Stainless steels are not indestructible materials, nor immune to all corrosive attack. However, the family of stainless steels are excellent combatants of corrosion and provide a wide choice of materials which, with careful selection and proper fabrication, enable the manufacture of cost effective critical components to meet the diverse needs of many industry sectors.

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 occurs 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°-859°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, very localised form of corrosion which results in small holes or perforations through the material, but with little measurable general metal loss. Some corrosive environments (commonly those containing the aggressive chloride ion) have the ability to attack localised weak points in the passive film. Due care in the selection of material should be taken if aggressive (eg chloride) ions are present, especially in acidic solutions at elevated temperatures. If conditions which promote pitting corrosion cannot be modified, materials with higher alloy content such as the duplex stainless steels and the stainless alloys will often give a solution to the problem.

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 aggravate the situation.

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 as occurring as a result of the metal forming, fabrication and welding procedure.

• A minimum temperature – generally SCC does not occur under 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 it acts as a local stress raiser, the pit yawns open and fresh electrolyte reaches the anodic tip of the pit. Corrosion occurs locally at the tip of the pit where further yawning and possible branching occurs.

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

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

 

Corrosion chart

R indicates the material is resistant to the named chemical up to the temperature shown, subject to limitations indicated by the footnotes.

NR indicates that the material is not recommended.

ND indicates that no data is available.

1 - not if chlorides present

2 - limited data

3 - depending upon the acid

4 - acid fumes dry, attack may occur if moisture builds up

5 - anhydrous

6 - depending upon concentration

7 - may discolour with time

8 - in strong solutions only when inhibited

9 - pitting possible in stagnant conditions

10 - possibility of pitting

11 - may cause stress corrosion cracking

12 - when free of SO2

13 - may cause contamination of product

14 - dilute hypochlorites can be used to sterilise some stainless steel with extreme care.

15 - general corrosion may become excessive

16 - 10%

17 - in the absence of impurities

18 - dilute

19 - some attack at high temperatures

 

  Stainless Steel Molybdenum
  18/8 Stainless Steel
  (304, 304L, 321) (316, 316L)
Temperature ºC20º60º100º20º60º100º
Aldehydes R1 R1 R1 R1 R1 R1
Acetic Acid (10%) R R R R R R
Acetic Acid (glac. & anh.) R R NR R R R
Acetic anhydride R2 NR NR R R NR
Ketones R R R R R R
Acetylene R R R R R R
Acid Fumes R3 R3 R3 R3 R3 R3
Alcohols R R R R R R
Alphatic Esters R R R R R R
Alkyl Chlorides R5 R5 R5 R5 R5 R5
Alum R R6 NR R R NR
Ammonia R R R R R R
Amyl Acetate R R R R R R
Aniline R R R R R R
Antimony Trichloride R5 NR NR R5 R5 NR
Aromatic Solvents R R R R R R
Atmospheric            
Industrial R7 ND ND R ND ND
Marine R7 ND ND R ND ND
Rural R ND ND R ND ND
Ascorbic Acid R1 R1 R1 R R R
Benzoic Acid R R R R R R
Boric Acid R R R R R R
Brines, saturated R8 NR NR R8 NR NR
Bromide (K) soln. R9 NR NR R9 R9 R9
Bromine (+ aqu.) NR NR NR NR NR NR
Butyl Acetate R R R R R R
Calcium Chloride NR NR NR R8 NR NR
Carbon Disulphide R R ND R R ND
Carbonic Acid R R R R R R
Carbon Tetrachloride R R R R R R
Caustic Soda & Potash R R R6 R R R6
Cellulose Paint R R R R R R
Chlorates of Na, K, Ba R1 R1 R1 R1 R1 R1
Chlorine, dry R R R R R R
Chlorine, wet NR NR NR NR NR NR
Chlorides of Na, K, Mg, Ca, R10 NR NR R9 R11 R11
Ni, NH4, Al, Sn, Zn            
Chlorosulphuric Acid NR NR NR NR10 NR NR
Chromic Acid (80%) NR NR NR NR NR NR
Citric Acid R6 R6 R6 R R R6
Cresylic Acids (50%) R R R R R R
Detergents, synthetic R R R R R R
Emulsifiers (all conc.) R R R R R R
Esters & Ethers R R R R R R
Fatty Acids (> C6) R R R R R R
Ferric Chloride NR NR NR NR NR NR
Fluorinated Refrigerants, R5 R R R5 R R
Aerosols e.g. Freon            
Fluorine, dry R ND ND R ND ND
Fluorine, wet NR NR NR R ND ND
Formic Acid R NR NR R R ND
Fruit Juices R12 R R R R R
Gelatine R1 R R R1 R R
Glycols R R R R R R
Hydrobromic Acid (50%) NR NR ND NR NR ND
Hydrochloric Acid (10%) NR NR NR NR NR NR
Hydrochloric Acid (conc.) NR NR NR NR NR NR
Hydrocyanic Acid R R ND R R ND
Hydrofluoric Acid NR NR NR NR NR NR
Hydrogen Peroxide (30%) R R R R R R
Hydrogen Sulphide R5 R5 R5 R5 R5 R5
Hypochlorite (Na 12-14%) R14 NR NR R14 ND ND
Lactic Acid (100%) R NR NR R R NR
Lead Acetate R R R R R R
Lead Perchlorate R1 R1 R1 R R1 ND
Lime (CaO) R R R R R R
Manganate, Potassium (K) R R R R R R
Meat Juices R R ND R R ND
Mercuric Chloride NR NR NR NR NR NR
Milk and its products R R R R R R
Molasses R R R R R R
Monoethanolamine R R R R R R
Naphthalene R R R R R R
Nitrates of Na, K, NH3, Ag R R R R R R
Nitric Acid (< 25%) R R R R R R
Nitric Acid (50%) R R R R R R
Nitric Acid (90%) R NR NR R NR NR
Nitric Acid (Fuming) R R2 NR R R2 NR
Oil, Diesel, Petroleum, Spirits R R R R R R
Oils, essential R R R R R R
Oil, Lube with aromatic adds. R R R R R R
Oils, vegetable and animal R R R R R R
Oxalic Acid R6 NR NR R6 R16 NR
Perchloric Acid NR NR NR NR NR NR
Phenoll R R R R R R
Phosphoric Acid (20%) R R R R R R
Phosphoric Acid (50%) R R NR R R R
Phosphoric Acid (95%) R R NR R R NR
Phosphorous Pentoxide R R R5 R R R5
Pyridine R R R R R R
Sea Water R9 NR NR R9 NR NR
Silicic Acid R R R R R R
Sodium Peroxide R16 NR NR R6 R16 R16
Sodium Silicate R R R R R R
Sodium Sulphide R R NR R R NR
Starch R R R R R R
Sugar, Syrups, Jams R12 R R R R R
Sulphamic Acid R18 NR NR R R19 NR
Sulphates (Na, K, Mg, Ca, Al, Fe) R R R R R R
Sulphates R R R R R R
Sulphur Dioxide, dry R R R R R R
Sulphur Dioxide, wet R NR NR R R NR
Sulphur Dioxide, aqu. soln. (96%) R NR NR R R R
Sulphur Trioxide NR NR NR R5 R5 R5
Sulphuric Acid (<50%) NR NR NR R16 NR NR
Sulphuric Acid (70%) NR NR NR NR NR NR
Sulphuric Acid (95%) R NR NR R NR NR
Sulphuric Acid, fuming R R2 NR R R NR
Tannic Acid (10%) R R R R R R
Tartaric Acid R R R R R R
Trichlorethylene R5 R5 R5 R5 R5 R5
Urea (30%) R R R R R R
Water, pure R R R R R R
Yeast R R R R R R
 

 

  Duplex 3CR12 / 5CR12
  Stainless Steel 
  (2205) 
Temperature ºC20º60º100º20º60º100º
Aldehydes R1 R1 R1 R1 R1 R1
Acetic Acid (10%) R R R R R ND
Acetic Acid (glac. & anh.) R R NR ND ND ND
Acetic anhydride R R R R2 NR ND
Ketones R R R R ND ND
Acetylene R R R R ND ND
Acid Fumes R4 NR NR NR NR NR
Alcohols R R R R R R
Alphatic Esters R R R R ND ND
Alkyl Chlorides R R R R5 ND ND
Alum R R NR ND ND ND
Ammonia R R R R R R
Amyl Acetate R R R R R R
Aniline R R R R R R
Antimony Trichloride R R NR NR NR NR
Aromatic Solvents R R R R R R
Atmospheric            
Industrial R ND ND R7 ND ND
Marine R ND ND R7 ND ND
Rural R ND ND R7 ND ND
Ascorbic Acid R R R R1 ND ND
Benzoic Acid R R R R R R
Boric Acid R R R R R R
Brines, saturated R R R NR NR NR
Bromide (K) soln. R ND ND NR NR NR
Bromine (+ aqu.) NR NR NR NR NR NR
Butyl Acetate R R R R R R
Calcium Chloride R10 R10 R10 NR NR NR
Carbon Disulphide R R R R R R
Carbonic Acid R R R R R9 NR
Carbon Tetrachloride R R R R R R
Caustic Soda & Potash R6 R6 ND ND R6 R6
Cellulose Paint R R R R R R
Chlorates of Na, K, Ba R R R ND ND ND
Chlorine, dry R R ND ND ND ND
Chlorine, wet NR NR NR NR NR NR
Chlorides of Na, K, Mg, Ca, R R ND R5 NR NR
Ni, NH4, Al, Sn, Zn            
Chlorosulphuric Acid ND ND ND ND ND ND
Chromic Acid (80%) NR NR NR NR NR NR
Citric Acid R R R NR NR NR
Cresylic Acids (50%) R R R R R R
Detergents, synthetic R R R R R R
Emulsifiers (all conc.) R R R ND ND ND
Esters & Ethers R R R R R R
Fatty Acids (> C6) R R R R R R
Ferric Chloride NR NR NR NR NR NR
Fluorinated Refrigerants, R R R R6 R NR
Aerosols e.g. Freon            
Fluorine, dry R R ND ND ND ND
Fluorine, wet ND ND ND NR NR NR
Formic Acid R R ND NR NR NR
Fruit Juices R R R R13 NR NR
Gelatine R R R R1 R1 ND
Glycols R R R R R R
Hydrobromic Acid (50%) NR NR ND NR NR NR
Hydrochloric Acid (10%) NR NR ND NR NR NR
Hydrochloric Acid (conc.) NR NR ND NR NR NR
Hydrocyanic Acid R R ND R ND ND
Hydrofluoric Acid NR NR NR NR NR NR
Hydrogen Peroxide (30%) R R R R R ND
Hydrogen Sulphide R5 R5 R5 R5 R5 R5
Hypochlorite (Na 12-14%) R14 ND ND R14 ND ND
Lactic Acid (100%) R R ND NR NR NR
Lead Acetate R R R R R R6
Lead Perchlorate ND ND ND NR NR NR
Lime (CaO) R R R R R R
Manganate, Potassium (K) R ND ND R6 ND ND
Meat Juices R R R R7 NR NR
Mercuric Chloride R R R NR NR NR
Milk and its products R R R R NR NR
Molasses R R R R R R
Monoethanolamine R R R R R R
Naphthalene R R R R R R
Nitrates of Na, K, NH3, Ag R R R R R R
Nitric Acid (< 25%) R R R R R15 NR
Nitric Acid (50%) R R R R R15 NR
Nitric Acid (90%) R NR ND R NR NR
Nitric Acid (Fuming) R NR NR R NR NR
Oil, Diesel, Petroleum, Spirits R R ND R R R
Oils, essential R R R R R R
Oil, Lube with aromatic adds. R R R R R R
Oils, vegetable and animal R R R R R R
Oxalic Acid R R R NR NR NR
Perchloric Acid NR NR NR NR NR NR
Phenoll R R R R R R
Phosphoric Acid (20%) R R R NR NR NR
Phosphoric Acid (50%) R R R NR NR NR
Phosphoric Acid (95%) R17 R17 R17 NR NR NR
Phosphorous Pentoxide R R R ND ND ND
Pyridine R R R R R R
Sea Water R NR NR NR NR NR
Silicic Acid R R R R R R
Sodium Peroxide R R R NR NR NR
Sodium Silicate R R R R R R
Sodium Sulphide R R NR R6 R6 NR
Starch R R R R R R
Sugar, Syrups, Jams R R R R12 R12 R12
Sulphamic Acid R R NR NR NR NR
Sulphates (Na, K, Mg, Ca, Al, Fe) R R R R R R
Sulphates R R R NR NR NR
Sulphur Dioxide, dry R R R R R R
Sulphur Dioxide, wet R R NR NR NR NR
Sulphur Dioxide, aqu. soln. (96%) R R R NR NR NR
Sulphur Trioxide R5 R5 R5 NR NR NR
Sulphuric Acid (<50%) R R NR NR NR NR
Sulphuric Acid (70%) R NR NR NR NR NR
Sulphuric Acid (95%) R NR NR R15 NR NR
Sulphuric Acid, fuming R R NR R2 R2 NR
Tannic Acid (10%) R R R R R NR
Tartaric Acid R R R NR NR NR
Trichlorethylene R5 R5 R5 R5 R5 R5
Urea (30%) R R R R R R
Water, pure R R R R R R
Yeast R R R R R R