Corrosion of stainless steel

In regard to corrosion, stainless steel is a genius material. It is exactly the high corrosion resistance combined with a fair price that long ago made stainless steel the most frequently used group of materials used for “critical” applications such as the food and medical equipment, households, as well as countless situations within the chemical industry.

The regular high corrosion resistance of stainless steel is a result of an ultra-thin film of oxides of especially chromium and iron. This film is only a few nanometres thick and completely invisible, however, it is so tight and strong that the steel is effectively isolated from the surrounding environment. If a situation occurs, despite all precautionary measures, where the protecting oxide film cracks, it will quickly regenerate itself and the steel is now protected once again.

Unfortunately, the way of the road is not always as anticipated. In unlucky situations, the oxide film can be degraded without it regenerating afterwards and this can cause critical corrosive attacks.

When the corrosion is started, a particularly fast complete corrosion can occur. Therefore, the use of stainless steel is often a sort of either-or scenario where the difference between the two extremes can even be very little.

If you can prevent the corrosion from even starting its attack, you nearly have a material for infinity. If not, critical corrosion rapidly will occur rapidly and the life span of one’s equipment can be uncomfortably short.

The corrosion forms that typically is a risk for when using stainless steel are:

  • General corrosion
  • Pitting and crevice corrosion
  • Stress corrosion cracking
  • Intergranular corrosion
  • Time is an important factor

General corrosion

Is also called acid corrosion as it is a corrosion form that most often is seen in strong acid but also strong alkaline mediums. Contrary to all other corrosion forms, general corrosion is characterised by the fact that the entire surface corrodes. Therefore, the loss of material expressed through grams per square meter is high while the complete corrosion often is slow.

General corrosion is, as stated, present in strong acid or (rarer) in strong alkaline media. Typical mediums are sulphuric acid, phosphoric acid, or similar. Besides the acid type and strength, the corrosion speed is especially dependent on the temperature and the number of impurities (especially chloride). Generally, the corrosion speed increases at increasing temperatures and increasing chloride concentration in the medium.

On the steel side of things, it is the austenitic stainless steel that performs most effectively especially with a high content of nickel and molybdenum. Lowly alloyed ferritic and, particularly, martensitic steel is normally not suitable for strong acids and bases.

Pitting corrosion and crevice corrosion

Pitting corrosion is a corrosion form that is caused by a local decomposition of the protective layer of oxides. At sufficiently powerful influence caused by the environment, the oxide film will normally not regenerate, and the corrosion gathers momentum. Pitting corrosion is the prime example of an either-or corrosion form and often results in a particularly quick complete corrosion.

Crevice corrosion is very similar to pitting corrosion but is found in crevices, cracks, or other places where there is little to none liquid replacements. These places, all transport is controlled by diffusion and compared to the “free surfaces”, the risk of corrosion in potential crevices is always higher.

Stainless 4301-sheet after a few days in a mixture of salt (NaCl) and hydrogen peroxide (H2O2). While 99% of the surface of the steel remains completely untouched, there is, nevertheless, occurrences of critical complete corrosion certain places. The picture on the right is a microscopic magnification of the framed area.

An old rule of thumb states that there is a risk of crevice corrosion at a temperature which is 20-25 °C lower than the temperature for pitting corrosion (= critical pitting temperature, CPT). In this scenario, the steel is at its corrosion related threshold and through design, it needs to be ensured that there are no crevices in the system. If this cannot be ensured, a more corrosion resistant steel needs to be picked.

The risk of pitting corrosion and crevice corrosion is strongly increased with:

  • Increasing chloride concentration
  • Increasing temperature
  • The concentration of oxidants
  • Low pH (acid conditions)

Regarding alloy elements, the resistance of the steel increases with increased contents of Cr, Mo, and N while the effect of Ni is relatively small. Non-metallic impurities as e.g. S and P lowers the corrosion resistance drastically.

Based on hundreds of experiments in practice, the resistance to pitting corrosion of the steel can be described through a Pitting Resistance Equivalent (PREN):

PREN = % CR + 3.3 x % Mo + 16 x % N

Based on experience, two steel types with the same PREN will have approximately the same resistance to pitting corrosion. The higher the PREN, the better. It is worth noting that it, theoretically, is irrelevant whether you add 1% Mo or 3,3% Cr. It is the increase of the PREN, which is decisive.

Usually, corrosion is worst when the steel is submerged into the medium but even above the waterline, splashes of saltwater can be enough to cause superficial pitting corrosion although these kinds of attacks rarely result in actual dysfunction. Corrosion above the waterline is usually “only” of cosmetic nature but this can indeed also be extremely annoying in the case of an expensive stainless mailbox or the front of an opera house.

Stress corrosion

Stress corrosion is a corrosion form that occurs in local cracking and results in extremely rapid complete corrosion even in thick goods. The term “stress corrosion” is connected to the fact that the corrosion occurs in places with inner tensile stress – that means places where the metal has been “pulled”. This can be caused by most types of mechanical machining e.g. welding, forging, polishing, etc.

In terms of the environment, the risk of stress corrosion is increased by the following factors:

  • Increasing chloride concentration
  • Increasing temperatures
  • Low pH (acid conditions)
  • Evaporation

Of the abovementioned, the single most important factor for stress corrosion is the temperature. Stress corrosion is dependent of exactly temperature more than any other corrosion form.

Stress corrosion is a corrosion form that almost selectively attacks the lowest alloyed austenitic steel types as e.g. the 4301-grade and normally the 4301 is in the danger zone at temperatures above 60-70 °C. In practice, however, it has occurred that 4301 has been the victim of stress corrosion at much lower temperatures – even all the way down to room temperatures. Due to the contents of Mo and Ni, the 4401-grade is far more resistant to stress corrosion and the guiding temperature limit is at approximately 100-110 °C. However, neither this temperature limit is safe as there have been reported stress corrosion in 4401 at temperatures as low as 30-40 °C.

Ferritic and duplex steel is far less sensitive to stress corrosion compared to austenitic steel so if stress corrosion is the primary corrosion risk, it is not a bad idea to consider using pipes in e.g. 4509 or 4521 instead of 4301 or 4404.

Time is an important factor

For all forms of corrosion, TIME is an important factor. Long-term exposure is always worse than short-term influences and often you can get away with exposing the steel to a very harsh environment – as long as the time of contact is ultra-brief. This is often seen by e.g. the disinfection of stainless tanks. As long as the disinfection can be limited to a few minutes all is well, while residual drops results in long-term exposure and frequent corrosion.

Even clearer, this is seen in corrosion above the waterline. For instance, stainless building construction should be finished in a way that all water can flow seamlessly off; otherwise, you risk remaining saline drops of water, which can result in all sorts of damages from cosmetically unfortunate superficial pitting corrosion (chilly conditions) to crevice corrosion at increased temperatures.

Almost all available corrosion data is based on long-term exposure. If the time of contact can remain brief, there is often a possibility of the steel lasting even better than described in the tables of data.