Corrosion of Stainless Steel
This section has been added to provide some clarity regarding the natural phenomenon of corrosion.
The first section is specific to stainless steel, as I have recently endured much in the way of consultant cross examination as to why, despite them specifying use of grade 316 for external use, that they are still witnessing what they chose to term as 'rusting'.
Thought for the day:
All metals except gold, platinum, and palladium corrode spontaneously
So to begin, lets start with the basics of the mechanical propoerties of stainless steel
Stainless steel types 1.4401 and 1.4404 are also known as grades 316 and 316L respectively. Grade 316 is an austenitic grade second only to 304 in commercial importance.
316 stainless steel contains an addition of molybdenum that gives it improved corrosion resistance. This is particularly apparent for pitting and crevice corrosion in chloride environments.
316L, the low carbon version of 316 stainless steel, is immune to grain boundary carbide precipitation (sensitisation). This makes it suited to use in heavy gauge (over about 6mm) welded components.
For elevated temperature applications the high carbon variant, 316H stainless steel and the stabilised grade 316Ti stainless steel should be employed.
The austenitic structure of 316 stainless steel gives excellent toughness, even at cryogenic temperatures.
Property data given below is typical for flat rolled products covered by ASTM A240/A240M. It is reasonable to expect specifications in these standards to be similar but not necessarily identical to those given in this datasheet.
Stainless steel grade 316Ti contains a small amount of titanium. Titanium content is typically only around 0.5%. The titanium atoms stabilise the structure of the 316 at temperatures over 800°C. This prevents carbide precipitation at the grain boundaries and protects the metal from corrosion. The main advantage of 316Ti is that it can be held at higher temperatures for a longer period without sensitisation (precipitation) occurring. 316Ti retains physical and mechanical properties similar to standard grades of 316.
Now to the most frequently specification referenced grade 316 (1.4401) typical composition by percentage:
Carbon (C): 0 - 0.08%
Chromium (Cr): 16.5 - 18.5%
Molybdenum (Mo): 2.0 - 2.5%
Silicon (Si): 0 - 1.0%
Phosphorous (P): 0 - 0.05%
Sulphur (S): 0 - 0.02%
Nickel (N): 10.0 - 13.0%
Magnesium (Mn): 0 - 2.0%
Iron (Fe): Balance
And now to the more proficient, but often overlooked grade 316L (1.4404):
Carbon (C): 0 - 0.04%
Chromium (Cr): 16.5 - 18.5%
Molybdenum (Mo): 2.0 - 2.5%
Silicon (Si): 0 - 1.0%
Phosphorous (P): 0 - 0.05%
Sulphur (S): 0 - 0.01%
Nickel (N): 10.0 - 13.0%
Magnesium (Mn): 0 - 2.0%
Iron (Fe): Balance
Types of stainless steel corrosion:
1. Uniform Attack - also known as general corrosion, this type of corrosion occurs when there is an overall breakdown of the passive film. The entire surface of the metal will show a uniform sponge like appearance. Halogens penetrate the passive film of stainless and allow corrosion to occur. These halogens are easily recognizable, because they end with "-ine". Fluorine, chlorine, bromine, iodine and astatine are some of the most active.
2. Crevice Corrosion - this is a problem with stainless fasteners used in seawater applications, because of the low PH of salt water. Chlorides pit the passivated surface, where the low PH saltwater attacks the exposed metal. Lacking the oxygen to re-passivate, corrosion continues. As is signified by its name, this corrosion is most common in oxygen restricted crevices, such as under a bolt head.
3. Pitting - See Galvanic Corrosion. Stainless that had had its passivation penetrated in a small spot becomes an anodic, with the passivated part remaining a cathodic, causing a pit type corrosion.
4. Galvanic Corrosion - Placing 2 dissimilar metals in a electrolyte produces an electrical current. A battery incorporates this simple philosophy in a controlled environment. The current flows from the anodic metal and towards the cathodic metal, and in the process slowly removes material from the anodic metal. Seawater makes a good electrolyte, and thus, galvanic corrosion is a common problem in this environment. 18-8 series stainless fasteners that work fine on fresh water boats, may experience accelerated galvanic corrosion in seawater boats, and thus it is suggested you examine 316 stainless.
5. Intergranular Corrosion - all austentic stainless steels contain a small amount of carbon. At extremely high temperature, such as welding, the carbon forces local chrome to form chromium carbide around it, thus starving adjacent areas of the chrome it needs for its own corrosion protection. When welding, it is recommended you consider low carbon stainless such as 304L or 316L.
6. Selective Leaching - Fluids will remove metal during a de-ionization or de-mineralization process. This usually happens inside a pipe and is rarely a fastener problem.
7. Erosion Corrosion - This corrosion happens when the velocity of an abrasive fluid removes the passivation from a stainless. Again, this is almost exclusively limited to pipe interiors and rarely a fastener problem.
8. Stress Corrosion - Also called stress corrosion cracking or chloride stress corrosion. Chlorides are probably the single biggest enemy of stainless steel. Next to water, chloride is the most common chemical found in nature. In most environments, the PPM are so small the effects on stainless are minute. But in extreme environments, such as indoor swimming pools, the effects can be extreme and potentially dangerous. If a stainless part is under tensile stress, the pitting mentioned above will deepen, and cracking may take place. If you are using stainless steel bolts under tensile stress, in an environment where chlorine corrosion is likely, you should examine the potential for stress corrosion cracking carefully.
And so to the conclusion, Stainless steel can corrode.
Action: Preventative maintenance will ensure the product remains fit for purpose, this is achieved through cleaning and passivation
Free iron, welding oxidation and embedded materials, such as dirt, sand, flux, metals other than steel or iron, etc. may be removed by either chemical cleaning or by abrasive cleaning. Chemical cleaning agents that will successfully remove free iron and most other contaminants are commercially available. These cleaning agents are acids which typically remove a little of the material (about 0.001 inches) from the surface to which they are applied. They need to be left on the surface long enough to remove any free iron and any visible oxides. Most contain nitric and hydrofluoric acids, so they must be handled using rubber gloves and other personal protective equipment, they must be thoroughly rinsed off the surface, and the rinse water should be neutralized with baking soda, baking powder, limestone or other basic material.
Stainless passivation is the process by which stainless steel will spontaneously form a chemically inactive surface when exposed to air or other oxygen-containing environments. Some important facts to know when considering stainless steel passivation:
The presence of free iron on stainless steel is readily detected by spraying the steel with water and letting it set overnight. This test is described in ASTM A-380, Standard Practice for Cleaning, Descaling, and Passivation of Stainless Steel Parts,
Equipment, and Systems. Spraying should be done in the shade and it should be done late in the day to maximize the time that the surfaces stay wet. The surface should wet completely without breaks in the water film; breaks in the film indicate surface contamination and further cleaning is required. Any areas of free iron will rust overnight, staining the surface. In humid or rainy weather, stainless steel only needs to left boldly exposed outdoors for a day and any free iron will turn to ugly rust. Only those areas exhibiting rust need to be cleaned as described above.
References made from:
Aalco
OutoKumpu
Sperko Engineering
The first section is specific to stainless steel, as I have recently endured much in the way of consultant cross examination as to why, despite them specifying use of grade 316 for external use, that they are still witnessing what they chose to term as 'rusting'.
Thought for the day:
All metals except gold, platinum, and palladium corrode spontaneously
So to begin, lets start with the basics of the mechanical propoerties of stainless steel
Stainless steel types 1.4401 and 1.4404 are also known as grades 316 and 316L respectively. Grade 316 is an austenitic grade second only to 304 in commercial importance.
316 stainless steel contains an addition of molybdenum that gives it improved corrosion resistance. This is particularly apparent for pitting and crevice corrosion in chloride environments.
316L, the low carbon version of 316 stainless steel, is immune to grain boundary carbide precipitation (sensitisation). This makes it suited to use in heavy gauge (over about 6mm) welded components.
For elevated temperature applications the high carbon variant, 316H stainless steel and the stabilised grade 316Ti stainless steel should be employed.
The austenitic structure of 316 stainless steel gives excellent toughness, even at cryogenic temperatures.
Property data given below is typical for flat rolled products covered by ASTM A240/A240M. It is reasonable to expect specifications in these standards to be similar but not necessarily identical to those given in this datasheet.
Stainless steel grade 316Ti contains a small amount of titanium. Titanium content is typically only around 0.5%. The titanium atoms stabilise the structure of the 316 at temperatures over 800°C. This prevents carbide precipitation at the grain boundaries and protects the metal from corrosion. The main advantage of 316Ti is that it can be held at higher temperatures for a longer period without sensitisation (precipitation) occurring. 316Ti retains physical and mechanical properties similar to standard grades of 316.
Now to the most frequently specification referenced grade 316 (1.4401) typical composition by percentage:
Carbon (C): 0 - 0.08%
Chromium (Cr): 16.5 - 18.5%
Molybdenum (Mo): 2.0 - 2.5%
Silicon (Si): 0 - 1.0%
Phosphorous (P): 0 - 0.05%
Sulphur (S): 0 - 0.02%
Nickel (N): 10.0 - 13.0%
Magnesium (Mn): 0 - 2.0%
Iron (Fe): Balance
And now to the more proficient, but often overlooked grade 316L (1.4404):
Carbon (C): 0 - 0.04%
Chromium (Cr): 16.5 - 18.5%
Molybdenum (Mo): 2.0 - 2.5%
Silicon (Si): 0 - 1.0%
Phosphorous (P): 0 - 0.05%
Sulphur (S): 0 - 0.01%
Nickel (N): 10.0 - 13.0%
Magnesium (Mn): 0 - 2.0%
Iron (Fe): Balance
Types of stainless steel corrosion:
1. Uniform Attack - also known as general corrosion, this type of corrosion occurs when there is an overall breakdown of the passive film. The entire surface of the metal will show a uniform sponge like appearance. Halogens penetrate the passive film of stainless and allow corrosion to occur. These halogens are easily recognizable, because they end with "-ine". Fluorine, chlorine, bromine, iodine and astatine are some of the most active.
2. Crevice Corrosion - this is a problem with stainless fasteners used in seawater applications, because of the low PH of salt water. Chlorides pit the passivated surface, where the low PH saltwater attacks the exposed metal. Lacking the oxygen to re-passivate, corrosion continues. As is signified by its name, this corrosion is most common in oxygen restricted crevices, such as under a bolt head.
3. Pitting - See Galvanic Corrosion. Stainless that had had its passivation penetrated in a small spot becomes an anodic, with the passivated part remaining a cathodic, causing a pit type corrosion.
4. Galvanic Corrosion - Placing 2 dissimilar metals in a electrolyte produces an electrical current. A battery incorporates this simple philosophy in a controlled environment. The current flows from the anodic metal and towards the cathodic metal, and in the process slowly removes material from the anodic metal. Seawater makes a good electrolyte, and thus, galvanic corrosion is a common problem in this environment. 18-8 series stainless fasteners that work fine on fresh water boats, may experience accelerated galvanic corrosion in seawater boats, and thus it is suggested you examine 316 stainless.
5. Intergranular Corrosion - all austentic stainless steels contain a small amount of carbon. At extremely high temperature, such as welding, the carbon forces local chrome to form chromium carbide around it, thus starving adjacent areas of the chrome it needs for its own corrosion protection. When welding, it is recommended you consider low carbon stainless such as 304L or 316L.
6. Selective Leaching - Fluids will remove metal during a de-ionization or de-mineralization process. This usually happens inside a pipe and is rarely a fastener problem.
7. Erosion Corrosion - This corrosion happens when the velocity of an abrasive fluid removes the passivation from a stainless. Again, this is almost exclusively limited to pipe interiors and rarely a fastener problem.
8. Stress Corrosion - Also called stress corrosion cracking or chloride stress corrosion. Chlorides are probably the single biggest enemy of stainless steel. Next to water, chloride is the most common chemical found in nature. In most environments, the PPM are so small the effects on stainless are minute. But in extreme environments, such as indoor swimming pools, the effects can be extreme and potentially dangerous. If a stainless part is under tensile stress, the pitting mentioned above will deepen, and cracking may take place. If you are using stainless steel bolts under tensile stress, in an environment where chlorine corrosion is likely, you should examine the potential for stress corrosion cracking carefully.
And so to the conclusion, Stainless steel can corrode.
Action: Preventative maintenance will ensure the product remains fit for purpose, this is achieved through cleaning and passivation
Free iron, welding oxidation and embedded materials, such as dirt, sand, flux, metals other than steel or iron, etc. may be removed by either chemical cleaning or by abrasive cleaning. Chemical cleaning agents that will successfully remove free iron and most other contaminants are commercially available. These cleaning agents are acids which typically remove a little of the material (about 0.001 inches) from the surface to which they are applied. They need to be left on the surface long enough to remove any free iron and any visible oxides. Most contain nitric and hydrofluoric acids, so they must be handled using rubber gloves and other personal protective equipment, they must be thoroughly rinsed off the surface, and the rinse water should be neutralized with baking soda, baking powder, limestone or other basic material.
Stainless passivation is the process by which stainless steel will spontaneously form a chemically inactive surface when exposed to air or other oxygen-containing environments. Some important facts to know when considering stainless steel passivation:
- Steels containing more than 11% Chromium are capable of forming an invisible, inert or passive, self-repairing oxide film on their surface. It is this passive layer that gives stainless steels their corrosion resistance.
- If a stainless steel surface is scratched, then more Chromium is exposed which reacts with oxygen allowing the passive layer to reform. However, if a particle of carbon steel is embedded in the scratch then the passive layer cannot reform and corrosion will occur when the metal is wetted or exposed to a corrosive environment.
- Stainless passivation is the chemical treatment of a stainless steel surface with a mild oxidant such as citric acid passivation solution. This process is to accelerate the process noted above in No. 1.
- Passivation is the removal of exogenous iron or iron compounds from the surface of stainless steel by means of a chemical dissolution, most typically by a treatment with an citric acid passivation solution that will remove the surface contamination but will not significantly affect the stainless steel itself.
- Passivation can also be accomplished by electropolishing. Electropolishing is an electrochemical process that is a super passivator of stainless steel and results in a more passive surface than the other methods mentioned above.
The presence of free iron on stainless steel is readily detected by spraying the steel with water and letting it set overnight. This test is described in ASTM A-380, Standard Practice for Cleaning, Descaling, and Passivation of Stainless Steel Parts,
Equipment, and Systems. Spraying should be done in the shade and it should be done late in the day to maximize the time that the surfaces stay wet. The surface should wet completely without breaks in the water film; breaks in the film indicate surface contamination and further cleaning is required. Any areas of free iron will rust overnight, staining the surface. In humid or rainy weather, stainless steel only needs to left boldly exposed outdoors for a day and any free iron will turn to ugly rust. Only those areas exhibiting rust need to be cleaned as described above.
References made from:
Aalco
OutoKumpu
Sperko Engineering