Forms of corrosion
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:
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 ºC | 20º | 60º | 100º | 20º | 60º | 100º |
| 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 ºC | 20º | 60º | 100º | 20º | 60º | 100º |
| 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 |