, when the coating is subjected to high temperatures, significant concentrations of lead fumes can be released. Because of the toxicity of lead, the regulatory bodies have publicized standards which are to be followed when welding, cutting, or brazing metals which contain lead or are coated with lead or lead alloys.
Long terne steel sheet has excellent paint adherence, which allows it to be painted using conventional systems, but this product is not usually painted. When painting is done, no prior special surface treatment or primer is necessary, except for the removal of ordinary dirt, oil, and grease. Oiled sheet, however, are to be thoroughly cleaned to remove the oil.
Long terne steel sheet is normally furnished dry and requires no special handling. It is to be stored indoors in a warm, dry place. Unprotected, outdoor storage of coils or bundles can result in white or gray staining of the terne coating, and if there are pores in the terne coating, rust staining can occur.
The phosphate coating of iron and steel consists of treatment with a dilute solution of phosphoric acid and other chemicals by which the surface of the steel, reacting chemically with the phosphoric acid, is converted into an integral layer of insoluble crystalline phosphate compound. This layer is less reactive than the steel surface and at the same time is more absorbent of lubricants or paints. Since the coating is an integral part of the surface, it adheres to the base steel closely. The weight and crystalline structure of the coating, as well as the extent of penetration of the coating into the base steel, can be controlled by the method of cleaning before treatment, the method of applying the solution, the duration of treatment, and the changes in the chemical composition of the phosphating solution. The two types of phosphate coatings in general use are zinc phosphate and iron phosphate. Within each type, chemical composition can be modified to suit various applications.
When zinc phosphate coatings are applied at the plant to galvanized sheets, the sheets are ready for immediate painting with the many paints readily available. The zinc phosphate coated product is often referred to as phosphatized. Minor cleaning with mineral spirits, paint thinner, or naphtha can be necessary to remove fingerprints, oils, or dirt picked up during fabrication or handling. When mill-phosphatized sheets which are to be baked after painting are exposed to humid storage conditions for long periods of time, pre-baking for several minutes at 150 deg C prior to painting can be required to prevent blistering during baking. The chief application for iron phosphate coatings is as a paint base for uncoated carbon steel sheet. Such a coating can be applied on coil coating lines.
The largest quantity of phosphate-coated steel is low-carbon flat-rolled material, which is used for applications such as sheet metal parts for automobiles and household appliances. Applications of the coatings range from simple protection to pre-paint treatments for painted products, such as pre-engineered building panels and the side and top panels of washing machines, refrigerators, and ranges.
Phosphate coatings need a clean surface. The cleaning stage preceding phosphating removes foreign matter from the surface and makes uniformity of coating possible. This involves removal of oils, greases, and associated dirt by solvent degreasing or alkaline cleaning followed by thorough rinsing. Phosphate coatings are applied by spray, immersion, or roller coating. A phosphate coating beneath a paint film helps prevent moisture and other corrosives which can penetrate the paint from reaching the metal. This prevents or delays the electro-chemical reactions which lead to corrosion or rust. If the paint film sustains scratches or damage which exposes bare steel, the phosphate coating confines corrosion to the exposed metal surface, preventing the corrosion from spreading underneath the paint film. In painting applications, coarse or heavy phosphate coatings can be detrimental. The coatings can absorb too much paint, thus reducing both gloss and adhesion, especially if deformation of the painted sheet steel is involved.
Primer paint coats are frequently applied to steel sheet at the steel plant or by a coil coating unit. Since their purpose is corrosion protection, they contain corrosion-inhibiting substances such as zinc powder, zinc chromate, or other compounds of zinc and / or chromium. Pre-primed sheets are especially useful for parts which have limited access after fabrication, rendering coating difficult. Parts made from pre-primed sheet can receive a top coat after fabrication. The plant-applied phosphate coatings can also be considered pre-priming treatments.
Formability – Pre-primed steel offers advantages in forming metal fabrication through (i) consistent surface morphology, (ii) reduced surface friction (reducing the flow over die surfaces) and reduced die wear, especially on the binder surfaces, (iii) reduced flaking and powdering (requiring less die maintenance), reduced need for metal finishing, and lesser surface defects, and (Iv) reduced galling. The painted surface acts as a cushion between sub-strate and stamping dies, which lessens the need for in-die lubrication and extends the life of the stamping die. The pre-primed, pre-painted surface can withstand severe forming and stretching. Hence, the need for lubricant is reduced or eliminated. This in-turn provides a clean process environment and reduces the need for extensive cleaning along with phosphating and electro-coating.
Zinc chromate primers – Zinc chromate pigments are useful as corrosion inhibitors in paint. They are used as after pickling coatings on steel and in primers. Zinc chromate pigments are unique since they are useful as corrosion inhibitors for both ferrous and nonferrous metals.
Zinc rich primers – In recent years, several priming paints have been developed which deposits films consisting mainly of metallic zinc which have many properties in common with the zinc coatings applied by hot dip galvanizing, electroplating, metal spraying, or mechanical plating methods. Such films provides some degree of sacrificial protection to the underlying steel if they contain 92 % to 95 % metallic zinc in the dry film and if the film is in electrical contact with the steel surface at a sufficient number of points. The type of zinc dust used in protective coatings is a heavy powder. It is light blue-gray in colour with spherically shaped particles having an average diameter of around 4 micro meters. Such powder normally contains 95 % to 97 % free metallic zinc with a total zinc content exceeding 99 %. Many zinc-rich paints are air drying, although oven-curable primers containing a high content of zinc dust are available.
Depending on the nature of the binder, zinc rich primers are classified as inorganic or as organic. The inorganic solvent-base types are derived from organic alkyl silicates, which, upon curing, become totally inorganic. The organic zinc-rich coatings are formed by using zinc dust as a pigment in an organic binder. This binder can be any of the well-known coating vehicles, such as chlorinated rubber or epoxy. The zinc dust is to be in sufficient concentration so that the zinc particles are in particle-to-particle contact through-out the film. In this way, zinc provides cathodic protection to the base steel. With the organic binder, there is no chemical reaction with the underlying surface, but the organic vehicle is to wet the surface thoroughly to achieve mechanical adhesion.
The inorganic zinc coating forms its film and adheres to the steel surface by quite different means. The chemical activity during coating is quite similar for either water-base or solvent-base inorganic binders. Zinc is the principal reactive element in the inorganic systems and is primarily responsible for the development of initial insolubility.
Zinc-rich primers offer a more versatile application of zinc to steel than galvanizing. Large, continuous, complex shapes and fabricated new or existing structures can be easily coated at production shops or in the field. The performance of zinc-rich primers has earned them a prominent place in the field of corrosion protection coatings. As an example, zinc-rich primers are being pre-applied to steel sheet as the first coat of a two-coat system for appliance applications such as refrigerator liners. However, the limitations of zinc-rich paints include cost and the required cleanliness of steel surfaces. They are to be top coated in severe environments (pH less than 6 and more than 10.5).
The following comparisons are helpful in selecting the binder system which is the most suitable for an application. Inorganics have superior solvent and fuel resistance. They can withstand temperatures to 370 deg C and are much easier to clean up after use. Inorganics do not blister upon exposure and are unaffected by weather, sunlight, or wide variations in temperature. They do not chalk, peel, or lose thickness over long periods of time. Also, they are easier to weld through and have excellent abrasion resistance and surface hardness. Organics use chlorinated rubber, epoxy, vinyl, phenoxy, or other coating vehicles, and the properties of the system are based on the characteristics of the vehicle used.
Organic composite coatings
Organic composite coated steels are coil coated products and normally use an electroplated zinc alloy base layer and a chemical conversion coating under a thin organic top coat containing a high percentage of metal powder. The thinness of the organic top coat allows for good formability without the risk of damaging the coating. Fig 4 compares the corrosion resistance of one of these organic composite coated sheet steels to cold-rolled steel and to Zincrometal. Test in Fig 4 consisted of 28-min cycles of dipping in 5 % saline solution at 40 deg C, humidifying at 50 deg C, and drying at 60 C.
Fig 4 Corrosion of heavily worked samples of a composite-coated steel, Zincrometal, and cold-rolled steel in a laboratory cyclic test
Another of these products uses an organic-silicate composite top coat only about 1 micrometer thick and has corrosion resistance and weldability superior to that of Zincrometal. A bake-hardenable version of this material with a 0.8 micro meters to 1.5 micro meters thick organic top coat has also been developed. The material possesses corrosion resistance, formability, and weldability equivalent to that of Zincrometal, which uses a 7 micro meters thick top coat.
A similar material developed has an electro-deposited zinc alloy base coat, a mixed intermediate layer of chromium oxide and zinc dust, and an organic top coat for barrier protection. In salt spray tests comparing this material to electro-deposited zinc-nickel and Zincrometal, zinc-nickel failed after 216 hours, Zincrometal at 480 hours, and the composite coating at 960 hours. This material has been developed to have weldability, formability, and adhesive compatibility similar to that of Zincrometal. Developmental work is continuing.
Organic-silicate composite coatings – Zinc-nickel electro-plated steel sheet coated with an organic-silicate composite has been developed in an attempt to combine a highly corrosion resistant base zinc-nickel coating with a protective surface layer to prolong the coating life. With a view to forming a thin film with high corrosion resistance, the protective layer has been designed as a two-layer protective film structure composed of a chromate film as a lower layer and the organic-silicate composite coating (the composite resin) as an upper layer. This protective film structure improves the corrosion resistance not only by the individual effects of each layer, such as the passivation of chromate film and the excellent corrosion resistance of the composite resin, but also by the suppression of excessive dissolution of Cr6+ from the lower chromate film layer by the sealing effect of the upper composite resin layer. This sealing effect sustains the passivation of chromate film more effectively in the corroding environment.
Pre-painted steel sheet is coated in coil form in a continuous coil painting facility. Lower production costs, improved product quality, elimination of production hazards in the shop, customer satisfaction, conservation of energy, elimination of ecological problems, and the ability to expand production without capital expenditure for new buildings and equipment are some of the advantages of pre-painted sheet over post-painting. Fabricated parts are readily joined by indirect projection welding, adhesives, tabs, and fasteners. Typical applications of pre-painted steel sheet include tool sheds, pre-engineered buildings, swimming pools, automobiles, lighting fixtures, baseboard heaters, truck vans, mobile homes, home siding, metal awnings, air conditioners, freezer, refrigerators, ranges, washers, and dryers.
Selection of paint system – A wide variety of paint systems are available on pre-painted sheet. In selecting the proper system for a particular application, the user is to consider fabrication requirements, the service life desired, and the service conditions, which are to be encountered. As an example, in an aggressive environment a plastisol coating can be needed. For a deep draw, a vinyl coating is to be used instead of polyester. For resistance to fading in sunlight, silicone polyester can be used instead of polyester or vinyl paint.
In the pre-engineered building industry, the paint system is to be capable of being roll formed and still perform over the years under a wide variety of conditions without chalking, fading, cracking, or blistering. In the automotive area, the drawing properties of the coating are to be considered in addition to corrosion protection from road salts. In the appliance industry, a high-gloss finish which bends without cracking is important, along with resistance to such materials as detergents, solvents, mustard, ketchup, shoe polish, grape juice, and grease. Other product requirements frequently considered when selecting appliance paint are colour, hardness, adhesion, resistance to abrasion, corrosion, humidity, heat, and pressure marking.
For severe corrosion service and decorative effects, heavier coatings are used, often by laminating or bonding a solid film to the metal substrate. Typical applications include buildings, roofing and siding near pickling tanks, chemicals and other corrosive environments, and storm drains and culverts which are subjected to corrosive soils, mine acids, sewage, and abrasion. These culvert coatings can be a thermo-plastic coal tar-base laminate 0.3 mm to 0.5 mm thick, or they can be a film of polyvinyl chloride.
Design considerations – In using pre-painted sheet, design is important. If necessary, binding radii, location of exposed edges, fastening methods, welding techniques, corner assembly, and other features are to be modified to make them compatible with the base metal and paint system. As an example, if a polyester paint is applied to bare steel sheet, a minimum bend radius of 3.2 mm is needed to minimize cracking and crazing of the paint. If hot dip galvanized sheet is the substrate, then minimum bend radius of 6.4 mm is to be used instead. Otherwise, the zinc coating can crack with sharper bending, and the paint may not be elastic enough to bridge the crack.
Paint is often cured at temperatures as high as 240 deg C. At the higher paint curing temperatures, the steel sheet can become fully aged and cause yield point elongation to return. The sheet is subject to the formation of stretcher strains during subsequent forming. Normally, return of yield point elongation is not objectionable in these applications. However, the formed part sometimes is given a critical amount of strain, and strain lines can become visible. Often, this problem can be overcome by proper shop practices, particularly if the part has been roll formed. At times, however, it is necessary to use killed steels, which are considered essentially non-aging.
Shop practices – Since a pre-painted surface is composed of an organic material, hence the abuse which this surface can withstand is less than that of a steel sheet surface. Thus, prior to using pre-painted sheet for the first time, it is advisable to train shop personnel in proper handling practices and to examine shop equipment to eliminate sources of scratches. For example, dies, brake presses, and roll-forming equipment are to have highly polished surfaces free of gouges, score marks, and so on.
Clearances of the dies are to be such that wiping of the paint film is avoided. Similarly, some care is needed when formed parts are put on carts or in containers for transfer from one location to another. It is not acceptable simply to pile one part on top of another. Good housekeeping is important to minimize the source of scratches. Frequent re-examination of shop equipment and parts containers is necessary to minimize scratches. Handling scratches can be refinished by retouching, which is costly and time consuming.
Packaging and handling – Shop and field conditions are to be considered when selecting packaging for pre-painted sheet. Transit pickoff and job-site corrosion from entrapped moisture can be serious problems. For pre-engineered building sheets, for example, packaging after roll forming is to include waterproof paper (no plastic wrapping), support sheets to prevent sagging, and pressure boards. Mixing sheets of different lengths in the bundle is to be avoided. Once the bundle of formed pre-painted sheets arrives at the job site, it is to be inspected to determine if the packages are still intact and resistant to the weather.
Wherever possible, sheets are to be erected on the day of delivery, or they are to be protected from water condensation. Under-roof storage is desirable. However, if this is not possible, the waterproof bundles are to be slanted so that any condensation can drain out. Damaged packages are to be opened, inspected, and the sheets separated to allow complete drying. In addition to the prevention of moisture entrapment, chips from drilling operations are to be brushed away to prevent rust spotting. Pre-painted sheets are to be installed with corrosion-resistant fasteners. Installations of sheets which are in contact with the soil are to be avoided. Oil, grease, fingerprints, and other contaminants are to be removed after installation.