Why does some stainless steel corrode and rust?

What is stainless steel?

Stainless Steel is the name given to iron based alloys that contain at least 10.5% chromium. Since its development elements have been added to stainless steel to increase the chromium content in order to improve its corrosion resistance. 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. There are now over 50 stainless steel different grades and a lot are know by their AISI numbering system (200, 300 & 400 grades).

A2 (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. A4 (Grade 316), has an addition of at least 2% molybdenum, which significantly increases the A4 (316) metal’s resistance to “salt” corrosion.

How is stainless steel different from carbon steel?

Carbon steel contains at least 95% iron with up to 2% carbon. The higher the carbon content, the stronger the steel. 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 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. A4 (316) contains an additional element, molybdenum, from 2 to 3%. It is these alloying elements added to the iron base that makes stainless steel very different from carbon steel.

How carbon steel corrodes

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. 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.

Carbon steel corrosion

How stainless steel resists corrosion

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. This is a very thin layer and will be subject to corrosion if it is removed by scratching or machining. 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. 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. Stainless steel will, however, corrode under certain conditions but not the same type of flaking corrosion that carbon steel has. 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. It usually occurs in very tiny dark brown pits on the surface. However stainless steel can also become subject to crevice corrosion when the deposits creates a “crevice” on the surface. 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. 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.

Stainless steel pitting corrosion

Salt in rainwater

Chlorides 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. 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.

Salt in rain water

Coastal salt exposure

Sea salt contains a mixture of salts including sodium chloride, calcium chloride, and magnesium chloride.
It is carried inland by wind, rain and fog. The distance salt is carried can vary significantly with local weather patterns. Generally, locations within five to ten miles (9 to 18 km) of salt water are at risk for corrosion by sea salts. In some locations, marine salt accumulations are only a factor within 0.9 miles or 1.5km from the shore. In others, salt deposits have been measured 27 miles (50 km) or more inland.

The distance that salt water travels inland – it can be up to 50 miles

Sea spray and deposits of dry salt particles can lead to pitting and unsightly rusting of some stainless steels. The performance of metals near the site should be evaluated prior to material selection. If possible, determine if there are salt (chloride) deposits on surfaces around the site. Portable chloride test kits can be used or a laboratory can provide a more accurate assessment. If a laboratory is used, they will need a sample that has been near the test site and has not been washed. Care must be taken, so that chlorides are not inadvertently removed from the surface before testing. If there is industrial or urban pollution, the level must be determined to assess corrosion potential. Evaporation and infrequent rain increase salt concentrations on exterior surfaces and corrosion rates. Sheltered locations generally have heavier salt deposits because the salt is not removed by rain. Humidity, fog and light rain
can dampen the deposited salt and create a concentrated, very corrosive salt solution on the surface. Salt solutions begin to form at temperatures above 00C (320F) and humidity levels above 45%. The most aggressive conditions are created by high salt concentrations combined with high ambient temperatures and moderate humidity.

Weather damage effects

Along with Salt water the influence of de-icing and corrosive pollutants like sulphur dioxide (SO2), nitrogen oxides (NOX), Hydrogen sulphide (H2S) and ammonium (NH4) also have a factor on this corrosion. SO2 and NOX can form sulphuric and nitric acid in the atmosphere and become acid rain. There is an example in America where A2 (304) stainless steel railings, were corroding after one winter in Pittsburgh (over 50 miles in land from the sea). They were near a busy highway with vehicle pollution (NOX) along with the deicing salt from the road, was all blown on to the rails as a form of mist during the winter months give them a pitted corrosion look with 12 month over being newly fitted. In addition this the rough surface finish of the railings, which had weakened the protective layer, made the corrosion worse.

De-icing salt exposure

In winter months, de-icing salt can be heavily used to clear road and path surfaces. These de-icing salts have sodium chloride and calcium chloride which are both corrosive. Unfortunately, salt accumulates over time and makes the environment around roads and walkways much more corrosive for all metals. Typically, de-icing salt (sodium chloride and calcium chloride) deposits can be heavier, subject to usage levels, than the sea salt deposits found in coastal areas. Both of these salts are corrosive to metals. Salt begins to absorb water from the air and forms a concentrated corrosive chloride solution above specific humidity and temperature levels. Calcium chloride becomes corrosive at 0C (freezing point) and 45% humidity and sodium chloride becomes corrosive at 100C and 76% humidity.

De-icing salt corrosion damge

Surface contamination with salt is not limited to metal immediately beside roads. Road mist and salt contaminated airborne dust can carry de-icing salt significant distances from busy roads. In America it has been recorded contaminating as high as the 12th or 13th floor of buildings in the surrounding areas of these busy roads. Once added to the environment, salt is present throughout the year. On building exteriors, salt concentrations and corrosion are usually greatest between street level and the third floor (which is the majority of all UK houses) but this can vary with the location.

Exterior wall panels and window frames

Again in America, A2 (304) window frames and A4 (316) wall panels fitted to a nearby buildings have been studied. Both have a smooth No. 4 finish (the passive layer not damage through scratching or machining) and both with situated on the second floor and were exposed to deicing salt in Minneapolis. The A2 (304) window frame was badly stained by corrosion where as the A4 (316) wall panel was fine. A2 (304) did not provide sufficient protection from salt corrosion unlike the A4 (316) that the wall panels were made from. In applications with moderate deicing salt exposure and urban pollution, A4 (316) is usually adequate to avoid this corrosion.

Street light corrosion

Other example are A4 (316) street lighting poles were installed at Jones Beach, New York in 1967 with a smooth No. 4 finish. Although they were in the parking area immediately adjoining the beach and were exposed to coastal salt, there is no sign of corrosion. A similar light pole of A2 (304), was installed in a sheltered location a few streets from Miami Beach, Florida. After one year, chloride corrosion was visible on these A2 (304) light poles. This again helps to illustrate the performance advantage of A4 (316) in a coastal location.


A4 (316) is recommended for most coastal applications, because it contains molybdenum, and it is this higher levels of molybdenum and chromium which increases A4 (316) resistance to corrosion pitting caused by salt exposure.
However, as already covered, that due to the travel of salt water mist inland, the salt levels in de-icing usage and urban pollution which all will have an effect on the damage to stainless steel anything less than A4 (316) will run the risk of corrosion. This is why we only use A4 (316) for all our POLYTOPS Nails and pins.

This information is based on the research from: