Corrosion is the gradual degradation of a metal by a chemical, often electrochemical, reaction with the surrounding environment. It impacts material properties such as mechanical strength, appearance, and impermeability to liquids and gases.
Although stainless steels are often chosen because of their resistance to corrosion, they are not immune to it. Whether a stainless steel is corrosion resistant in a specific environment depends on a combination of its chemical composition and the aggressiveness of the environment.
How corrosion occurs
The corrosion resistance of stainless steel is attributed to the thin passive film that forms spontaneously on its surface in oxidizing environments if the steel has a minimum chromium content of approximately 10.5%.
As the film adheres strongly to the metal substrate and protects it from contact with the surrounding environment, the electrochemical reactions that cause corrosion are effectively stopped. If locally destroyed, for example by scratching, the film can 'heal' by spontaneously repassivating in an oxidizing environment.
All types of corrosion affecting stainless steel are related to permanent damage of the passive film, through either complete or local breakdown. Factors such as the chemical environment, pH, temperature, surface finish, product design, fabrication method, contamination, and maintenance procedures can all affect the corrosion behavior of steel and the type of corrosion that may occur.
Corrosion can be divided into two categories: wet corrosion and high temperature corrosion.
Wet corrosion
Wet corrosion refers to corrosion in liquids or moist environments, and includes atmospheric corrosion. It is an electrochemical process that involves an anode and a cathode, connected by an electrolyte. The metal oxidizes (corrodes) at the anode, forming rust or some other corrosion product. At the cathode, a reduction reaction takes place – typically the reduction of oxygen or hydrogen evolution. Preventing corrosion involves stopping these reactions taking place.
Typically, stainless steel does not corrode in the same manner as carbon or low-alloy steel, which rust due to constantly changing anodes and cathodes on the whole surface. In order for this process to occur on stainless steel, the passive film needs to be completely broken down in environments such as non-oxidizing acids like hydrochloric acid. More commonly, the passive film is attacked at certain points, causing various types of localized corrosion.
Wet corrosion forms
There are several different forms of wet corrosion, including:
- Pitting corrosion
- Crevice corrosion
- Uniform corrosion
- Environmentally assisted cracking
- Stress corrosion cracking
- Sulfide stress cracking
- Hydrogen induced stress cracking
- Corrosion fatigue
- Atmospheric corrosion
- Intergranular corrosion
- Galvanic corrosion
Common causes of pitting and crevice corrosion
Pitting and crevice corrosion have very similar causal factors. Stainless steels are particularly susceptible to pitting and crevice corrosion in media containing halide ions such as chlorides. Therefore, high-risk environments for pitting and crevice corrosion include seawater and process solutions containing high chloride concentrations.
Other factors that increase the probability for pitting and crevice corrosion are increased temperature, low pH, and the addition of oxidative chemicals, for example by chlorination. Both pitting and crevice corrosion can have serious consequences and therefore must be avoided.
Pitting corrosion
This type of corrosion is highly localized, with discrete pits on the free surface of stainless steels. If the passive layer is damaged or locally weak, pitting corrosion can initiate, and the small area that is unprotected by the passive film becomes the anode. As this anodic area is very small compared to the large cathode area of the undamaged passive film, the corrosion rate is high and a pit is formed.
Crevice corrosion
As its name implies, this type of corrosion occurs in crevices and confined spaces. Crevices can be caused by component design or joints such as flanges and threaded connections, but also under deposits formed on the surface during maintenance.
As the oxygen content is limited inside a tight crevice, the passive layer is weakened and, just as with pitting, dissolved metal ions in the crevice lower the pH and allow chloride ions to migrate into the crevice. Eventually the passive layer breaks down and the aggressive environment facilitates the corrosion attack. Compared to pitting, crevice corrosion results in larger but shallower attacks.
Resistance to pitting and crevice corrosion
It is well known that increasing the chromium content and adding molybdenum and nitrogen as alloying elements increases stainless steels' resistance to pitting and crevice corrosion.
The Pitting Resistance Equivalent (PRE), often also given as PREN to indicate the influence of nitrogen in the steel, can be used in order to rank and compare the resistance of different stainless steels in terms of their resistance to pitting corrosion. It takes into account the effect of the most important alloying elements. One frequently used equation for stainless steels is PRE = % Cr + 3.3 × %Mo + 16 × %N.
It is important to remember that the calculated PRE only gives an indication of the resistance of stainless steels and gives no information on their behavior in real environments. Therefore, it should only be used for roughly comparing the pitting corrosion resistance of different grades.
Avoiding pitting and crevice corrosion
There are a number of measures that can be taken in order to avoid pitting and crevice corrosion. These include:
- Selecting a highly alloyed stainless steel grade
- Lowering the chloride content of the corrosive environment
- Increasing the pH
- Decrease the content of oxygen and other oxidizing species from the environment, or eliminate them altogether
- Use a design that avoids the need for tight crevices and which discourages stagnant conditions and the formation of deposits
- Employ good fabrication practices that produce smooth and clean surfaces, and ensure that weld oxides are removed
Uniform corrosion
Uniform corrosion occurs when the passive layer is destroyed on the whole, or a large part, of the steel surface. This means the anodic and cathodic reactions occur on the same surface at constantly changing locations, much like corrosion on carbon steel. The result is more or less uniform removal of metal from the unprotected surface. Uniform corrosion can occur on stainless steels in acids or hot alkaline solutions.
In an environment with constant temperature and chemical composition, uniform corrosion occurs at rather a constant rate. As a result, in contrast to pitting and crevice corrosion, the corrosion rate can be measured. This rate is often expressed as loss of thickness over time, for example mm/year. Stainless steel is normally considered to be resistant to uniform corrosion in a specific environment if the corrosion rate does not exceed 0.1 mm/year.
Resistance to uniform corrosion generally increases with increasing levels of chromium, nickel, and molybdenum. However, in strongly oxidizing environments molybdenum has proved to be detrimental to corrosion resistance.
Uniform corrosion is considered to be easier to predict than localized corrosion. While every effort should be made to avoid pitting and crevice corrosion completely, some degree of metal loss due to uniform contamination can often be tolerated. The exception is in applications where contamination is unacceptable, such as for hygiene reasons in food-handling equipment.
Environmentally assisted cracking
This phenomenon is caused by the combined action of mechanical stress and a corrosive environment. Once initiated, crack propagation can be very rapid and result in critical failure. Environmentally assisted cracking can be caused by a number of environmental species [CR2] including chlorides, hydrogen, and hydroxides. In order for cracking to occur the mechanical tensile stresses must exceed a critical level. These need not necessarily be applied stresses, but can also be residual stresses from manufacturing operations such as forming and welding.
Stress corrosion cracking
Like pitting and crevice corrosion, stress corrosion cracking (SCC) most frequently occurs in chloride-containing environments. Elevated temperatures (> 60 °C for chloride environments and > 100 °C for alkaline environments) are normally required for SCC to occur in stainless steel. Nevertheless, there are cases where cracking can occur at temperatures as low as 30 °C, for example in swimming pool environments.
A common cause of SCC is evaporation on hot stainless steel surfaces. Liquids with low chloride content that would normally be considered harmless can cause chloride concentrations high enough to cause SCC. One example of where this can occur is underneath thermal insulation on piping.
Standard austenitic grades, such as 4307 and 4404, are generally sensitive to chloride-induced stress corrosion cracking. High nickel and molybdenum content increases the resistance of austenitic stainless steels, which is why the high alloyed austenitic grades 904L, 254 SMO®, and 654 SMO® show excellent resistance to chloride-induced SCC. Stainless steels with a duplex microstructure generally have high resistance to SCC, as have ferritic grades.
Sulfide stress cracking
Sulfide stress cracking (SSC), a form of hydrogen-induced cracking, is the cracking of a material under the combined action of mechanical tensile stress and corrosion in the presence of water and hydrogen sulfide (H2S). It is of particular importance in the oil and gas industry, as natural gas and crude oil can contain considerable amounts of hydrogen sulfide (often referred to as sour service).
To assess the corrosivity of process fluids containing hydrogen sulfide, the partial pressure of hydrogen sulfide has to be considered, together with pH, temperature, chloride, carbon dioxide, and oxygen content.
Susceptibility to hydrogen embrittlement is most severe at or below ambient temperature, whereas chloride-induced SCC is most severe at high temperatures. Consequently, the combined risk of cracking due to hydrogen sulfide and chlorides tends to be most severe for austenitic, and especially duplex, stainless steel grades in the 80–100 °C range.
Hydrogen-induced stress cracking
Another hydrogen embrittlement failure mode that can be a concern in the oil and gas industry is hydrogen-induced stress cracking (HISC), where hydrogen is introduced when the material is under cathodic protection in seawater. The hydrogen is a result of the increased cathodic reaction, hydrogen ion reduction, on the stainless steel surface.
Even high-alloyed stainless steels can be subjected to full cathodic protection in offshore applications, as these steels are typically connected to carbon steel and other low-alloyed steels already under protection. Ferritic, martensitic, and duplex stainless steels are generally more susceptible than the austenitic grades to hydrogen embrittlement.
Corrosion fatigue
A material that is subjected to a cyclic load can fail due to fatigue at loads far below the ultimate tensile strength. If the material is simultaneously exposed to a corrosive environment, failure may occur at even lower load levels – and even more quickly.
Failure resulting from a combination of cyclic load and a corrosive environment is known as corrosion fatigue. In many cases there is no pronounced fatigue limit, as observed in air, but a gradual lowering of the fatigue strength with an increasing number of load cycles.
Corrosion fatigue cracks are usually less branched than stress corrosion cracks, although both forms of corrosion cause brittle failures. Corrosion fatigue can occur at ambient temperature and in environments that could be considered harmless with regard to other forms of corrosion.
As with stress corrosion cracking, residual stresses from manufacturing processes can adversely affect resistance to corrosion fatigue. Increasing the mechanical strength of stainless steels also increases their resistance to corrosion fatigue so duplex stainless steels are often superior to conventional austenitic grades.
Atmospheric corrosion
Atmospheric corrosion is a collective term to describe the corrosion on metal surfaces in the atmosphere. The atmosphere may be indoor or outdoor, and many different corrosion forms may be involved.
Stainless steel that is exposed to an aggressive atmospheric environment is primarily affected by staining, sometimes referred to as tea staining. However, not all discoloration is necessarily the result of corrosion. It can also be discoloration from dirt or extraneous rust caused, for example, by iron particles on the surface. However, if the chloride level is high enough, stainless steel can, over time, also be attacked by localized corrosion such as pitting and crevice corrosion.
According to the ISO 9223 standard, the corrosivity of the environment is classified C1 to CX, where C1 is the least corrosive and CX the most aggressive. The corrosivity classes are a good tool for selecting materials that suffer uniform corrosion under atmospheric conditions, such as carbon steel or zinc.
However, with their passive layer, stainless steels exhibit a totally different corrosion mechanism. This means it is not easy to apply the corrosivity classes in ISO 9223 to stainless steels, and they are therefore not the best tool for selecting stainless steels for atmospheric conditions. The higher the corrosion class, the higher alloyed stainless steel needs to be used, with the range going from ferritic grades up to superaustenitic and superduplex grades.
How the stainless steel is exposed to the atmosphere is also of great importance. In areas with rainfall, sheltered conditions prevent rinsing, and corrosivity is increased. In dry areas with little or no rainfall, sheltering will protect steel from aggressive pollutants and thus decrease corrosivity.
Surface condition and surface roughness can affect the performance of stainless steels. On a coarse surface, dirt, particles, and corrosive chemicals are easily retained, increasing the susceptibility to atmospheric corrosion. A smooth surface will facilitate rinse-off and is therefore less susceptible. Rinse-off is also facilitated if a ground or polished surface is vertically orientated. The lower the alloying levels of the stainless steel, the greater the impact of the surface finish on the resistance to atmospheric corrosion.
Intergranular corrosion
This type of corrosion was previously a potential risk for stainless steel because of its high carbon content (0.05–0.15%). Modern steelmaking methods, and especially the use of AOD (Argon Oxygen Decarburization), have enabled lower carbon content, meaning that intergranular corrosion is rarely a problem today.
Nevertheless, it can occur if stainless steels are exposed to temperatures in the range 550–850 °C. Chromium carbides with very high chromium content can then be precipitated along the grain boundaries, causing the material nearby to be depleted of chromium and thus become less corrosion resistant. A stainless steel which has been heat-treated in a way that produces such grain boundary precipitates and adjacent chromium-depleted zones is said to be sensitized.
Sensitization can be a result of welding or of hot forming at an inappropriate temperature. Sensitization might also increase sensitivity to other forms of corrosion such as pitting, crevice corrosion, and stress corrosion cracking.
Measures against intergranular corrosion by preventing carbide precipitation include:
- Using low-carbon stainless steel (< 0.05%)
- Using steel that is stabilized with e.g. titanium or niobium (which bind the carbon as titanium or niobium carbides and prevent chromium carbides from forming)
- Ensuring the shortest possible holding time in the 550–850 °C temperature range
- Solution annealing at 1,000–1,200 °C, at which temperature chromium carbides are dissolved, followed by rapid cooling in water or air
Galvanic corrosion
Galvanic corrosion can take place if two dissimilar metals are electrically connected and exposed to a corrosive environment. Galvanic corrosion, is usually not a problem for stainless steels but can affect other metals in contact with them.
In their passive state, stainless steels are nobler than the majority of other metallic construction materials in most environments. Galvanic coupling to metals such as carbon steel, galvanized steel, copper, and brass can therefore increase the corrosion rate of these metals. Galvanic corrosion between different stainless steel grades is generally not a problem providing that each grade remains passive in the environment in question.
If the surface of the less noble metal is small relative to the nobler metal, the corrosion rate can become very high. This is the case if carbon steel bolts are used to fasten stainless steel sheets, which can lead to severe galvanic corrosion on the bolts. Similarly, defects in coating or paint on a less noble material can result in a small anodic area and lead to high corrosion rates. It is therefore preferable to coat or paint the nobler metal in a galvanic couple in order to reduce the risk of galvanic corrosion.
Problems with galvanic corrosion can often be avoided by proper design and through electrically insulating dissimilar metals.
High temperature corrosion
In addition to the electrochemically based wet corrosion, stainless steels can suffer high temperature corrosion and oxidation. This can occur when a metal is exposed to a hot atmosphere containing oxygen, sulfur, halogens, or other compounds able to react with the material.
As with wet corrosion, stainless steel used for high-temperature applications must rely on the formation of a protective oxide layer at the surface. The environment must be oxidizing in order to form the protective layer, which consists of oxides of one or several of the alloying elements. An environment is often termed oxidizing or reducing as meaning "with respect to iron", since a so-called reducing atmosphere can oxidize elements such as aluminum and silicon, and often even chromium.
Oxidation
When stainless steels are exposed to an oxidizing environment at elevated temperatures, an oxide layer is formed on the surface, acting as a barrier between the metal and the gas. Chromium increases the oxidation resistance of stainless steels by the formation of a chromia (Cr2O3) scale on the surface.
When the chromium content is increased from 0 to 27%, the maximum service temperature increases from around 500 °C to 1,150 °C. At temperatures above 1,000 °C, aluminum oxides are more protective than chromium oxides. The amount of aluminum required for the formation of a protective layer will, however, make the alloy rather brittle and hence fabrication will be difficult and costly.
The sensitivity to temperature variations can be reduced by the addition of small amounts of so-called reactive elements such as yttrium, hafnium, and rare earth metals (REM) such as cerium and lanthanum. Small REM additions will lead to the formation of a tougher and more adherent oxide layer, improving cyclic oxidation resistance, erosion-corrosion resistance, and oxide spallation resistance. These are important properties when the component is subjected to temperature changes or mechanical deformation.
Although oxides as a rule are beneficial, there are a few elements that tend to form liquid or gaseous oxides, leading to so-called catastrophic oxidation. Catastrophic oxidation generally occurs in the 640–950 °C temperature range and for this reason molybdenum, which forms low-melting-point oxides and oxide-oxide eutectics, should be avoided in material for service at temperatures above 750 °C.
Sulfidation
Different sulfur compounds are often present as contaminants in flue gases and some process gases. Chemically, sulfidation is similar to oxidation. However, sulfides have a lower melting point than the corresponding oxide, so there is risk of forming molten corrosion products.
Nickel in particular can form nickel-sulfur compounds with a low melting point, resulting in a rapid deterioration of the alloy. In addition, sulfide scales are generally much less protective than the corresponding oxide scales, leading to a faster corrosion rate. In order to avoid nickel-sulfur compounds, nickel-free materials, such as ferritic high temperature grades, should be selected in reducing sulfur environments.
Under conditions where it is difficult to form a protective oxide layer (reducing environment), the corrosion resistance is considerably lower and is directly dependent on the bulk chemical composition of the alloys. In these conditions, steels with high chromium content and little or no nickel are superior.
Carburization and nitridation
Carburization of stainless steels can take place in carbon monoxide, carbon dioxide, methane, and other hydrocarbon gases at high temperature. The degree of carburization is governed by the levels of carbon and oxygen in the gas, the temperature, and the steel composition. Excessive uptake of carbon or nitrogen has a detrimental effect on material properties. The precipitation of carbides and nitrides leads to embrittlement – a reduction in toughness and ductility – especially at room temperature.
Resistance to carburization and nitridation is improved primarily by increasing the nickel content, but also by increasing the silicon and chromium content. Experience shows that it takes only small amounts of oxygen in the gas (even in the form of carbon dioxide or steam) to produce a thin, tough oxide layer on steel grade 253 MA®, which provides good protection against pickup of both carbon and nitrogen.
A special form of carburization is metal dusting, sometimes also called catastrophic carburization or carbon rot. This occurs at lower temperatures – typically between 450 °C and 700 °C – in, for example, heat treating, refining, and petrochemical processing. The attack is severe and leads to the disintegration of the material into carbon dust and metal, characterized by the formation of pits and holes in the material.
Good storage and handling practices for maintaining corrosion resistance
Stainless steel products should be handled and stored in such a way that they are not damaged. The level of the demands depends on the product and the intended future use. If the material is to undergo a fabrication sequence that includes both heat treatment and pickling, slight surface damage may be tolerated. On the other hand, if the stainless steel product will be directly installed, the storage and handling demands are much stricter. A high standard of cleanliness, good order, and common sense regarding how various operations will affect the material is usually enough to achieve appropriate handling conditions.
As it is the surface that gives stainless steel its corrosion resistance, it is important to properly protect it. This includes taking care to avoid any mechanical damage. Scratches or other damage introduced in fabrication shops are a common cause of passive film deterioration.
All kinds of contamination should be avoided. The easiest way to avoid contamination is to keep stainless steel products separated from carbon steel and other metals.
Different types of contamination have different effects on the stainless steel:
- Carbon steel particles give rise to rust stain
- Paint, grease, and oil can give rise to intergranular attack after welding or a heat treatment operation
- Low-melting metals such as copper, zinc, lead, aluminum, and brass can give rise to cracks in weld or heat-treated areas – known as LME (Liquid Metal Embrittlement)
Storage and packaging
Store stainless steel indoors if possible, as this helps to protect it from external pollution. If indoor storage is not possible, the steel should be covered. This is especially important if a wrapping that might absorb water and stain the surface, such as cardboard, has been used.
Packaging should not be broken unnecessarily, as it usually provides good protection. This is particularly important if the surface is susceptible to damage – for example, a polished or ground surface.
The use of strippable plastic film coatings on the stainless steels can help to avoid surface contamination. If the stainless steel has a protective film cover, it should be left on as long as practically possible and removed just before handover. Special packaging measures may be needed for protecting stainless steel components in transit in order to protect the surface. For example, care is needed when components are being secured to pallets or vehicles for transport to avoid damage to surfaces from strapping.
Suitable protective materials, such as wood, should be placed between the stainless steel and the securing straps. If carbon steel strapping is to be used to secure items to pallets or in bundles, some form of wrapping or padding is required to prevent the strapping from damaging the edges or surface of the stainless steel components.
Sheet and plate storage
To maintain flatness and avoid permanent deformation, sheet should be stored in wooden boxes and should be covered to prevent airborne contamination. Plate should be stored vertically on racks in a covered, dry location to minimize contamination and avoid the possibility of footprints. Racks of carbon steel should be protected by wooden, rubber, or plastic battens, or sheaths to avoid contamination of stainless steel.
Extra precautions for pipes
The rules for storing tubes and pipes are the same as for storage of sheet and plate, but one should remember that it is much more difficult to clean the inside of a pipe than an open plate or sheet surface. Extra precautions are therefore needed to avoid inside contamination, especially for thin pipes with small diameter. One effective way to protect the pipe inside is to use end-plugs.
Preserving corrosion resistance with cleaning
Stainless steel products need to be cleaned to maintain a pristine appearance and preserve corrosion resistance. Stainless steel will not corrode under normal atmospheric conditions provided the correct grade has been selected and appropriate fabrication and post-treatment procedures are followed. However, lack of cleaning can lead to accumulation of corrosive substances that surpass the corrosion resistance of the selected grade, leading to staining and, in more severe cases, initiation of corrosion.
Usually, discoloration is the first indication of corrosion. In this case, it is no longer sufficient to remove visible stains by means of usual cleaners. In the tiny pits, which may hardly be perceptible to the naked eye, corrosive media or corrosion products may be trapped, which will cause new stains to form. In such cases, it is advisable to use a cleaning agent that has a pickling and/or passivating effect. These kinds of cleaning agents are often very aggressive and consequently health, safety, and environmental precautions have to be taken. If the corrosion attack is more severe, with deep pits or cracks, grinding and weld repair might be needed.
Glossary
Passive
The state of a metal in which a surface reaction product causes a marked decrease in the corrosion rate relative to that in the absence of the product. A passive metal usually exhibits a higher electrode potential than one that is undergoing active dissolution.
Atmospheric corrosion
Corrosion of a metal by contact with substances present in the atmosphere, such as water, carbon dioxide, water vapor, and sulfur and chloride compounds.
Anode
The electrode of an electrochemical cell at which oxidation occurs.
Electrolyte
A chemical substance containing free ions that makes it electrically conductive.
Reduction
Gain of electrons in a chemical reaction.
Uniform corrosion
Also termed general corrosion. Uniform corrosion is characterized by corrosive attack proceeding evenly over the entire surface area, or a large fraction of the total area, with general thinning of the material as a result.
Pitting resistance Equivalent (PrE, PrEN)
A number developed to reflect and predict the pitting corrosion resistance of stainless steels based on chemical composition. Several formulas exist, but we use: PRE = % Cr + 3.3 x %Mo + 16 x % N.
Stress Corrosion Cracking (SCC)
Cracking of a material produced by the combined action of corrosion and tensile stress (residual or applied).
Sour service
Environment with natural gas and crude oil containing considerable amounts of hydrogen sulfide (H2S).
Cathodic protection (CP)
A technique used to prevent the corrosion of metal by making it the cathode of an electrochemical cell. This can be achieved by connecting the metal to be protected to a less noble metal, i.e. a galvanic anode, which corrodes instead of the protected metal. Alternatively, imposed electrochemical polarization can be achieved by using an external DC power source.
Ultimate tensile strength, Rm
The largest stress on the tensile testing curve characterizing the maximum obtainable engineering stress.
Surface roughness
The surface roughness is often expressed by the Ra value (μm). A low Ra value indicates a smooth surface.
AOD
Argon Oxygen Decarburization. A method of reducing the carbon content in the stainless steel melt.
Noble
A higher or more positive electrode potential. Noble metals include gold and platinum.
Oxidation
(1) Loss of electrons in a chemical reaction.
(2) Corrosion of a metal that is exposed to an oxidizing gas at elevated temperatures. The stainless steel reacts with O2, H2O, CO2 and forms an oxide on the stainless steel surface.
Spallation
Spallation is a state where the oxide formed on the stainless steel surface breaks and spalls off.
Eutectic
Mixture of two or more compounds with a lower melting point than any of the compounds themselves.
Sulfidation
A type of high temperature corrosion, in which the stainless steel reacts with sulfur compounds in the environment.
Carburization
A type of high temperature corrosion, in which the stainless steel reacts with, for example, CO or CO/CO2 gas present in the environment.
Nitridation
A type of high-temperature corrosion, in which the stainless steel reacts with nitrogen compounds in the environment.
Toughness
The ability to absorb energy in the plastic range.