CORROSION OF WELDS
Different types of corrosion common in metals and alloys are shown in Fig. 9.6. Some of these are related to welds. Their causes and remedies will be briefly discussed in the following paragraphs.
i. Selective leaching j. Intergranular k. Stress cor - l. Corrosion
rosion cracking fatigue
Fig. 9.6 Types of corrosion commonly found in metals and alloys 9.8.1 Galvanic Corrosion
This corrosion occurs when two metals in contact are exposed to a conductive medium. The electrical potential difference acts as a driving force to corrode one of the metals in the couple as electric current flows. Active metals corrode more than the noble metals.
Galvanic corrosion can occur in welds when the filler metal is of different composition than the base metal. It may occasionally occur because of cast weld metal and wrought base metal. Comparatively larger area of the noble compared to active metal will accelerate the attack. This situation is shown in Fig. 9.7.
Fig. 9.7 Galvanic corrosion in a welded join Top: weld Metal less noble than base metal Bottom: Weld metal more noble than base metal
9.8.2 Crevice Corrosion
In a crevice the environmental conditions may become more aggressive with time as compared to the nearby open surface. Crevices in welded joints may occur in various ways: surface porosity, cracks, undercuts, inadequate penetration and design defects. Some materials are more susceptible to it than others. Materials that form oxide film for protection e. g., aluminium and stainless steel are such examples. These materials may be alloyed to change their behaviour, together with designing to minimize crevices and maintenance to keep surfaces clean are some of the ways to combat the problem.
9.8.3 Intergranular Corrosion
The atomic mismatch at the grain boundaries makes it a favoured place for segregation and precipitation. Corrosion generally occurs because the corrodent prefers to attack regions that have lost an element that is necessary for adequate corrosion resistance. Susceptibility to intergranular attack is usually a by product of a heat treatment for example chromium carbides precipitate at the grain boundaries when the steel is heated to 650°C. This results in intergranular corrosion in a band array from weld where the temperature reached is 650°C. This problem can be avoided by post weld annealing.
9.8.4 Stress Corrosion
A combination of tensile stress and corrosive medium gives rise to cracking of a metal. Many alloys are susceptible to this attack, but fortunately the number of alloy-corrodent combinations that cause it are relatively few. Stresses that cause this arise from residuals stresses due to cold work, welding, thermal treatment and may be due to externally applied forces during assembly and service. Cracks may follow intergranular or transgranular path. There is a tendency of crack branching. The following list gives some characteristics of stress corrosion cracking:
(a) Stress corrosion requires a tensile stress. Below a threshold stress cracks do not occur.
(b) Cracking appears macroscopically brittle even though the material may be ductile in the absence of corrodent.
(c) Stress corrosion depends on metallurgical conditions of the alloy.
(d) In a given alloy a few specific corrodents cause cracking.
(e) Stress corrosion may occur in environments otherwise mild for uniform corrosion.
(f) Long time periods (often years) may pass before cracks become visible. The cracks then propagate fast and may cause unexpected failure.
(g) Stress corrosion is not yet understood in most cases, although there is now a large amount of data to help avoid this problem.
Methods of fighting stress corrosion problem include: stress relieving, removing critical environmental species or selecting a more resistant material.