Environmentally induced cracking often occurs unexpectedly because the environment associated with the failure was not considered fully when setting the materials specifications. Stress corrosion cracking (SCC) is a subset of this type of failure mechanism and is induced by the synergistic combination of a susceptible metal, tensile stress and a specific environment. The tensile stress may be either applied or residual, such as from assembly, forming or welding. Certain combinations of alloys and environments are required for SCC to occur, such as chlorides with austenitic stainless steel, ammonia with brass, and caustic with steel.
The phenomenon of SCC can be divided into two stages. The first is referred to as the incubation period, where the appropriate local chemistry for the aggressive environment is established. This is often the most time consuming stage. The second stage is the actual crack propagation. (Learn more about crack propagation in the article The Effects of Stress Concentration on Crack Propagation.)
Types of Stainless Steel Susceptible to Stress Corrosion Cracking (SCC)
The austenitic stainless steels are the most widely used commercially, and because of their high levels of chromium (Cr) and nickel (Ni), they have a different crystal structure than that of other Fe-based alloys. (Read The Crystalline Structure of Metals for an in-depth explanation.) Such a structure and chemistry results in some unique properties. For example, in the annealed condition austenitic stainless steels are non-magnetic. With Cr levels around 18% for austenitic alloys, a uniform 1-3 nm thick Cr2O3 film can passivate the surface of the metal, which will minimize staining and corrosion.
The most widely used austenitic stainless steel alloy is Type 304. The addition of molybdenum (Mo) to an austenitic stainless steel such as Type 316 provides improved corrosion resistance. To improve intergranular (down the grain boundaries) corrosion resistance at elevated temperature, tantalum (Ta) or niobium (Nb) are added to minimize carbide formation in the grain boundaries with subsequent Cr depletion along the grain boundaries. It is the Cr depleted zone that is susceptible to corrosive attack. There are a substantial number of specialty austenitic stainless steels with varying corrosion resistance and properties that are superior to that of Type 304.
Stress Corrosion Cracking Failure Modes
For austenitic stainless steels there are three common SCC failure modes; these are chloride, polythionic acid and caustic cracking.
Chloride stress corrosion cracking
Chloride stress corrosion cracking (Cl-SCC) is the most prevalent because of the ubiquitous presence of chloride ions. Temperature is an important parameter in the initiation and propagation of Cl-SCC. Many references suggest that a temperature above 60°C (140°F) is required for Cl-SCC; however, there are many observed Cl-SCC failures reported in the literature at 50°C (122°F) and lower. Above 80°C (176°F) the initiation of Cl-SCC becomes fairly rapid.
The presence of embedded steel particles from the fabrication process can enhance the susceptibility of austenitic stainless steels to Cl-SCC by increasing the concentration of chloride from the environment at a specific site. Thus, there is no threshold concentration of chloride below which the alloy would be immune to SCC because it is the local environment and not the bulk environment that is the critical factor. (Learn more in the related article Chloride Stress Corrosion Cracking of Austenitic Stainless Steel.)
Accumulation of chloride ions can occur inside crevices or under corrosion deposits on the metal surface. Evaporative concentration of chlorides from continuous condensation droplets is one of the most common sources. Often, chlorides will concentrate at the base of a pit with the crack ultimately initiating there.
All austenitic stainless steel alloys are susceptible to Cl-SCC, although the increased pitting resistance of the Mo containing alloys provides a small amount of improvement in susceptibility or time to initiate a crack.
The relationship between chloride concentration, temperature and tensile stress to initiate a crack is complex. In general, a higher chloride concentration requires less tensile stress for crack initiation and vice versa for a given temperature. Lower levels of chloride and tensile stress take longer to crack.
Identifying whether a fracture is the result of Cl-SCC requires metallographic and/or scanning electron microscopy (SEM) fractographic analysis. The crack propagates by a transgranular cleavage mechanism with the SEM fracture surface morphology exhibiting fan-shaped features consisting of facets and steps. In polished cross-section the crack(s) tends to be branched and propagates through the grains. Multiple cracks may be present if there are many initiation sites. Detection of the element Cl on the fracture surface by SEM energy dispersive analysis is not always observed because of the high solubility of the chloride ion in water and its easy loss.
Polythionic acid stress corrosion cracking
Polythionic acid stress corrosion cracking (PTA-SCC) of austenitic stainless steel alloys requires not only a tensile stress and a specific environment but also a unique microstructure.
This phenomenon is associated with environments such as those in a refinery or petrochemical plant that can produce a sulfide scale on the metal surface. These feeds either contain sulfur contaminants or sulfur components that are specifically added to minimize the formation of metal catalyzed coke in the unit. The sulfur species react with H2 to form H2S, which then reacts with the metal surface to form an iron sulfur scale. During a shutdown when the unit is opened for inspection, the sulfide scale on the metal surface can react with moisture and oxygen in the air and form what is referred to as polythionic acid (H2SxO6) that can attack the metal down the grain boundaries.
Tetrathionic acid (H2S4O6) is considered to be the species that induces the PTA-SCC. One of the necessary requirements for PTA-SCC is that the alloy has a sensitized microstructure where there is chromium depletion adjacent to the grain boundaries. Sensitization occurs when the metal is exposed to high temperatures (from about 400°C to 800°C) for a period of time with the precipitation of Cr-rich carbides in the grain boundaries. Even stabilized and low carbon grades of austenitic stainless steel can sensitize.
One of the approaches to minimize PTA-SCC is to apply a soda ash wash per NACE SP0170 prior to opening the susceptible equipment to the atmosphere.
Austenitic stainless steels are used in various chemical and manufacturing processes such as those in the pulp and paper industry. While these alloys demonstrate good general corrosion resistance in caustic media, they are susceptible to caustic embrittlement or cracking. At a sodium hydroxide (NaOH) concentration of 50% and a temperature of about 121°C (250°F) these alloys can crack, while at 93°C (200°F) they can exhibit severe general corrosion. At very low and at very high NaOH concentrations, the temperature for cracking is substantially higher. Impurities in the caustic can depress the critical temperature for cracking. On the other hand, potassium hydroxide (KOH) at the same concentration as NaOH requires a higher cracking on set temperature.
The susceptibility of Types 304 and 316 to caustic cracking appears to be quite similar. Crack propagation is by transgranular cleavage and thus the environmental conditions must be known in order to differentiate whether the failure mechanism was by Cl-SCC or caustic cracking.