Is coating required on stainless-steel components?
Stainless steel is a generic term used for a large group of corrosion-resistant alloys containing at least 10.5% chromium (Cr), and possibly containing other alloying elements like nickel (Ni), molybdenum (Mo), manganese (Mn) and nitrogen (N).
Stainless steel is a generic term used for a large group of corrosion-resistant alloys containing at least 10.5% chromium (Cr), and possibly containing other alloying elements like nickel (Ni), molybdenum (Mo), manganese (Mn) and nitrogen (N).
Stainless steels are divided into five categories based on their microstructure and properties:
- Austenitic stainless steel
- Ferritic stainless steel
- Martensitic stainless steel
- Duplex stainless steel
- Precipitation-hardened stainless steels.
These grades are further subdivided into super austenitic, super ferritic and super duplex stainless steel. Super (austenitic, ferritic and duplex) stainless steel grades may contain higher Cr, Ni, Mo and N, depending on their material type and grade.
Corrosion in stainless steels
In stainless steels, Cr content above 10.5% is necessary to form a stable passive chromium oxide (Cr2O3) layer. This passive layer protects underlying material from corrosion damage from the surrounding environment. The passive Cr2O3 film is extremely thin, approximately 10–100 atoms thick (approximately 2 nm); however, it prevents further oxygen diffusion into the base metal. A Cr2O3 passive layer in stainless steel acts as a defender of the material. Any mechanical activity, like grinding or cutting, damages the passive Cr2O3 layer. However, Cr has very high affinity with oxygen, and so it immediately reacts with surrounding oxygen and forms a passive Cr2O3 film, thereby protecting stainless steels. In a corrosive environment, damage to the passive layer cannot be restored, which leads to corrosion of the underlying material.
Generally, stainless steel suffers from corrosion damage in the presence of halides, such as chlorides, bromides, etc. Damage due to chloride presence is common in stainless steels around industrial areas, buried vessels under soil and/or water, and vicinity to a marine environment. These locations contain high chlorides and lead to chloride-related damage mechanisms.
Chloride ions from wet and humid environments can combine with Cr of the passive layer, forming soluble chromium chloride. As Cr dissolves, free iron (Fe) is exposed to chemically reactive surroundings containing chlorides, and the surface reacts with the corrosive environment to initiate corrosion. In this way, the chloride ion acts as a nemesis of the material.
Damage mechanisms of corrosion types
Depending on chloride concentration, temperature and operating conditions, chloride ions can cause three different corrosion damages in stainless steels:
- Crevice corrosion
- Pitting corrosion
- Stress corrosion cracking.
Crevice corrosion is highly localized corrosion that occurs within the crevices and shielded areas on metal surfaces exposed to corrosives. A typical example of crevice corrosion is under-deposit corrosion, which forms below sand, dirt and corrosion products.
Pitting corrosion is an extremely localized attack that results in holes on metal surfaces. Pitting is one of the most destructive and insidious form of corrosion. It causes equipment to fail because of perforations with only a small percent of weight loss of the entire structure.
The damage mechanism for crevice and pitting corrosion is the same result: the passive Cr2O3 layer is damaged, and the chloride ion forms hydrochloric acid within the pits/crevices, thereby enlarging the pits and crevices over time.
FIG. 1A shows a typical crevice and pitting corrosion mechanism. FIG. 1B and FIG. 1C show how crevice and pitting corrosion looks in stainless steel.
FIG. 1. Crevice and pitting corrosion mechanism (A), crevice corrosion appearance in stainless steel (B), pitting corrosion appearance in stainless steel (C).
Stress corrosion cracking involves the cracking and sudden rupture of equipment without any warning, which makes stress corrosion cracking in stainless steel more damaging to equipment, to personnel working around the equipment and to the surrounding environment. Austenitic stainless steel is susceptible to chloride stress corrosion cracking if the temperature is above 60°C (140°F). FIG. 2 shows chloride stress corrosion cracking in austenitic stainless steel.
FIG. 2. Chloride stress corrosion cracking in austenitic stainless steel.
Critical factors that increase susceptibility for chloride damage in stainless steel involve chloride concentration, temperature, pH, oxygen and residual stresses for stress corrosion cracking. Increasing the chloride content and temperature increases the susceptibility of stainless steel to corrosion. Susceptibility to corrosion increases with lower pH, and corrosion tendency decreases with higher pH. Excessively stressed and low-temperature components are highly susceptible to chloride stress corrosion cracking. Oxygen and oxidizers increase chloride stress corrosion cracking susceptibility; however, stagnant conditions and low oxygen availability increase the susceptibility for crevice and pitting corrosion.
In terms of chloride-related corrosion resistance, duplex and ferritic stainless steels offer better resistance. Precipitation and martensitic stainless steels offer the least resistance. Austenitic stainless steel offers intermediate resistance. However, material selection should be performed only after considering all of the previously specified critical factors.
Duplex stainless steel has better resistance against chloride-related corrosion damage mechanisms; however, it is not immune to these damages. Its resistance against chloride is also dependent on the critical factors previously specified.
Microbiologically influenced corrosion (MIC)
MIC is another form of corrosion that occurs in stainless steels. This type of corrosion is caused by living organisms such as bacteria, algae and/or fungi. It is often associated with the presence of tubercles or slimy organic substances. Often, the bacteria produce localized corrosion in the form of crevice or pitting corrosion. MIC is found in aqueous environments where stagnant or low-flow conditions exist and promote the growth of microorganisms.
These microbes fall into two basic groups: aerobic and anaerobic. The two groups are based on the environment the microbes prefer (with or without oxygen). Slime-forming bacteria comprise a diverse group of aerobic bacteria. Common anaerobic bacteria include sulfate-reducing bacteria (SRB) and organic acid-forming bacteria. In stainless steels, anaerobic bacteria are more harmful since they do not require oxygen for growth and, therefore, do not allow oxygen to reach the metal surface for passive film formation.
Different organisms thrive on different nutrients, including inorganic substances (e.g., sulfur, ammonia, Fe, sulfate compounds and H2S) and organic substances (e.g., hydrocarbons and organic acids). In addition, all organisms require a source of carbon, nitrogen and phosphorous for growth. Corrosion is often blamed on iron-oxidizing bacteria or sulfate-reducing bacteria. However, these organisms typically are only part of a complex colony of multiple types of interdependent organisms, each capable of creating byproducts that might be a food source for others.
These microbes tend to form colonies, with different characteristics appearing on the outside and inside. On the outside, slime-forming bacteria may produce polymers (slime) that attract inorganic material, making the colony appear as a pile of mud and debris. These aerobic organisms can efficiently use up all available oxygen, giving anaerobic microbes inside the colony a hospitable environment, which encourages enhanced corrosion under the colony. FIG. 3 shows MIC damage in stainless steel pipe.
FIG. 3. Examples of MIC corrosion damage in stainless steel pipe.
Coatings to prevent corrosion
Material selection is carried out based on the extreme corrosive conditions from the process fluid side and/or the external atmosphere. When stainless steel is selected, internal coatings are usually not required because the stainless steel corrosion resistance will protect against the internal process fluids. However, the possibility of corrosion from the external atmosphere still exists, which could require coatings on the external surfaces of stainless steel components to mitigate against chlorides and/or microbial corrosion damage.
The following conditions require external coatings on stainless steels to guard against external corrosion damage:
- Equipment buried underground, where wet soil with chlorides and microbes is in contact with stainless steel
- Equipment submerged in wet pits, where salty water could be present and microbes are in contact with stainless steel
- Equipment enclosed in insulation, where salty water from a marine environment or rain water containing some chlorides may accumulate on stainless steel.
All these conditions may cause accumulation of chlorides on the external surface. Similarly, underground buried and submerged equipment may encounter microbial activity. Both equipment scenarios can create an environment that is conducive for pitting, crevice and microbial corrosion, and even stress corrosion at moderate temperatures.
To maintain corrosion resistance against external crevice, pitting, stress corrosion cracking and MIC, the severity of corrosion should be assessed. One option to mitigate against corrosion is to select corrosion-resistant material to ensure internal and external corrosion-resistant conditions. The second option is to implement an external coating on stainless steel surfaces to avoid direct contact of corrosive species with the equipment external surface. The most economical way to protect against external damage is to apply a coating to exposed external surfaces, which will avoid direct contact of corrosive chlorides and microbes with the stainless steel components.
It is important that the coating is applied as per the manufacturer’s coating application guidelines to avoid coating defects. Surface preparation and application are key to ensuring the coating integrity. For buried equipment, where regular coating inspection is not possible, the application of sacrificial or impressed-current cathodic protection, along with coating, can help improve design life and equipment integrity. Cathodic protection additionally protects equipment in case of coating damage due to defect or physical damage. Note: The selection of cathodic protection type and design is a wide subject and is not covered here.
Various international standards and company specifications prescribe coatings on stainless steel if corrosion conditions exist. For example, coating systems are used as specified in TABLE 1 and TABLE 2 for austenitic and duplex stainless steels, respectively.
Buried, submerged and insulated stainless steel components can encounter chloride and microbial corrosion attack. Stainless steels in these scenarios require external coating; otherwise, external coating on stainless steel components is not necessary. HP
The Author
Kadikar, R. - ITT Corp., Vadodara, India
Ratnakar Kadikar is Global Manager, Materials for ITT Corp. Industrial Process Group. He provides metallurgical and materials technology for ITT Corp.’s business, as well as technical support to ITT customers and product specialists. He has more than 21 yr of experience in corrosion monitoring, corrosion evaluation, material selection, failure investigation, heat treatment, micro-structural analysis and training on metallurgical aspects. He has also worked in corrosion loop risk-based inspection study, integrity operating window, cathodic protection, painting/coatings and residual life assessment of static equipment. Mr. Ratnakar holds BS and MS degrees in metallurgical engineering. He is a qualified NACE International Senior Corrosion Technologist and is API 571/API 653 certified.
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