Galvanizing of Steel
Galvanizing of Steel
Galvanizing of steel means application of zinc on the steel for corrosion protection. The major types of commercially available methods for applying zinc coatings are hot dip galvanizing, continuous galvanizing process, electro-galvanizing, zinc plating, mechanical plating, zinc spraying, and zinc painting. Each of these zinc coating methods has unique characteristics. These characteristics not only affect applicability but also the relative economics and expected service life. The method of processing, adhesion to the base metal, protection afforded at corners, edges, and threads, hardness, coating density, and thickness can vary greatly among the different coatings.
The recorded history of galvanizing dates back to 1742 when P.J. Malouin, a French chemist described a method of coating iron by dipping it in molten zinc in a presentation to the French Royal Academy. Thirty years later, Luigi Galvani, galvanizing’s namesake, discovered more about the electrochemical process which takes place between metals. Galvani’s research was furthered in 1829 when Michael Faraday discovered zinc’s sacrificial action, and in 1836, French engineer Sorel obtained a patent for the early galvanizing process. By 1850, the British galvanizing industry was using 10,000 tons of zinc annually for the protection of steel, and in 1870, the first galvanizing plant started in USA. Today, galvanizing is found in almost every major application and industry where iron or steel is used. Hot dip galvanized steel has a proven and growing history of success in innumerable applications worldwide.
Hot dip galvanizing is one of the most widely used means of protecting steel. In this process application of zinc coating is by a hot dip process. In the process a zinc coating on iron and steel products is obtained by immersion of the material in a bath of liquid zinc. Zinc can be applied as coating to steel in a hot dip process because it has low melting point. Since the steel material is immersed in molten zinc, the zinc flows into recesses and other areas difficult to access, coating all areas of complex shapes thoroughly for corrosion protection. Before the coating is applied, the steel is cleaned to remove all oils, greases, soils, mill scale, and rust. Hot dip galvanized coatings are used on a multitude of materials ranging in size from small parts such as nuts, bolts, and nails to sheets, pipes, and very large structural shapes. The size of available zinc baths and material handling restricts the size of steel which can be galvanized.
Since the galvanizing process involves total immersion of the material into cleaning solutions and molten zinc, the entire interior and the exterior surfaces are coated. This includes the insides of hollow and tubular structures, and the threads of fasteners. Complete coverage is important since corrosion tends to occur at an increased rate on the inside of some hollow structures where the environment can be extremely humid and condensation generally occurs. Hollow structures which are painted have no corrosion protection on the inside. Additionally, fasteners with no protection on the threads are susceptible to corrosion, and corroded fasteners can lead to concerns about the integrity of structural connections.
The galvanizing process naturally produces coatings at least as thick at the corners and edges as the coating on the rest of the part. This is because the reaction between iron and zinc is a diffusion reaction and thus the crystalline structure of the coating forms perpendicular to the steel surface. As coating damage is most likely to occur at the edges, this is where added protection is needed most. Brush- applied or spray-applied coatings have a natural tendency to thin at corners and edges.
Zinc coating and corrosion
Corrosion can simplistically be viewed as the tendency for the steel, after production and shaping, to revert to its lower, more natural energy state of ore. This tendency is known as the ‘law of entropy’. Corrosion protection methods employed to protect steel include (i) altering the metal by alloying, (ii) changing the environment by lowering its humidity or by using inhibitors, (iii) controlling electro-chemical potential by applying cathodic or anodic currents and applying organic and metallic coatings.
Zinc has a number of characteristics which make it a well-suited corrosion protective coating for iron and steel products in most of the environments. In addition to creating a barrier between steel and the environment, zinc has also the ability to cathodically protect the base metal. Zinc, which is anodic to iron and steel, preferentially corrodes and protect the iron or steel against rusting when the coating is damaged. If the galvanized coating is physically damaged, it continues to provide cathodic protection to the exposed steel. If individual areas of underlying steel or iron become exposed by upto a 6 mm diameter spot, the surrounding zinc provide these areas with cathodic protection for as long as the coating lasts.
The outstanding field performance of zinc coatings results from its ability to form dense, adherent corrosion product films and a rate of corrosion considerably below that of ferrous materials (some 10 times to 100 times slower depending upon the environment). While fresh zinc surface is quite reactive when exposed to the atmosphere, a thin film of corrosion products develops rapidly, which greatly reduces the rate of further corrosion.
Hot dip galvanizing provides superior corrosion protection to steel especially in harsh environments. It provides three levels of corrosion resistance to steel namely (i) barrier protection, (ii) cathodic protection, and (iii) the zinc patina.
Barrier protection is the first line of corrosion defense. Such as paints, the hot dip galvanized coating provides protection by isolating the steel from the electrolytes in the environment. As long as the barrier is intact, the steel is protected and corrosion does not occur. However, if the barrier is breached, corrosion begins. Since a barrier is to remain intact to provide corrosion resistance, two important properties of barrier protection are (i) adhesion to the base metal, and (ii) abrasion resistance. The tightly-bonded, impervious nature of zinc metal makes it a very good barrier coating. Coatings such as paint which generally have pin holes are susceptible to penetration by elements causing under-film corrosion to spread rapidly.
In addition to the barrier protection, hot dip galvanizing also protects steel cathodically, which means zinc is preferentially corroded to protect the underlying base steel. The Galvanic Series of Metals (Fig 1) is a list of metals arranged in order of electrochemical activity in seawater (the electrolyte). This arrangement of metals determines which of the metal is going to be the anode and cathode when the two are put in a galvanic or electrolytic cell (Fig 1). Metals higher on the list are anodic to the metals below them meaning they provide cathodic or sacrificial protection when the two are connected. Hence, zinc protects steel. In fact, this cathodic protection ensures even if the galvanized coating is damaged to the point bare steel is exposed (upto 6 mm in diameter), no corrosion begins until all the surrounding zinc is consumed.
In a galvanic cell (Fig 1), there are four elements necessary for corrosion to occur. They are (i) anode which is the electrode where the anode reaction(s) generates electrons and the material corrodes, (ii) cathode which is the electrode which receives electrons and it gets protected from corrosion (iii) electrolyte which is the conductor through which ion current is carried and they include water solutions of acids, bases and salts, and (iv) return current path which is the metallic pathway connecting the anode to the cathode and is often the underlying metal. All four elements, anode, cathode, electrolyte, and return current path, are necessary for corrosion to occur. Removal of any one of these elements stops the current flow and corrosion does not occur. Substituting a different metal for the anode or cathode can cause the direction of the current to reverse, resulting in a change as to which electrode experiences corrosion.
Fig 1 By –metallic couple and galvanic series of metal
The final factor in the galvanizing of steel for its long lasting corrosion protection is the development of the zinc patina. The zinc patina is the formation of zinc corrosion by-products on the surface of the steel. Zinc, like all metals, begins to corrode when exposed to the atmosphere. As galvanized coatings are exposed to both moisture and free flowing air, corrosion by-products are naturally formed on the coating surface. The formation of these by-products (zinc oxide, zinc hydroxide, and zinc carbonate) occurs during natural wet and dry cycles in the environment. The zinc patina, once fully developed, slows the corrosion rate of zinc to around 1/30th of the rate of steel in the same environment and acts as an additional passive, impervious barrier for the hot dip galvanized coating.
The process of galvanizing
The galvanizing process (Fig 2) consists of three basic steps namely (i) surface preparation, (ii) galvanizing, and (iii) inspection.
Fig 2 Process of galvanizing
Surface preparation – Surface preparation is the most important step in the application of any coating. In most occasions, incorrect or inadequate surface preparation is generally the cause of a coating failure before its expected service lifetime. The surface preparation step in the galvanizing process has its own built-in means of quality control in that zinc simply does not metallurgically react with a steel surface which is not perfectly clean. Any failures or inadequacies in the surface are immediately be apparent when the steel is withdrawn from the molten zinc since the unclean areas remain uncoated and immediate corrective actions are required to be taken.
Surface preparation for galvanizing typically consists of three stages namely (i) caustic cleaning, (ii) acid pickling, and (iii) fluxing. During the caustic cleaning stage a hot alkali solution is generally used to remove organic contaminants such as dirt, grease and oil from the metal surface. Epoxies, vinyls, asphalt, paint or welding slag are to be removed before galvanizing by grit-blasting, sand-blasting or other mechanical means. During pickling stage, scale and rust are normally removed from the steel surface by pickling in a dilute solution of hot sulphuric acid (H2SO4) or in a ambient temperature hydrochloric (HCl) acid. Surface preparation also can be accomplished using abrasive cleaning as an alternative to or in conjunction with chemical cleaning. Abrasive cleaning is a process whereby metallic shot or grit is propelled against the steel material by air blasts or rapidly rotating wheels.
The third stage is fluxing which is the final surface preparation stage in the galvanizing process. Fluxing removes oxides and prevents further oxides from forming on the surface of the metal prior to galvanizing. The method of applying the flux depends upon whether during the galvanizing operation the wet or dry galvanizing process (Fig 2) is used. In the dry galvanizing process, the steel or iron is dipped or pre-fluxed in an aqueous solution of zinc ammonium chloride. The material is then dried prior to immersion in molten zinc. In the wet galvanizing process, a blanket of liquid zinc ammonium chloride is floated on top of the molten zinc. The iron or steel being galvanized passes through the flux on its way into the molten zinc.
Galvanizing – In this step, the material is completely immersed in a bath consisting of a minimum of 98 % pure molten zinc. The bath chemistry is to be as per the specifications specified by the national or international standards. The bath temperature is maintained at around 450 deg C to 460 deg C. Fabricated steel items are immersed in the bath until they reach bath temperature. The zinc metal then reacts with the iron on the steel surface to form a zinc-iron intermetallic alloy. The articles are withdrawn slowly from the galvanizing bath and excess zinc is removed by draining, vibrating and/or centrifuging.
The metallurgical reactions which result in the formation and structure of the zinc-iron alloy layers continue after the articles are withdrawn from the bath, as long as these articles are near the bath temperature. The articles are cooled in either water or ambient air immediately after withdrawal from the bath. Because the galvanizing process involves total material immersion, it is a complete process i.e. all the surfaces are coated. Galvanizing provides both outside and inside protection for hollow structures.
Factors which influences the thickness and appearance of the galvanized coating include (i) chemical composition of the steel, (ii) steel surface condition, (iii) cold working of steel prior to galvanizing, (iv) bath immersion time, (v) bath withdrawal rate, and (vi) steel cooling rate.
Galvanizing is performed at the plant under any weather or humidity conditions. Most brush- applied and spray-applied coatings depend upon proper weather and humidity conditions for correct application. This dependence on atmospheric conditions often translates into costly construction delays.
Inspection – The two properties of the hot dip galvanized coating which are closely scrutinized after galvanizing are coating thickness and coating appearance. A variety of simple physical and laboratory tests can be performed to determine thickness, uniformity, adherence and appearance. Products are galvanized according to national and international standards. These standards cover everything from minimum required coating thicknesses for various categories of galvanized items to the composition of the zinc metal used in the process.
The inspection process for galvanized items is simple and fast, and requires minimal labour. This is important since the inspection process required to assure the quality in case of many brush-applied and spray-applied coatings is highly labour-intensive and uses expensive skilled labour.
The coating thickness is generally tested by using a magnetic thickness gauge. The minimum coating thicknesses and sampling requirements are normally available in national and international standards. The standards also provide the guidelines for the number of samples which are to be measured based on the total lot size.
The most accurate and arguably the most easy thickness gauge to operate is an electronic magnetic thickness gauge. No individual reading of a sample can be less than one coating grade lower than the required coating grade and the average is to be equal to or more than the required coating grade.
An adherence test is generally not part of the standards but can be performed using a stout knife. If the galvanized coating cannot be removed by pressing firmly with a stout knife, then it is sound.
Physical quality of the galvanized coating
The physical quality of the galvanized coating depends on the metallurgical bond. Galvanizing forms a metallurgical bond combining the zinc and the underlying steel or iron, creating a barrier that is part of the metal itself. During galvanizing, the molten zinc reacts with the iron in the steel to form a series of zinc-iron alloy layers. Fig 3 gives a photomicrograph of a galvanized steel coating’s cross-section and shows a typical coating microstructure consisting of three alloy layers and a layer of pure metallic zinc. The galvanized coating is adherent to the underlying steel on the order of 250 kg/ sq cm. Other coatings typically offer adhesion rated around 20 to 45 kg/sq cm, at the best.
The formation of the galvanized coating on the steel surface is a metallurgical reaction, in that the zinc and steel combine to form a series of hard intermetallic layers, prior to the outside layer being, typically, 100 % zinc (eta layer). The photomicrograph (Fig 3) is a cross-section of a galvanized steel coating. The first zinc-iron alloy layer above the steel surface is the gamma layer which is having around 75 % zinc (Zn) and 25 % iron (Fe). The next layer, the delta layer, is having around 90 % zinc and 10 % iron. The third layer, the zeta layer, is having around 94 % zinc and 6 % iron. The last layer (eta layer), which forms as the material is withdrawn from the zinc bath, is identical to the zinc bath chemistry, i.e. pure zinc. It can be seen in the micrograph, that the gamma, delta and zeta layers form around 60 % of the total galvanized coating, with the eta layer making up the balance.
In the Fig 3, below the name of each layer, its respective hardness has been expressed by a ‘diamond pyramid number’ (DPN). The DPN is a progressive measure of hardness. The higher is the number, the greater is the hardness. Typically, the gamma, delta and zeta layers are harder than the underlying steel. The hardness of these layers provides exceptional protection against coating damage through abrasion. The eta layer of the galvanized coating is quite ductile, providing the coating with some impact resistance. Hardness, ductility and adherence combine to provide the galvanized coating with unmatched protection against damage caused by rough handling during transportation to and/or at the job site as well as during its service life. The toughness of the galvanized coating is extremely important since barrier protection is dependent upon coating integrity. Correctly applied galvanized coatings are impermeable.
Fig 3 Photomicrograph of a galvanized coating
During the reaction of the steel with the molten zinc in the galvanizing bath, two factors have a predominant effect on the growth of the coating. The galvanized coating thickness is primarily determined by both the thickness of the steel and the chemical composition of the steel being coated. This is important for two reasons namely (i) in general, the thicker the zinc coating, the longer the corrosion protection provided, and (ii) excessively thick coatings can have less adherence and bond than coatings of normal thickness.
Steels suitable for galvanizing
Most of the steels can be satisfactorily hot dip galvanized. However, reactive elements in the steel, such silicon and phosphorus can affect the hot dip galvanizing process. An appropriate selection of the steel composition can hence give more consistent quality of coating with regard to appearance, thickness and smoothness. The prior history of the steel (e.g. whether hot rolled or cold rolled) can also affect its reaction with the zinc melt. Where aesthetics is important, or where particular coating thickness or surface smoothness criteria exist, special attention on steel selection is needed prior to the hot dip galvanizing.
The steel chemistry, particularly the levels of silicon, phosphorus, manganese, and carbon, has influence on the coating characteristics. Silicon especially can have a profound effect on the growth of galvanized coatings. Phosphorus and manganese also increase the reactivity of the steel, and in combination with specific silicon levels and can also produce a thicker matte gray coating. The carbon, sulphur, and manganese content of the steel also may have a minor effect on the galvanized coating thickness.
The chemical composition of the steel being galvanized is very important. The amount of silicon and phosphorus present in the steel strongly influences the thickness and appearance of the galvanized coating. A silicon level of 0.04 % or higher or a phosphorous level of 0.05 % or higher in the steel generally result in thick coatings consisting primarily of zinc-iron alloys. For highest quality galvanized coatings, silicon levels are to be less than 0.04 % or between 0.15 % and 0.23 %. Steels outside these ranges, considered reactive steels, can be galvanized, and typically produce an acceptable coating. However, these steels often form a thicker coating, thus a darker appearance is to be expected.
Influence of silicon and phosphorus on steel reactivity – During steel production, silicon or aluminum is added to remove oxygen. These steels are known as killed steels. Since the content of silicon affects the hot dip galvanizing reaction, the silicon content is always be taken into consideration for steels which are to be galvanized. Aluminum killed steels suitable for galvanizing has low silicon content, below 0.03 %. Silicon killed steels with silicon content above 0.14 % also works well in galvanizing, but gives a thicker coating than aluminum killed steels. The phosphorus content of the steel also influences on the reactivity, especially for cold rolled steels. Other alloying elements in the steel have no major influence on the coating.
The Sandelin range – Steels with a silicon + phosphorus content in the range of 0.03 % to 0.14 % are called ‘Sandelin steels’ in galvanizing terminology. These steels are either to be avoided or special types of galvanizing baths are to be used. In a conventional zinc bath the reaction between this type of steel and zinc is very strong and the coating becomes thick and irregular, often with poor adherence. It is the crystals in the outermost alloy layer, the zeta-phase, which grow as small, thin grains. Molten zinc diffuses rapidly between the grains and the growth of the coating is very fast. If zinc baths with suitable alloy additions are not available, this type of steel is to be avoided for hot dip galvanizing.
Studies have shown that the bottom limit for the Sandelin range is lower than earlier suggested. It has also been shown that the phosphorus content has a large influence on the reactivity for cold rolled steels. These studies have given the following recommendations:
If the appearance of the galvanized surface is very important, for example in architectural applications, the expression which is recommended for cold rolled steel is ‘silicon less than 0.03 % and Si + 2.5 x P less than 0.04 %’.
For hot rolled steel the silicon content is even more critical, but the phosphorus content is of less importance, and the expression which is recommended is ‘silicon less than 0.02 % and Si + 2.5 x P less than 0.09 %’.
However, in most cases, steel with silicon + phosphorus content less than 0.03 % is adequate and gives an acceptable surface finish in both the cold rolled and hot rolled conditions.
Aluminum killed steel – Aluminum killed steels also contain low levels of silicon, which is important for the reactivity. In recent years, aluminum killed steel with so-called ultra-low silicon content, below 0.01 %, and aluminum content above 0.035 %, has become more common. These steels have many positive properties when it comes to cutting and formability. However the low silicon content in combination with the high aluminum content makes the zinc layers thinner than stated in the hot dip galvanizing standards.
If galvanizing is performed in a nickel alloy bath, which is common today since nickel is considered to add several positive properties, the reactivity is further decreased, with thinner layers as a result. A deviation from the standard for such steels can be agreed between customer and galvanizer. If a deviation cannot be accepted, this type of steel must be blasted before galvanizing.
Hot dip galvanizing of steels with low silicon content or phosphorus content gives light and shiny zinc coatings. In outdoor environments, the surface colour changes to dull and light grey after some time. Steels with silicon content in the range 0.15 % to around 0.22 % – 0.23 % normally gives light, shiny coatings. Silicon contents around 0.25 % can give grey surfaces or a grey network on an otherwise bright surface. If a nickel alloyed zinc bath is used, the reaction between zinc and iron are reduced, and the coating is usually bright upto 0.22 % of percent silicon.
Steel with higher silicon contents (greater than 0.25 %) normally gives dull, grey coatings, which gets darker with increasing silicon content. The dark grey colour is only an aesthetic effect, since the corrosion protection is the same or even better as long as the coating thickness is the same. Dark grey coatings are often thicker than bright coatings, since they usually are coarser due to the higher reactivity between iron and zinc, and hence give longer corrosion protection. The colour of the coating is determined by the proportion of iron-zinc crystals that are mixed with pure zinc on the outer surface of the coating which is the purer zinc. The lighter is the surface, the higher is the iron-zinc content, the darker the surface.
When a zinc coating with high iron content corrodes, the iron is released and oxidized, which can give the surface a reddish-brown dis-colouration. The reddish-brown colour increases when a larger part of the iron-zinc coating corrodes. Thus, a reddish-brown dis-colouration of the surface does not mean that the zinc coating is gone.
Zinc coatings with an outermost layer of pure metallic zinc and a light appearance can also develop reddish-brown dis-colouration when the pure zinc layer has corroded away. The time for the reddish-brown dis-colouration to form is longer in this case, depending on the thickness of the pure zinc layer. On steels with a silicon + phosphorus content of higher than 0.03 % the pure zinc content is generally 30 % -50 % of total coating thickness.
Continuous galvanizing process
The continuous hot dip coating process is a widely used method originally developed around 1960s for galvanizing of products such as steel sheet, strip, and wire. The molten coating is applied onto the surface of the steel in a continuous process. The steel is passed as a continuous ribbon through a bath of molten zinc at speeds upto 200 metres per minute. The size of the steel sheet can range from 0.25 mm to 4.30 mm thick, and upto 1830 mm wide.
This continuous hot dip coating process begins by cleaning the steel in a process unit which typically uses an alkaline liquid combined with brushing, rinsing, and drying. Then, the steel passes into the heating or annealing furnace to soften it and impart the desired strength and formability. In this annealing furnace, the steel is maintained under a reducing gas atmosphere, composed of hydrogen and nitrogen, to remove any oxide which can be on the steel surface. The exit end of the furnace is connected with a vacuum chamber, known as a ‘snout’, to the molten coating bath to prevent any air from re-oxidizing the heated steel product. In the bath, the steel product is sent around a submerged roll and reacts with the molten metal to create the bonded coating, and then removed in a vertical direction. Once the product is removed from the bath, high pressure air is used to remove any excess molten zinc to create a closely controlled coating thickness. Then the steel is cooled to allow the metal to solidify onto the steel surface, which is done before the steel contacts another roll to avoid transferring or damaging the coating.
The hot dip process for sheet product is used today to make seven different types of hot dip coated products, including galvanized (zinc), galvannealed (90 % – 92 % zinc and 8 % – 10 % iron alloy), two alloys of zinc and aluminum (55 % aluminum and 45% zinc alloy, and 95 % zinc and 5 % aluminum alloy), two aluminum based alloys (100 % aluminum, and 89 % – 95 % aluminum and 5 % – 11 % silicon alloy), and the terne coating (85 % – 97 % lead and 3 % – 15 % tin alloy).