Sensitization and Intergranular Corrosion of Stainless Steels
What are stainless steels?
Stainless steels are alloys of iron and carbon containing a minimum of about 10.5% chromium. The presence of chromium ensures the formation of a ultra-thin, invisible, protective, self-healing passive film on the steel surface, mainly made up of chromium oxides and hydroxides; Cr2O3 and Cr(OH)3. This oxide film is insoluble, compact and adheres well to the substrate thereby protecting the underlying steel. This is the reason why this group of steels is known as “stainless steels” and is free from rusting.
Chromium is the most important alloying element in stainless steels. It is the element that gives the stainless steels their basic corrosion resistance. As the chromium content increases the corrosion resistance increases. Although all stainless steels contain chromium as the principal alloying element, other elements are often added to enhance their properties. Nickel is added to promote an austenitic structure. Nickel also increases ductility and toughness and stabilizes the chromium oxide film. Carbon is a strong austenite former and strongly promotes an austenitic structure. It also increases the mechanical strength. However, carbon reduces the resistance to intergranular corrosion. Molybdenum when added substantially increases the resistance to both general and localized corrosion. Titanium, niobium and tantalum are ferrite stabilizers and strong carbide formers. In austenitic steels they are added to increase the resistance to intergranular corrosion.
Stainless steels have traditionally been divided into five categories based on their microstructure at room temperature, viz. austenitic, ferritic, martensitic, precipitation hardening steels and duplex stainless steels.
Austenitic stainless steels are iron-chromium-nickel alloys and are most common stainless steel used today. Type 304 (Fe +18% Cr + 8% Ni) is the basic 300 series austenitic stainless steel.
Mechanism of sensitization and intergranular corrosion:
When austenitic stainless steel of normal carbon content (0.03-0.08% C) is exposed in the range of approximately 425 to 850 C for a period of time, or when the steel has been heated to higher temperatures and allowed to cool through that temperature range at a relatively slow rate, the chromium and carbon in the steel combine to form precipitates of chromium carbide (M23C6) particles along the grain boundaries of the steel. The exposure of the steel to this critical temperature range can result from improper annealing, during stress relieving, or heating during forming and welding. The chromium content of these carbides can be as high as 70%, while the chromium content in the steel is as per nominal amount i.e. around 18%. Eventually a narrow band of material around the carbides therefore becomes depleted in chromium (< 9% Cr) to such an extent that the corrosion resistance decreases. Carbides precipitate when heating occurs, such as during welding, heat treatment or metal fabrication. Formation of the carbide particles at the grain boundaries depletes the surrounding metal of chromium and reduces its corrosion resistance, allowing the steel to corrode preferentially along the grain boundaries. Steel in this condition is said to be “sensitized” and the phenomena of carbide precipitation along the grain boundaries of the steel is called “sensitization”(Figure 1(a)). Sensitization refers to the precipitation of carbides at grain boundaries in a stainless steel making it susceptible to intergranular corrosion. Figure 1(b) and 1(c) show microstructures of stainless steel before and after sensitization, respectively. Sensitization leads to the loss of alloy integrity. It results from chromium depletion in the vicinity of carbides precipitated at grain boundaries.
Sensitization often occurs in austenitic steels but ferritic & martensitic stainless steels are also prone to this phenomenon. However, the sensitive temperature range at which carbide precipitation along the grain boundaries occurs is higher, generally around 900 – 950 C.
Figure 1: Mechanism of sensitization in stainless steel
Figure 2: normal un-sensitized
Figure 3: sensitized microstructure of stainless steel
The extent of carbide precipitation depends upon carbon content, temperature and time at temperature. Sensitization in austenitic stainless steels occurs only in the temperature range 550-850 C. Rapid cooling of the steel through the sensitizing temperature range does not provide adequate time for the process of sensitizing to occur. The degree of sensitization increases with increasing carbon content and decreasing chromium content.
If the sensitized steel is then exposed to an aggressive environment (such as an acidic environment) the chromium depleted region is attacked, and the material along the grain boundaries is corroded away. The result is that the alloy disintegrates, the grains may drop out of the steel or in extreme cases the grains are only mechanically locked together while the stiffness and strength of the material is totally lost. Any sensitized microstructure will undergo selective localized corrosion along grain boundaries leading to phenomena called intergranular corrosion (IGC). Intergranular corrosion is a thus a localized attack at and adjacent to grain boundaries, with little corrosion within the grains. The driving force of intergranular corrosion is the difference between the Electrode potentials of the grain boundary and the grain itself, which form a galvanic cell. The grain boundaries are anodically attacked in the presence of an electrolyte. The alloy, sensitive to IGC, disintegrates and/or loses its strength when exposed to corrosive environments.
Thus, in case of a sensitized steel the grain boundaries are more susceptible to corrosion than that the rest of the grain. The chromium depleted areas around the grain boundaries can be preferentially/selectively dissolved in certain corrosive environments. Intergranular corrosion occurs where the carbon content in the stainless steel is high, or where cooling rates in welding or heat treatment are slow.
Intergranular corrosion of stainless steels that occurs due to sensitization in the heat affected zone (HAZ) (at appreciable distance from the weld) during welding is known as “weld decay”. Weld decay occurs in normal un-stabilized steels. Somewhat different from this, the “Knifeline attack” is a localized form of intergranular corrosion occurring only a few grain diameters nearer to the weld zone in austenitic stainless steels. Here, a narrow band in parent metal near to the fusion line becomes susceptible to the intergranular cracking. It occurs in stabilized grades.
Oxalic acid etch test is a rapid observation method of screening specimens for susceptibility to intergranular attack associated with chromium carbide participates. The test is used for acceptance but not for rejection of material. The ASTM A923 and ASTM A262 and other similar tests are often used to determine when stainless steels are susceptible to intergranular corrosion.
How to prevent sensitization and intergranular corrosion (IGC)?
The problem of sensitization and intergranular corrosion in stainless steels & during welding can be overcome by taking the following measures:
- Using extra low carbon (e.g. 304L, 316L) grades of stainless steel. They have carbon less than 0.03%. This ensures insufficient carbon in the alloy to form large amounts of chromium carbides and thus reduces chromium in the grain boundaries.
In case of welding, use of a low carbon base metal and filler metal is recommended to reduce or eliminate carbon in the welding. However, this is not always practicable.
- Solution annealing treatment: It is possible to reclaim the steel that has undergone sensitization by heating it to 1050 – 1100 C temperature followed by quenching (rapid cooling) in water. During the heating stage the carbides dissolve and their formation / reprecipitation is suppressed by fast cooling. However, this method is unsuitable for treating large assemblies, and also ineffective where welding is subsequently used for making repairs or for attaching other structures.
In case of welded components, the post-weld heat treatment (PWHT) dissolves the carbides and puts chromium back into the solution.
- Stabilization treatment: Using special grades of steel known as stabilized grades such as Type 321 (stabilized with titanium), 347 (stabilized with niobium) and 348 (stabilized with tantalum & niobium). Stainless steels can be stabilized against chromium carbide precipitation & sensitization by addition of strong carbide forming elements like titanium, niobium, or tantalum, which form titanium carbide, niobium carbide and tantalum carbide, respectively, in preference to chromium carbide. These elements have a much greater affinity for carbon than chromium. They tie up the carbon and reduce the carbon available in the alloy for formation of chromium carbides; thereby preventing chromium carbide formation and resultant chromium depletion at the grain boundaries. Since chromium remains in solution in steel, it ensuring full corrosion resistance. The titanium and niobium carbides are dispersed in the matrix of the grains and not localized at grain boundaries to promote intergranular corrosion.
However, there is a problem associated with stabilization treatment where welding is involved. In the area closest to the weld, the temperature during welding can be so high that titanium or niobium carbides are dissolved. In turn they do not get adequate time to re-precipitate before the material has cooled sufficiently to allow the formation of chromium carbides at the grain boundaries. This leads to so-called ‘knife line attack’ in which a narrow zone of material very close to the weld suffers intergranular corrosion. Since the carbon level in stabilized steels is often quite high (0.05-0.08% C) this can result in serious attack.
Use of filler metals having less than 0.02% carbon or having special alloying ingredients (Ti and Nb) can prevent the formation of chromium carbides.