Technical Information about Stainless Steel - Corrosion

Stainless steels are not indestructible materials, nor immune to all corrosive attack. However, stainless steels are excellent combatants of corrosion. With the correct selection of alloy and application of good design principles the few problems associated with corrosion in stainless steel can be overcome.

The more common forms of corrosion which affect all metals and alloys, including stainless steel, are briefly outlined.

General Corrosion

General corrosion is a uniform attack of the entire metal surface. It is the least dangerous because rates of corrosion can be measured and predicted. Stainless steels have very low general corrosion rates in many aggressive environments.

Galvanic Corrosion

Galvanic corrosion may occur when two different metals are in electrical contact and immersed in the same corrosive solution. Stainless steels are noble metals and therefore seldom suffer increased corrosion rates as a result of galvanic corrosion.

Erosion/Abrasion Corrosion

Erosion/abrasion corrosion is a combination of mechanical and corrosive attack. Abrasive particles in suspension, or high velocities, expose fresh metal surfaces which then suffer high rates of corrosion. Stainless steels offer a high resistance due to the tenacious and stable passive film on their surface.

Intergranular Corrosion

Intergranular corrosion is due to the formation of chromium carbides at high temperatures (450° - 850°C).

These form preferentially at the grain boundaries thus reducing the chromium content and resulting in a path of lower corrosion resistance around the grains. With correct choice of material ("L" or stabilised grades) and care during fabrication, this form of corrosion should not occur.

Pitting Corrosion

Pitting corrosion is a dangerous localised form of corrosion which results in small pits or perforations, but with little general metal loss. The chloride ion, which can breakdown the passive chromium rich oxide film, is usually responsible for pitting corrosion. Stainless steels alloyed to resist pitting corrosion can be selected for environments where it is likely to occur.

Embedded iron may also give rise to pitting corrosion. This is one of the most common forms of corrosion seen on stainless steel. Iron particles deposited on the surface of stainless steel through mechanical contact with, usually, carbon steels, quickly corrode and form a rust stain. If chloride ions are present this can give rise to an environment likely to cause pitting corrosion in the basic grades of stainless steel. Stainless steel favricators are cautioned against using the same tools on stainless steel as carbon steels. Embedded iron can be removed by passivation treatment.

Crevice/Shielded Corrosion

Crevice/shielded corrosion occurs where the surface of stainless steel is shielded or occluded thus preventing the free access and availability of oxygen to the surface. The passive film therefore tends to break down in these areas. Any conditions which give rise to a "crevice" should be avoided.

Microbiologically Induced Corrosion (MIC)

MIC results from the attraction and adherence of bacteria to the surface of the metal. A condition similar to a crevice is thereby produced. Certain bacteria produce aggressive metabolic products which also contribute.

Stress Corrosion Cracking (SCC)

Both pitting and crevice corrosion can lead to SCC under certain conditions. Stress corrosion cracking is a brittle fracture occurring in an otherwise ductile material.

The austenitic crystal structure is prone to SCC whereas the ferritic crystal structure prevents its development.

SCC requires the following three factors for development

• The presence of tensile stress. This can be either applied or residual stress occurring as a result of the metal forming, fabrication and welding procedure.
• A minimum temperature - generally SCC does not occur below 60°C
• The presence of a particular ion, eg the chloride ion. It is often difficult to quantify the exact chloride concentration needed, but localised concentration of chlorides will often initiate SCC.

These three factors are synergistic to a degree. SCC is a process whereby initiation may take considerable time. However, once the pit has developed its acts as a local stress raiser, the pit yawns open and fresh electrolyte reaches the anodic tip of the pit where further yawning and possible branching occurs.

Eventually, the process of attack reaches such a degree that the yield stress of the material is exceeded due to the lack of residual sound material.

High resistance to SCC is obtained by use of duplex stainless steels, stainless alloys and super ferritic stainless steels.