Stainless steels are stainless because a protective layer spontaneously forms on their surfaces and reduces the rate of corrosion to almost negligible levels. Under normal conditions, this layer heals very rapidly if scratched, so that if stainless steels only suffered from uniform corrosion, they could survive for literally millions of years.
This thin, invisible surface layer is an oxide that protects the steel from attack in an aggressive environment. As chromium is added to steel, a rapid reduction in corrosion rate is observed to around 10% because of the formation of this protective layer or passive film.
Corrosion and Corrosion Properties of Stainless Steels: Part One
Stainless steels represent the most diverse and complex family of all steels. The single most important property of stainless steels,
and the reason for their existence and widespread use, is their corrosion resistance. Stainless steels are stainless because a protective
layer spontaneously forms on their surfaces and reduces the rate of corrosion to almost negligible levels. Under normal conditions, this
layer heals very rapidly if scratched, so that if stainless steels only suffered from uniform corrosion, they could survive for literally
millions of years.
As mentioned above, the reason for the good corrosion resistance of stainless steels is that they form a very thin, invisible surface film
in oxidizing environments. This film is an oxide that protects the steel from attack in an aggressive environment. As chromium is added to steel,
a rapid reduction in corrosion rate is observed to around 10% because of the formation of this protective layer or passive film. In order to
obtain a compact and continuous passive film, a chromium content of about 17% is needed. This is the reason why many stainless steels contain
Figure 1: The effect of chromium content on passivity
The most important alloying element is therefore chromium, but a number of other elements such as molybdenum, nickel and nitrogen also
contribute to the corrosion resistance of stainless steels. Other alloying elements may contribute to corrosion resistance in particular
environments, for example copper in sulfuric acid or silicon, cerium and aluminum in high temperature corrosion in some gases.
A stainless steel must be oxidized in order to form a passive film; the more aggressive the environment the more oxidizing agents are
required. The maintenance of passivity consumes oxidizing species at the metal surface, so a continuous supply of oxidizing agent to
the surface is required. Stainless steels have such a strong tendency to passivate that only very small amounts of oxidizing species
are required for passivation. Even such weakly oxidizing environments as air and water are sufficient to passivate stainless steels.
The passive film also has the advantage, compared to for example a paint layer, that it is self-healing. Chemical or mechanical
damage to the passive film can heal or repassivate in oxidizing environments. It is worth noting that stainless steels are most
suitable for use in oxidizing neutral or weakly reducing environments. They are not particularly suitable for strongly reducing
environments such as hydrochloric acid.
Corrosion can nevertheless occur if the passive film breaks down, locally or uniformly. This can happen by different mechanisms depending
on the conditions of use. The most common types of corrosion are the following.
Uniform corrosion of stainless steels can occur in acidic or hot alkaline solutions. It results in uniform loss which can easily be
predicted and allowed for.
General corrosion resistance is increased with increasing chromium content, but other elements can be detrimental. In particular, sulfur
in solid solution is believed to make passivation more difficult and therefore is generally undesirable for good corrosion properties.
Unfortunately, sulfur makes welding considerably easier and also improves machinability. In the case of welding, sulfur appears to modify
the surface tension of the weld pool and therefore alters its shape significantly. Austenitic grade 316 with sulfur content lower than
0.007 wt% tend to have a high width-to-depth ratio while higher sulfur contents lead to a narrower, deeper weld pool (specifying the
sulfur content of 316L for welding).
Some of the standard grades contain a quantity of sulfur deliberately greater than the typical 0.003 that can otherwise routinely be achieved
with modern steel-making processes (the free machining grades).
Nickel significantly improves the general corrosion resistance of stainless steels, by promoting passivation. The austenitic stainless steels
series therefore possesses a corrosion resistance superior to that of martensitic or ferritic stainless steels (no nickel), particularly
with mineral acids.
Pitting corrosion is the result of the local destruction of the passive film and subsequent corrosion of the steel below.
It generally occurs in chloride, halide or bromide solutions. If a fault in the passive layer or a surface defect results in the local
destruction of the former, dissolution of the steel underneath leads to a build-up of positively charged metallic ions, which in turn
causes negatively charges (e.g. chloride ions) to migrate near the defect. Even in a neutral solution, this can cause the pH to drop
locally to 2 or 3, and can prevent regeneration of the passive layer.
In the passive condition, the current density is in the scale of nanoampers/cm2; in the pit, however, it may be above
1A/cmp2. Similarly, the concentration in chloride ions can be thousands of times greater than that in the solution.
Figure 2: Schematic illustration of pitting corrosion
The Figure 2 illustrates the process: the anodic dissolution of the steel leads to introduction of M+ in solution, which
causes migration of Cl- ions. In turn, metal chloride reacts with water following:
M + Cl- + H2O ⇒ MOH + H + Cl-
This causes the drop of pH. The cathodic reaction, on the surface near the pit follows:
O2 + 2H2O ⇒ MOH + 4OH-
While the propagation phenomenon is well understood, the mechanism of pit initiation is still debated. The initiation of pitting has
long been associated with the presence of MnS inclusions which are difficult to avoid in the steel making process. It has recently been
shown that these inclusions are surrounded by a Cr depleted region which is believed to cause the initiation.
The pitting resistance of a stainless steel is affected by its composition. Increasing the Cr content or adding both the molybdenum
and/or the nitrogen enhances the pitting resistance, though they are not equally potent in this respect. For comparison purposes, an index
is often used to represent the combined effect of these elements:
Pitting index = Cr + 3.3Mo + 16N
where Cr, Mo and N are given in weight percent.
One obvious environment where pitting corrosion is of concern is marine applications. AISI type 316 (an 18Cr-12Ni austenitic stainless
steel with 2-3% Mo) is often the material of choice. In this case, although the severe conditions met in offshore platforms, for example,
call for heavily alloyed steels with up to 6% Mo.
Street furniture is another case where pitting resistance might be relevant, particularly in colder areas where salt de-icing is common.