Corrosion in Steels – Its Types and Testing
Corrosion in Steels – Its Types and Testing
Corrosion is a universal natural process. The effect of corrosion is seen in every-day life in the form of rusted steel parts. Corrosion has a huge economic impact. About a fifth of the global annual steel production goes towards simply replacing steel parts damaged by corrosion. Even though it involves higher up-front cost, correct and efficient corrosion protection at the source helps save money and resources in the long run. Failure due to corrosion can result into dramatic consequences.
Corrosion is the gradual degradation of a metal by chemical, often electrochemical reaction with the surrounding environment. Corrosion results into loss of material properties such as mechanical strength, appearance, and impermeability to liquids and gases. Whether steel is corrosion resistant in a specific environment depends on the combination of the chemical composition of steel and the aggressiveness of the environment.
As per ISO 8044:2010, corrosion is the physicochemical interaction between a metal and its environment, which results in changes in the metal’s properties and which may lead to significant functional impairment of the metal, the environment, or the technical system of which they form a part. Corrosion takes place when there is a change in the steel’s or system’s properties which may lead to an undesirable outcome. This can range simply from visual impairment to complete failure of technical systems which cause great economic damage and even present a hazard to the people.
The typical corrosion process can be regarded as the thermodynamically favoured reverse reaction of the metal-winning (extraction) process (Fig 1). Like all chemical reactions, corrosion processes take place when conditions favour the related chemical reactions (thermodynamics). Then, potential other factors drive the speed of the reaction (kinetics).
Fig 1 Chemical reactions of iron during the metal extraction and corrosion process
Types of corrosion reactions
A distinction is made between the types of corrosion, generally describing the interaction between the steel and the environment, and forms of corrosion describing the phenomenological appearance.
Chemical reaction – It is a typical chemical corrosion reaction occurs at high temperatures, where the steel reacts with hot gases and forms an oxide layer.
Metallo-physical reaction – An example of this is the embrittlement caused by hydrogen (H2) which diffuses into steel, possibly leading to failure of a component. Embrittlement can be the result of a careless manufacturing process, e.g. when surface coatings such as electrochemical zinc (Zn) plating are not applied properly on high-strength steel products (primary embrittlement). It can also be initiated by corrosion processes (metal dissolution). In the latter case, reference is made to corrosion-induced hydrogen assisted cracking (secondary embrittlement).
Electrochemical reaction – It is the most frequent type of corrosion. Such reactions denote an electrical exchange by way of electrons in the metal and ions in a conducting electrolyte, such as a water film on its surface. The overall reaction can be separated in the following two partial reactions.
- Metal dissolution, also known as oxidation or anodic Fe=(Fe2+) + 2 (e-)
- Reduction or cathodic reaction which mainly involves the oxygen O2) present in the air with water O2 + 2 H2O + 4 (e-) = 4 OH.
These two partial reactions can take place on the steel surface in a fairly homogenous distribution leading to uniform attack or can occur locally and separately, leading to localized forms of corrosion such as pitting corrosion. The mechanism and the electrochemical nature of the corrosion reaction define the necessary requirements for corrosion to take place. These are (i) a conducting metal, (ii) an electrolyte (a thin moisture film on the surface is already sufficient), and (iii) O2 for the cathodic reaction.
The basic corrosion mechanism of iron under a drop of water shows that both metal dissolution and O2 reduction reactions take place with slight separation on the surface, and their products (Fe-ions and OH-ions) react in the water drop to form red rust.
With the simple model of the corrosion reaction, as seen in Fig 2, many forms of corrosion can be explained and measures to mitigate corrosion can be deducted. The overall corrosion rate is also reduced by preventing or slowing down one of the partial reactions.
Fig 2 Corrosion of iron under a drop of water
Forms of corrosion
Different types of corrosions are described below.
Uniform corrosion or shallow pitting corrosion
Uniform corrosion is a form of corrosion where the surface is removed almost evenly. The partial reactions (metal dissolution and O2 reduction) are statistically distributed over the surface, leading to more or less homogenous dissolution of the metal and uniform formation of corrosion product (red rust). The extent of this form of corrosion can usually be well estimated on the basis of previous experience. The rate of corrosion is usually given in micrometers per annum. Using these average values, it is possible to calculate the life expectancy of a component, and thus to enhance its life expectancy by increasing its thickness. Uniform corrosion takes place on unprotected carbon steel and on galvanized steel under atmospheric conditions. However, in reality, purely homogenous corrosion attack is unlikely to take place. There are always areas, especially on complex metal parts, which will corrode faster than others leading to a more or less rough surface with an irregular covering of corrosion.
Pitting corrosion is a localized form of corrosion which leads to the creation of small holes (pits) in the metal. This form of corrosion is mainly found on passive metals. Passive metals and alloys, such as aluminum, titanium and stainless steel owe their corrosion resistance to a thin oxide layer on the surface with a thickness of only a few nanometers. The corrosion initiating process starts with a local break-down of the passive layer. Local corrosive attack can be initiated on stainless steels, for example, by chloride ions. Pitting corrosion can be quite problematic. Whereas uniform corrosion can be seen clearly on the surface, pitting corrosion often appears only as small pinholes on the surface. The amount of material removed below the pinholes is generally unknown, as hidden cavities may form, making pitting corrosion more difficult to detect and predict. Technically, there is no reasonable way to control pitting corrosion. This form of corrosion is required to be excluded right from the start through proper designing and use of the right material. In addition, pitting corrosion can often be the starting point for more severe forms of corrosion such as stress corrosion cracking (SCC). Phases of pitting corrosion in stainless steel are shown in Fig 3.
Fig 3 Phases of pitting corrosion on stainless steel
Crevice corrosion refers to the corrosion occurring in cracks or crevices formed between two surfaces (made from the same metal, different metals or even a metal and a non-metal). This type of corrosion is initiated by the restricted entrance of O2 from the air by diffusion into the crevice area leading to different concentrations of dissolved O2 in the common electrolyte (the so-called aeration cell). Again, the two partial reactions take place on different parts of the surface. Oxygen reduction takes place in the outer areas with higher O2 concentrations easily accessible by the surrounding air, whereas the anodic metal dissolution occurs in the crevice area resulting in localized attack (e.g. pitting). It may also occur under washers or gaskets, when the entry of water underneath is not prevented). There are lower and upper limits to the size of a crevice in which corrosion is induced. If the crevice is too tight, no electrolyte for corrosion is introduced. If the crevice is too wide to reduce O2 entrance, the aeration cell and consequently different concentrations of O2 cannot develop. However, the critical crevice width depends on several factors such as the type of metals involved, the corroding environment and wet/dry cycles.
Environmental induced cracking
Stress corrosion cracking
Stress corrosion cracking (SCC) is a combined mechanical and electrochemical corrosion process which results in cracking of certain materials. It can lead to unexpected sudden brittle failure of normally ductile steels subjected to stress levels well below their yield strength. Internal stresses in a material can be sufficient to initiate an attack of SCC. SCC is not simply an overlapping of corrosion and mechanical stresses, but can be understood as an autocatalytic, self-accelerating process leading to high metal dissolution rates (anodic reaction). Initially, a small pit is formed and develops into a crack due to applied or residual stress in the material. The crack formation opens up a new active (non-passive) metal surface, which again corrodes very easily. This leads to further crack propagation and again to the exposure of new highly active metal surfaces in the crack. Metal dissolution in the crack advances rapidly until mechanical failure occurs. SCC is a highly specific form of corrosion which happens only when the three different requirements are fulfilled at the same time. These requirements are (i) mechanical (load, stress), (ii) material (susceptible alloy – e.g. austenitic stainless steel), and (iii) environment (highly corrosive, chlorides). It is well known that certain grades of austenitic stainless steel can suffer SCC in harsh environments such as indoor swimming pools. In most of these cases, corrosion is initiated by chlorides attacking the passive layer.
Hydrogen assisted cracking
Hydrogen (H2) assisted cracking is caused by the diffusion of H2 atoms into the metal. The presence of H2 in the lattice weakens the mechanical integrity of the metal and leads to crack growth and brittle fracture at stress levels below the yield strength. Like SCC, it can lead to sudden failure of metal parts without any detectable warning signs. In common applications, H2 damage is usually only relevant for high strength steel (HSS) with a tensile strength of around 1000 N/sq mm or higher. As for SCC, three different conditions are to be present at the same time for H2 assisted cracking. These are (i) mechanical (load, stress), material (hardness structure), and environmental (external, internal H2). The source of H2 can be the production process such as steelmaking, pickling and electro-galvanizing (primary H2). A secondary source can be the H2 formed during the corrosion process. During the corrosion process, H2 is formed and diffuses into the steel. This H2 intake leads to a decrease in the toughness or ductility of the steel.
Inter-crystalline (inter-granular) corrosion
Inter-crystalline corrosion is a special form of localized corrosion, where the corrosive attack takes place in a quite narrow path preferentially along the grain boundaries in the metal structure. The most common effect of this form of corrosion is a rapid mechanical disintegration (loss of ductility) of the material. Usually it can be prevented by using the right material and production process. A well-known example relevant to the construction industry is the so called sensitization of stainless steel. When certain grades of stainless steel are kept at a temperature within the range of 500 deg C to 800 deg C for a considerable time, e.g. during a welding process, chromium-rich carbides are formed, resulting in chromium depletion at the grain boundaries. Consequently, the grain boundaries possess a lower degree of corrosion resistance than the residual material, leading to localized corrosion.
Galvanic (contact) corrosion
Galvanic corrosion refers to corrosion damage where two dissimilar metals have an electrically conducting connection and are in contact with a common corrosive electrolyte. In the electro-chemical model of corrosion, one of the two partial reactions (anodic metal dissolution and cathodic O2 reduction) takes place almost exclusively on one metal. Generally, the less noble metal is dissolved (anodic metal dissolution), whereas the more noble part is not attacked by corrosion (serves only as the cathode for O2 reduction). Where galvanic corrosion takes place, the rate of corrosion of the less noble metal is higher than it would be in a free corroding environment without contact to another metal. Using thermodynamic data and taking common experience gained in typical applications into account, it is possible to predict which material combinations are affected by galvanic corrosion. A positive example of active utilization of the galvanic corrosion phenomenon described here is the way zinc protects carbon steels and low alloy steels. Zinc is the less noble metal which actively protects steel by being corroded.
Corrosion performance assessment
Comprehensive laboratory and field corrosion tests are necessary to assess the corrosion protection of the steel products. A wide variety of tested corrosion protection solutions for different environmental conditions are generally available. Many methods for testing corrosion resistance are specific to particular materials and are based on conditions prevailing in certain environments. A large number of factors affect corrosion behaviour. Hence, there is not a unique and universal corrosion test covering all aspects of materials in use. The most reliable indicator of corrosion behaviour is service history, but this information is not always available exactly as needed. For that reason, other tests are required, i.e. ranging from accelerated laboratory tests to field tests. Additionally, it is necessary to test the products in the condition they are going to have in the application for which they are to be used.
Corrosion tests are a suitable method for assessing new products and to compare them with known corrosion protection systems. However, such tests alone are not sufficient to qualify a product for a certain application as corrosivity of the environment can differ greatly from one application to another. In the end, it is the responsibility of the user to choose the right corrosion protection based on detailed information about the application, long-term experience and fundamental knowledge of the corrosion.
Purpose of corrosion testing
Corrosion lab tests are state-of-the-art when it comes to evaluating the performance of materials, as they represent standardized and reproducible conditions and allow an assessment to be made after a short period of testing (days to weeks). One major use of this type of test is in the quality control of corrosion protection coatings. In order to pass quality control, the product is to show the required performance (e.g. two days without showing red rust). Moreover, these types of tests are very useful in product development activities, where screening and classifying new coatings and materials for new product is crucial.
The most relevant laboratory corrosion tests are given below.
Neutral salt spray test
The salt spray test is one of the oldest and most widely-used accelerated corrosion tests. The samples are exposed permanently to a saline fog made from a 5 % sodium chloride (NaCl) solution. The salt spray test is not directly representative of corrosion protection in real atmospheres because of the high chloride concentration and missing dry periods. However, it is a practical test primarily used for process qualification and quality acceptance. The salt spray test is normally used to check the homogeneity of zinc coatings on steel parts for the purpose of quality control during production.
Cyclic corrosion test
In the cyclic corrosion test, temperature and relative humidity are varied to simulate typical wet/dry cycles like the ones taking place in natural outdoor environments. Additionally, the samples are sprinkled with a dilute NaCl solution (1 %) twice a week to induce corrosion. Though this test is still not directly representative of most real atmospheres, due to the wet/dry cycles as well as the lower chloride concentration, it is much better suited for triggering natural corrosion processes than the simple salt spray test. However, the test requires longer test times (several weeks).
Cyclic corrosion test with exposure to ultra violet (UV) radiation
This test in addition exposes the samples to high-energy UV radiation. It is combined with water condensation, with chloride exposure and a freezing period. Organic polymers such as paints and varnishes can show degradation when exposed to sun light. This test is therefore mainly used for products with organic coatings. Apart from corrosion testing, it is also used to check the ageing effect on UV sensitive products.
Humidity test and Kesternich-test (sulphur dioxide)
In the humidity test, samples are exposed to an atmosphere with 100 % relative humidity. This test can be combined with the addition of a certain amount of sulphur dioxide gas. This causes a highly corrosive and acidic environment for simulating the effect of heavy pollution normally associated with industry.
The electrochemical nature of the corrosion process renders electrochemical techniques a useful method for the investigation of corrosion reactions of certain materials. Besides the accelerated corrosion tests, the corrosion laboratory is normally equipped with electrochemical testing equipment (potentiostat) which is mainly used for the examination of pitting corrosion and the re-passivation behaviour of stainless steel.
Accelerated corrosion tests as well as field exposure tests are required to be supported by various analytical methods for proper interpretation of the results. In the metallography laboratory required testing equipments are scanning electron microscope (SEM) and elemental analysis equipment for deeper investigation.
Outdoor field tests
Corrosion protection of products can be assessed more accurately by exposure tests of specimens and products in real atmospheric environmental conditions. For this purpose, multiple outdoor field tests are conducted in conditions ranging from cold temperate to tropical, and from coastal to industrial and even offshore atmospheres.