Corrosion Properties Of Stainless Steel
In metallurgy, stainless steel, also known as inox steel or inox from French “inoxydable”, is defined as a steel alloy with a minimum of 10.5% to 11% chromium content by mass.
Stainless steel does not corrode, rust or stain with water as ordinary steel does. However despite the name it is not fully stain-proof, most notably under low oxygen, high salinity, or poor circulation environments. It is also called corrosion-resistant steel or CRES when the alloy type and grade are not detailed, particularly in the aviation industry. There are different grades and surface finishes of stainless steel to suit the environment the alloy must endure. Stainless steel is used where both the properties of steel and resistance to corrosion are required.
Stainless steel differs from carbon steel by the amount of chromium present. Unprotected carbon steel rusts readily when exposed to air and moisture. This iron oxide film (the rust) is active and accelerates corrosion by forming more iron oxide, and due to the dissimilar size of the iron and iron oxide molecules (iron oxide is larger) these tend to flake and fall away. Stainless steels contain sufficient chromium to form a passive film of chromium oxide, which prevents further surface corrosion and blocks corrosion from spreading into the metal’s internal structure, and due to the similar size of the steel and oxide molecules they bond very strongly and remain attached to the surface.
Passivation only occurs if the proportion of chromium is high enough and in the presence of oxygen.
Corrosion Resistance Properties of Stainless Steel
Stainless steel is one of those metals that form a protective oxide layer.
Grades of Stainless Steel
Stainless is available in numerous grades, depending on the quantity and types of metals it is alloyed with. The most commonly used grades for electrical conductors are 310 and 316.
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 17-18% chromium.
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 sulphuric 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, sulphur in solid solution is believed to make passivation more difficult and therefore is generally undesirable for good corrosion properties.
Unfortunately, sulphur makes welding considerably easier and also improves machinability. In the case of welding, sulphur appears to modify the surface tension of the weld pool and therefore alters its shape significantly. Austenitic grade 316 with sulphur content lower than 0.007 wt% tend to have a high width-to-depth ratio while higher sulphur contents lead to a narrower, deeper weld pool (specifying the sulphur content of 316L for welding).
Some of the standard grades contain a quantity of sulphur 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
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.