Selection of Stainless Steel Corrosion Resistance

Stainless steel react with corrosive deterioration phenomenon is known as corrosion. Common metal corrosion of intergranular corrosion, pitting corrosion, stress corrosion, fatigue, corrosion and crevice corrosion.

Intergranular corrosion 
Intergranular corrosion of stainless steel corrosion in a particular medium of steel along the grain boundary of a local selective corrosion occurred. If this corrosion is the metal surface, will form micro-cracks and deep inside, until the cause breakage. In certain corrosive media (such as nitric acid, phosphoric acid, sulfuric acid, lactic acid, formic acid, hydrofluoric acid and copper sulfate, etc.), will occur along the grain boundary corrosion of stainless steel. There exist in the case of stress, intergranular corrosion may develop as intergranular stress corrosion cracking. To avoid corrosion, generally use ultra-low carbon (ω (C) ≤ 0.03%) austenitic stainless steel, duplex stainless steel and containing Titanium, Nb stabilized austenitic stainless steel.

Pitting corrosion 
Point, also known as pitting corrosion, is the most common form of localized corrosion. It is because the metal surface to a partial role in the micro-cell, some corrosion holes, to depths of development. Presence of impurities in the stainless steel surface, dirt and bug parts to rust or damage of passive film on stainless steel used in seawater, that there will be pitting. Pitting and pitfalls of large destructive. Corrosion occurred, although the weight loss is not metal, but the anode area is very small, the anode corrosion current density flowing through a large, resulting in higher metal dissolution rate, serious damage can perforated metal equipment. Pitting also make intergranular corrosion, erosion, stress corrosion cracking and corrosion fatigue increased, in many cases the origin of these localized corrosion. 

To prevent the occurrence of pitting corrosion of stainless steel (pitting), should be selected with high chromium, nickel, molybdenum, nitrogen stainless steel, to improve the purity and lower stainless steel non-uniformity. Choose strong passivation material and passivation of stainless steel to prevent corrosion of effective measures point.

Crevice corrosion 
Under the action of the corrosive medium, stainless steel crevice corrosion, the crevice corrosion. Crevice corrosion is generally based on the shape of the gap has a different shape. Minor, the crevice corrosion in general, more serious point to sheet erosion or ulcers form. 

All can cause corrosion of the media can cause crevice corrosion. If the surface of stainless steel and non-metallic inclusions in the metal (metal micro-materials, dust, dirt, sand material, marine life), or structural reasons, such as screws riveting, riveting, gaskets and other non-metallic contacts formed gap, in these cases and corrosion of stainless steel parts, if the media contact, so that crevice corrosion occurs. Especially in the Cl-of environment, crevice corrosion is most common. 

Crevice corrosion of stainless steel was mainly due to acidification of the crevice solution, hypoxia caused by passive film damage. Prevention of crevice corrosion of stainless steel so strong measures are: choice of high chromium, nickel, molybdenum, nitrogen stainless steel, to improve the stability of passive film and passivation, re-passivation ability to improve the purity of stainless steel, and stainless steel is not reduced uniformity.

Stress corrosion 
Stress corrosion of stainless steel in the permanent tensile stress (including the external load, thermal stress and cold, hot working or welding residual stress after, etc.) and specific joint action of the corrosive medium appears brittle cracking. Local corrosion damage of stainless steel, it is the most common form of the most damaging kind of corrosion. 

Characterized by stress corrosion of stainless steel corrosion cracks or faults occur, the origin point of fracture is often the corrosion pits or corrosion holes in the bottom; cracks have spread along the grain boundaries, transgranular and mixed three, usually perpendicular to the stress of the main crack direction, mostly branching; sharp crack tip, crack walls and metal surfaces are usually very minor corrosion, cracking the end of the proliferation of fast fracture has the characteristics of brittle fracture. Lead to stress corrosion of stainless steel is the most common media containing Cl-and oxygen atmosphere and industrial water, seawater, etc., in the tensile stress exceeds the critical value (including stress) and high temperature result of the role. 

Cr-Ni austenitic stainless steel caused by stress corrosion common media are: 

1. various chloride or a solution containing chloride. 
2. water, salt water, river water, well water, water, solar terms, and marine atmosphere. 
3. hydroxide, such as NaOH, KOH aqueous solution. 
4. nitric acid and nitrate; HNO3 + HF and HNO3 + HCl + HF acid lotion. 
5. hydrofluoric acid, fluorine-containing F-acid aqueous solution. 
6. sulfate and sulfite; even more sulfuric acid; hydrogen sulfide solution. 

For stress corrosion cracking, usually can be controlled through proper selection. 

1. chloride in high concentration, usually choose a high nickel content stainless steel, high silicon Cr-N stainless steel and ferritic stainless steel. 

2. chloride in the intergranular stress corrosion environment, the option with Ti, Nb austenitic stainless steel (by stabilizing treatment), control of ultra-low carbon or nitrogen austenitic stainless steel. 

3. chlorine ions in aqueous solution, is lower than 60 ℃ when the low concentration of chloride ions medium, no enrichment or enrichment can be used 18-8,18-12-2 type austenitic stainless steel, AISI 444 ferritic stainless steel and 18-5-Mo duplex stainless steel; lower than 60 ℃ in the low concentration of chloride ions medium (concentration, enrichment), the choice of 444 ferritic stainless steeland 18-5-Mo Double phase stainless steel; in less than 60 ℃, the medium of high concentration of chloride ions (with concentration, enrichment), the choice of XM27, AISI 444 ferritic stainless steel, and ω (C) 22% ~ 25% and Mo containing duplex stainless steel ; in the 60 ~ 200 ℃ low concentration of chloride ions medium (concentration, enrichment), the choice of type 18-5 ,22-5 ,25-5 with Mo duplex stainless steel and high Cr, Mo, Ni stainless steel 904L UNS N08904 at 200 ~ 350 ℃ low concentration of chloride ions (concentration, enrichment) medium, can be used Incoloy 800 iron - nickel alloy, Inconel 69 nickel-based alloys. 

4. aqueous solution containing NaOH, when ω (NaOH) ≤ 20%, non-chlorine sub-boiling temperature ≤ 120 ℃, optional or 18-12-2 type 18-8 austenitic stainless steel; when ω (NaOH) ≤ 50%, ω (NaCl) = 2.5%, NaOH aqueous solution at 85 ℃, you can use ultra-low carbon austenitic stainless steel 18-8, UNS S44629 ferritic stainless steel. 

5. duplex stainless steel especially for the point of corrosion (pitting) caused by stress corrosion cracking of the occasion, such as S32750, S32205 S31803 has an excellent resistance to stress corrosion properties of duplex stainless steel.

Fatigue corrosion 
Corrosion fatigue of stainless steel parts in the corrosive medium and the combined effect of alternating stress corrosion occurs thereby causing part of the damaged. Characterized by fatigue, corrosion pits produced a large number of cracks, so the metal mechanical fatigue limit ceased to exist; crack mostly through the grain, generally unbranched; crack tip blunt; fracture of corrosion products covered most of the small department was brittle fracture damage. 

Activity causes fatigue, corrosion media are acidic medium, chloride, containing H2S, SO2 and O2 to produce gas and other corrosive media. 

In order to prevent the generation of fatigue, stainless steel should have a good pitting resistance and high strength, it should be selected with Cr, Mo high stainless steel, super austenitic stainless steel, and high chromium, molybdenum and nitrogen content of the double phase stainless steel typically has: S31254 1.4547, S31050 1.4466, S32750 and so on. 

Of course, the corrosion resistance of stainless steel in the selection process, according to the actual situation, analyze the causes of corrosion and corrosion characteristics under test, summed up best and then select the good corrosion resistance, economically rational, the market has stainless steel material. 

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