Stainless steel has excellent resistance to acid and alkali corrosion. However, this does not imply that they are universally resistant to all media. The forms of stainless steel corrosion include pitting corrosion, crevice corrosion, intergranular corrosion, and stress corrosion cracking (SCC).
Pitting and Crevice Corrosion
Pitting and crevice corrosion are localised attacks that strike specific spots on a metal surface. In stainless steels, these are the two most common forms of corrosion, both arising when the protective passive film is breached in a particular area. Pitting tends to occur on open, relatively clean surfaces; the metal around the pit usually shows little or no damage. Crevice corrosion, by contrast, hides away in narrow gaps or under deposits where the bulk solution cannot easily reach—think threaded joints, lap overlaps, O-ring seals, gaskets, fasteners, pipe connections, or even accumulated dirt, grease, marker pen, tape, protective films, scale or corrosion products.
Inside a crevice, the chemistry soon diverges from the surrounding environment. Reactions between the metal and the trapped solution, combined with slow diffusion in and out of the gap, drive up chloride levels, drop the pH value and set up small galvanic cells. The result is a rapid attack inside the crevice and almost none on the exposed surfaces outside.
The main alloying elements that fight pitting in high-performance austenitic stainless steels are chromium, molybdenum and nitrogen, the higher their contents, the better the resistance. Engineers capture this relationship in an empirical index called the Pitting Resistance Equivalent Number (PREN). For austenitic and duplex grades the usual formula is:
PREN = %Cr + 3.3 %Mo + 0.5%W) +16(%N)
PREN is useful for selecting stainless steels to cope with chloride environments. If a stainless steel fails in a certain service environment due to pitting or crevice corrosion, a grade with a higher PREN must be selected. It is currently impossible to determine how much PREN increase is required to avoid corrosion, but selecting a stainless steel with low PREN will definitely fail. If a certain grade of stainless steel is suitable for a chloride environment but unavailable on the market, selecting an available grade with equivalent or higher PREN is a feasible strategy for finding the most economical alternative.
Pitting initiators include increased chloride and oxidant content in the environment, elevated temperature, and stronger acidity, but are insufficient to cause uniform corrosion. Poor design or unfavorable operating environments include inherent crevices and deposit residues caused by poor system cleanliness. Carelessness during processing can also lead to localized corrosion; roughly ground surfaces are prone to corrosion. Welding defects and heat tint are sites prone to pitting. Sometimes, improper operations during smelting or welding can lead to harmful non-metallic inclusions or intermetallic phases, both of which impair localized corrosion resistance.
Stress Corrosion Cracking
Tensile stress, a specific environment, and susceptible stainless steel—the simultaneous presence of these three elements leads to a corrosion morphology known as stress corrosion cracking (SCC). For austenitic stainless steels, SCC failure most commonly occurs in chloride-containing environments, hence also called chloride stress corrosion cracking (CSCC). Although CSCC is the most common type of SCC, other substances can also cause stress corrosion cracking in austenitic stainless steels, such as caustic soda, hydrogen sulfide, and polysulfuric acids.
When annealed austenitic stainless steel undergoes CSCC, it typically manifests as dendritic transgranular cracks, with the morphology determined by the stress mode. Sensitization increases SCC susceptibility, promoting the formation of intergranular cracks. High stress, high temperature, high chloride, and low pH value increase the likelihood of stress corrosion cracking. Standard austenitic stainless steels, including 304/304L, 316/316L, and 200-series stainless steels, are highly susceptible to this corrosion, even under relatively low-temperature and chloride-containing conditions. Although high-nickel austenitic stainless steels have excellent resistance to chloride stress corrosion cracking, they cannot completely prevent it; in high-temperature, high-chloride environments, it can still occur.
The nickel content in 304 and 316 stainless steels is 8% ~ 12%; austenitic stainless steels with higher nickel content exhibit better resistance to chloride stress corrosion cracking. High-performance austenitic stainless steels contain more chromium, molybdenum, and nickel, thus offering particularly good resistance to chloride stress corrosion cracking.
Stress Corrosion Cracking in Other Environments
Strong alkaline environments can cause a type of stress corrosion in stainless steels called “caustic cracking.” At temperatures above 100°C, standard grades such as 304, 316, and similar grades are prone to caustic cracking. Caustic cracking is essentially intergranular cracking, difficult to distinguish from CSCC. Increasing nickel content can improve resistance to caustic cracking.
Intergranular Corrosion
Intergranular corrosion forms rapidly at grain boundaries or in their vicinity, with little or no effect on the grains themselves. For standard austenitic stainless steels, this corrosion is usually associated with sensitization occurring during welding or heat treatment. Different degrees of sensitization and environmental corrosivity can lead to various forms of corrosion. If sensitization is moderate, pitting may form. If a continuous chromium carbide network forms along the grain boundaries, intergranular corrosion occurs, leading to surface grain dropout. The combined action of sensitized microstructure, corrosive environment, and tensile stress produces intergranular stress corrosion cracking. Designers and fabricators can reduce sensitization and subsequent intergranular corrosion through the following three methods:
Adopt low-carbon grades such as 201L, 304L, 316L, or 317L. High-performance austenitic stainless steels are all low-carbon grades. Low carbon content delays the onset of chromium carbide precipitation (sensitization) during cooling. Thus, during hot forming and welding, the material does not sensitize, even in large cross-section workpieces. However, if exposed for long periods in the critical sensitization temperature range, L-grades cannot resist sensitization.
Adopt stabilized austenitic grades such as 321 or 347. These contain a stabilizing alloy element that firmly binds carbon. Titanium (Ti) and niobium (Nb) are typically used; these grades require appropriate stabilization heat treatment to preferentially form titanium carbide or niobium carbide, avoiding chromium carbide formation, thereby retaining chromium in solid solution for corrosion protection.
Solution annealing restores corrosion resistance in sensitized material. The recommended solution annealing temperature range is 1040°C ~ 1175°C. Solution annealing followed by rapid quenching must be the final heat treatment step for the component. For components exposed to high temperatures for extended periods, solution annealing cannot eliminate the risk of sensitization.
Weld Corrosion
During stainless steel welding, the weld metal and surrounding metal are heated to liquid metal temperature or near liquid metal temperature. During heating and cooling, the weld metal and heat-affected zone spend time in the sensitization temperature range. Whether welding causes steel sensitization depends on the alloy itself, section thickness, and heat input.
High-performance austenitic stainless steels generally do not undergo chromium carbide sensitization during welding. However, the welding thermal cycle promotes intermetallic compound formation, reducing pitting resistance. Therefore, welding parameters such as heat input and interpass temperature should be adjusted to minimize time at critical temperatures and avoid multiple welding passes whenever possible. Lower heat input and interpass temperature facilitate rapid cooling, shortening exposure time at critical temperatures.
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Knife-Line Corrosion
Knife-line corrosion is a unique form of sensitization that occurs only in stabilized grades such as 321 and 347. If the welding process dissolves titanium carbide or niobium carbide, subsequent heat treatment may form chromium carbide. In this case, a narrow sensitized zone appears around the weld, hence the name knife-line corrosion. Most austenitic stainless steels and high-performance austenitic stainless steels are not stabilized, so they are not prone to knife-line corrosion.
Galvanic Corrosion
When metals are immersed in a corrosive environment, corrosion current is generated, directly related to the corrosion rate of the metal in that environment. Metals with very low corrosion rates have more positive or more inert corrosion potentials; metals with high corrosion rates have more negative or more active corrosion potentials. If two or more different metals or alloys are immersed in a corrosive environment, in mutual contact or electrically connected, and their corrosion potentials in the environment differ significantly, galvanic corrosion is highly likely.
When galvanic corrosion occurs, the more active metal corrodes at a faster rate, while the inert metal is often protected, with a corrosion rate lower than when the metal is alone in the environment. For example, if carbon steel is connected to passivated stainless steel, the carbon steel corrodes rapidly—i.e., carbon steel undergoes galvanic corrosion—because carbon steel is more active, and the corrosion potential difference between them is large.
An important parameter for assessing the expected intensity of galvanic corrosion is the relative size of the exposed surfaces of the two metals. As the relative surface area of the more stable metal increases, the area for the cathodic reaction increases, leading to a higher rate of metal dissolution or corrosion on the more active metal surface. Due to this surface area effect, the galvanic corrosion rate increases with the area ratio of inert metal to active metal. Using carbon steel rivets to connect stainless steel plates is a common example of an incorrect area ratio.
Measures to reduce galvanic corrosion include avoiding the connection of dissimilar metals, using insulating materials to separate dissimilar metals, selecting dissimilar metals close in the galvanic series, adjusting the surface areas of dissimilar metals, and increasing the exposed area of the metal most likely to corrode.
If a dissimilar metal connection is unavoidable, spraying a coating on the more stable metal surface near the connection can prevent galvanic corrosion. It is not advisable to apply anti-galvanic corrosion coatings on the more active metal surface, because any coating defects will create a highly unfavorable inert metal/active metal area ratio, leading to accelerated corrosion at the coating defect.
