The performance of metals near the site should be evaluated prior to material selection. Sea spray and deposits of dry salt particles can lead to pitting and unsightly rusting of some stainless steels. The distance that salt water travels inland – it can be up to 50 miles In others, salt deposits have been measured 27 miles (50 km) or more inland. In some locations, marine salt accumulations are only a factor within 0.9 miles or 1.5km from the shore. Generally, locations within five to ten miles (9 to 18 km) of salt water are at risk for corrosion by sea salts. The distance salt is carried can vary significantly with local weather patterns. It is carried inland by wind, rain and fog. Sea salt contains a mixture of salts including sodium chloride, calcium chloride, and magnesium chloride. Generally, locations within five to ten miles of salt water are considered at risk for chloride-related corrosion, however the distance airborne salt is carried can vary significantly depending on wind patterns, even up to and above 50 miles inland. Stainless steel pitting corrosion Salt in rainwaterĬhlorides in airborne sea spray, rain, and dry salt particles carried by wind may cause pitting and rusting of stainless steels, unless a sufficiently corrosion resistant grade like A4 (316) is chosen. In most cases it should not affect the mechanical properties of the stainless steel but it will show brown rust stains which can effect the attractiveness of the steel and any material that it has contact with. It is similar to pitting but over a larger area or whole area where the ability of the passive layer has been attacked by the environmental conditions. However stainless steel can also become subject to crevice corrosion when the deposits creates a “crevice” on the surface. It usually occurs in very tiny dark brown pits on the surface. When stainless steel corrodes, it is normally in the form of “pitting”corrosion, when the environment penetrates the stainless steel’s passive layer film normally when the film has been damage through scratching or machining. Stainless steel will, however, corrode under certain conditions but not the same type of flaking corrosion that carbon steel has. The addition of molybdenum (2% minimum) in A4 (316) further increases this passivity layer range and further improves corrosion resistance, in particular with acetic, sulphuric, and sulphurous acids and in neutral chloride solutions including sea water. The addition of nickel to the structure, 8% minimum in A2 (304) and 10% minimum in A4 (316), increases the passive layer and therefore corrosion resistance. This is a very thin layer and will be subject to corrosion if it is removed by scratching or machining. The 10.5% chromium in stainless steel means iron with the stainless steel is changed to produce an oxide that resists further oxidation and forms a passive layer on the surface. Carbon steel corrosion How stainless steel resists corrosion Rusting creates a layer of oxide on the surface that is several times thicker than the original iron present and often results in a spalling or flaking of the surface, reducing the steel thickness and therefore its strength. It is the iron in carbon steel that reacts with the oxygen in the atmosphere to produce “iron-oxides” which we can see as “red rust” on the steel surface. It is these alloying elements added to the iron base that makes stainless steel very different from carbon steel. A4 (316) contains an additional element, molybdenum, from 2 to 3%. The 300 series stainless steel grades, A2 (304) and A4 (316) contain nickel from 8 to 14% in addition to the chromium that must be present. Stainless steel also contains iron, but in addition it must contain at least 10.5% chromium and the carbon content is very low, usually 0.08% maximum. The higher the carbon content, the stronger the steel. How is stainless steel different from carbon steel?Ĭarbon steel contains at least 95% iron with up to 2% carbon. A4 (Grade 316), has an addition of at least 2% molybdenum, which significantly increases the A4 (316) metal’s resistance to “salt” corrosion. There are now over 50 stainless steel different grades and a lot are know by their AISI numbering system (200, 300 & 400 grades).Ī2 (Grade 304 – 18% chromium, 8% nickel), is the most common of the 300 series and has corrosion resistance in most applications but is vulnerable to salt water. Chromium is added during the melting of the steel and forms a mixture with the iron and other alloying elements, such as nickel and molybdenum, which improves the metal’s resistance to corrosion. Since its development elements have been added to stainless steel to increase the chromium content in order to improve its corrosion resistance. Stainless Steel is the name given to iron based alloys that contain at least 10.5% chromium.
0 Comments
Leave a Reply. |