Weld decay

Identification. The primary identif3dng feature is confinement of metal loss to the weld bead (Fig. 15.3), although in advanced stages base metal immediately adjacent to the weld bead may also be affected. Note that this feature seems to distinguish galvanic corrosion of welds from other weld-related corrosion, such as weld decay, which preferentially attacks the immediately adjacent base metal (Fig. 15.4). 

Note that sensitization will not result in weld decay in all environments. Stainless steels may be used in environments that do not require the full corrosion resistance of the alloy. In these cases, weld decay may not occur even though sensitization has taken place. 

Locations. Weld decay may affect welded stainless steels that have normal carbon contents and are not specifically inhibited for sensitization. Weld decay affects only the immediate weld area. 

Critical factors. The critical factors governing weld decay include the use of a sensitized stainless steel and the exposure of this metal to an environment that is sufficiently aggressive to cause degradation in the sensitized region. 

Identification. Weld decay characteristically produces distinct, narrow zones of disintegration immediately adjacent and parallel to the weldment (Fig. 15.4). Attack will occur on either side of the weld. The weld itself and surrounding metal will be unattacked. The corroded area frequently has a granular or sugary appearance and feel. This is due to corrosion along the boundaries of individual grains that causes them to be released from the metal surface. Affected metal may have lost its metallic ring. 

Elimination. The sensitization that leads to weld decay is a reversible process, and elimination can be achieved by appropriate heat treatment of the affected metal. Weld decay may also be avoided by specifying proper materials and welding techniques. 

Cautions. Sensitization is a metallurgical condition. It can be identified by certain specialized nondestructive testing techniques or by destructive metallurgical examinations, but it cannot be identified by simple visual examination. It becomes visually apparent only after exposure of the sensitized metal to a sufficiently aggressive environment produces corrosion that is, weld decay. 

Note that low carbon or stabilized grades of stainless steel do not possess intrinsically greater corrosion resistance than their unadjusted counterparts. Their sole value in typical cooling water systems results from their resistance to sensitization and potential weld decay that can result when the metals are welded. It is therefore not economically justifiable to specify low carbon or stabilized grades of stainless steel for typical cooling water system components that are not to be welded. 

When stress-relief-annealing 300 series stainless steel components, care must be taken to avoid slow cooling through the sensitization range (see Weld Decay in this chapter). 

There is one major pitfall which must be avoided in using stainless-steel components joined by welding it is known as weld decay. It is sometimes found that the heat-affected zone - the metal next to the weld which got hot but did not melt - corrodes badly. 

Slides Covering pipelines with polymeric films cathodic protection of pipelines, ships, etc.. With zinc bracelets use of inert polymers in chemical plant galvanic corrosion in architecture (e.g. A1 window frames held with Cu bolts) weld decay. 

Weld Decay—localized corrosion of weld metal. 

Fig. 1.9(a) Curves showing the variation of temperature with time at various points adjacent to a fusion weld in an austenitic stainless steel and (b) weld decay in an unstabilised austenitic... 

The ferritic steels may also undergo intercrystalline corrosion as a result of grain boundary carbide formation. In the normal softened state (treated i 800 C) the carbon is largely precipitated and the ferrite composition homogenised so that further heating at lower temperatures has no adverse effect. During solution treatment above 950 C, however, carbon is redissolved. Sensitisation can then occur at lower temperatures but the rate is so rapid that it can only be suppressed by very rapid cooling which is not practically feasible. Thus weld decay is very possible in service unless a remedial... 

In the ferritic steels the effects of nitrogen and carbon are indistinguishable one from the other and the normal incidental level is sufficient to cause weld decay susceptibility. Thus in the super ferritic group both carbon and nitrogen are controlled to a low practicable level and sufficient titanium is added to stabilise both elements. 

The corrosion of stainless steel welds has probably been studied more fully than any other form of joint corrosion and the field has been well reviewed by Pinnow and Moskowitz , whilst extensive interest is currently being shown by workers at The Welding Institute. Satisfactory corrosion resistance for a well-defined application is not impossible when the austenitic and other types of stainless steels are fusion or resistance welded in fact, tolerable properties are more regularly obtained than might be envisaged. The main problems that might be encountered are weld decay, knifeline attack and stress-corrosion cracking (Fig. 9.29). 

The effect of the welding process on the severity of weld decay varies according to the process and the plate thickness so that no single recommendation is possible for every thickness of plate if resistance to attack is essential. The severity of weld decay correlates quite well with sensitisation times as calculated from recorded weld heating cycles. 

The Ni-28Mo alloy provides a special case of selective corrosion analogous to the weld-decay type of attack it may be removed by solution treatment or using an alloy containing 2%... 

Weld Decay localised attack of austenitic stainless steels at zones near a weld, which results from precipitation of chromium carbides. 

Type 304 (the so-called 18/8 stainless steels) the most generally used stainless steel. It contains the minimum Cr and Ni that give a stable austenitic structure. The carbon content is low enough for heat treatment not to be normally needed with thin sections to prevent weld decay (see Section 7.4.4). 

Intergranular corrosion (weld decay) and stress corrosion cracking are problems associated with the use of stainless steels, and must be considered when selecting types suitable for use in a particular environment. Stress corrosion cracking in stainless steels can be caused by a few ppm of chloride ions (see Section 7.4.5). 

Figure 16.6 Creation of microscopic anodic areas in the weld decay of a typical stainless steel.

There are several ways in which weld decay can be avoided ... 

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