Figure 10. Elongated pitting attack on a 316 stainless-steel heat exchanger plate.

Figure 10. Elongated pitting attack on a 316 stainless-steel heat exchanger plate.

which the solution will cause pitting within a 24-h test period, the results being expressed as the critical pitting temperature or CPT (13). However, as stated, both of these methods are used to rank the susceptibility of a range of alloys rather than define the performance of a material in a service environment. Electrochemical methods have also been used.

As with crevice corrosion, alloy composition has a profound effect on the resistance of a material to pitting attack. Greene and Fontana (14) summarized the effect of various elements as shown in Table 4.

Intergranular Corrosion. A fact not often appreciated is that metals and alloys have a crystalline structure. However, unlike crystalline solids such as sugar or salt, metallic crystals can be deformed or bent without fracturing; in other words, they are ductile. In the molten state, the atoms in a metal are randomly distributed but on cooling and solidification, they become arranged in crystalline form. Because crystallization occurs at many points in the solidification process, these crystals or grains are randomly orientated and the region where they meet are grain boundaries. In thermodynamic terms, the grain boundaries are more susceptible to corrosion attack because of their higher free energy, although in practice the difference in free energy of the grain boundaries and the main crystals or grains in a homogeneous alloy are too small to be significant. However, when the metal or alloy has a heterogeneous structure, preferential attack at or adjacent to the grain boundaries can occur. This is intergranular corrosion as shown in Figure 11.

When austenitic stainless steels are heated to and held in the temperature range of 600-900°C (1100-1650°F), the material becomes sensitized and susceptible to grain boundary corrosion. It is generally agreed that this is due to chromium combining with carbon to form chromium carbide, which is precipitated at the grain boundaries. The net effect is that the metal immediately adjacent to the grain boundaries is denuded of chromium and instead of having a composition of, say, 18% chromium and 8% nickel, it may assume an alloy composition where the chromium content is reduced to 9% or even lower. As such, this zone depleted in chromium bears little similarity to the main metal matrix and has lost one of the major alloying elements on which it relied for its original corrosion resistance. Indeed, the lowering of corrosion resistance in this zone is so great that sensitized materials are subject to attack by even mildly corrosive environments.

As supplied from the steel mills, stainless steels are in the so-called solution annealed condition, ie, the carbon is in solution and does not exist as grain boundary chromium carbide precipitates. During fabrication where welding is involved, the metal adjacent to the weld is subjected to temperatures in the critical range (600-900°C/1100-1650°F) where sensitization can occur. As such, therefore, this zone may be susceptible to the development of intergranular carbide precipitates. Because the formation of chromium carbides is a function of time, the longer the dwell time in the critical temperature zone, the greater the propensity for carbide formation. Hence, the problem is greatest with thicker metal sections due to the thermal mass and slow cooling rate.

By heating a sensitized stainless steel to a temperature of 1050°C (1950°F), the carbide precipitates are taken into solution and by rapidly cooling or quenching the steel from this temperature, the original homogeneous structure is reestablished and the original corrosion resistance restored.

The first stainless steels were produced with carbon contents of up to 0.2% and as such, were extremely susceptible to sensitization and in-service failure after welding. In consequence, the carbon levels were reduced to 0.08%, which represented the lower limit attainable with steel making technology then available. Although this

Table 4. The Effect of Alloying on Pitting Resistance of Stainless-Steel Alloys


Effect on pitting resistance


Effect on pitting resistance

Chromium Nickel

Titanium/Niobium Silicon

Increases Increases

No effect in media other than ferric chloride Decrease or increase depending on the absence or presence of molybdenum

Molybdenum Nitrogen

Sulfur (and selenium) Carbon

Increases Increases Decreases

Decreases if present as grain boundary precipitates

Figure 11. Scanning electron micrograph of the surface of sensitized stainless steel showing preferential attack along the grain boundaries.

move alleviated the problem, it was not wholly successful, particularly when welding thicker sections of the metal. Solution annealing of the fabricated items was rarely a practical proposition and there was a need for a long-term solution. It was shown that titanium or niobium (colum-bium) had a much greater affinity for the carbon than chromium and by additions of either of these elements, the problem was largely overcome. The titanium or niobium carbides that are formed remain dispersed throughout the metal structure rather than accumulating at the grain boundaries.

Grade 321 is a type 304 (18 Cr, 8 Ni) with titanium added as a stabilizing element, while grade 347 contains niobium. By far, the most commonly used is 321, grade 347 being specified for certain chemical applications.

Modern steelmaking techniques such as AOD (air-oxygen decarburization) were developed to reach even lower levels of carbon, typically less than 0.03%, to produce the "L" grades of stainless steel. These are commercially available and routinely specified where no sensitization can be permitted. With these advances in steel making technology, even the standard grades of stainless steels have typically carbon levels of 0.04-0.05% and, generally speaking, are weldable without risk of chromium carbide precipitates at metal thicknesses of up to 6 mm (1/4 in.). Above this figure or where multipass welding is to be employed, the use of a stabilized or "L" grade is always advisable.

Stress Corrosion Cracking. One of the most insidious forms of corrosion encountered with the austenitic stainless steels is stress corrosion cracking (SCC). The morphology of this type of failure is invariably a fine filamen tous crack that propagates through the metal in a transgranular mode. Frequently, the crack is highly branched as shown in Figure 12, although sometimes it can assume a single-crack form. Factors such as metal structure, environment, and stress level have an effect on crack morphology. The disturbing feature of SCC is that there is virtually no loss of metal and frequently, it is not visible by casual inspection and is only apparent after perforation occurs. Some claim that as much as 50% of the failures of stainless steel are attributable to this cause.

Another characteristic of SCC in stainless steels is that once detected, repair by welding is extremely difficult. Crack propagation frequently occurs below the surface of the metal, and any attempt to weld repair results in the crack opening up and running ahead of the welding torch. The only practical method of achieving a satisfactory repair is to completely remove the affected area with a 1525 cm (6-9 in.) allowance all around the area of visible damage and replace the section. Even then, there is no guarantee that the damaged zone has been entirely removed. In most cases, there are three prerequisites for the initiation of SCC.

• Tensile Stress. This may be either residual stress from fabricating operations or applied through the normal operating conditions of the equipment. Furthermore, it has been observed that a corrosion pit can act both as a stress raiser and a nucleation site for SCC.

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