Weldment corrosion

Additionally, specific environmental conditions can induce localized corrosion such as temperature, conductivity of the corrosive fluid, or thickness of the liquid corrosive film in contact with the metal. In some cases, both metallurgical and geometric factors will influence behavior, such as in stress-corrosion cracking. Preferential weldment corrosion of carbon steels has been investigated since the 1950s, commencing with the problems on icebreakers, but the problem continues today in different applications. (Bond)5... 

Weldment corrosion has clear microstructural dependence, and studies on HAZ show corrosion to be more severe when the material composition and welding parameters are such that hardened structures are formed. It is a well-known fact that hardened steel corrodes more rapidly in acid solutions than fiilly tempered steel, as local microcathodes on the hardened surface stimulate the cathodic hydrogen evolution reaction (4). 

D.G. Enos, S, R. Taylor. Influence of sulfate-reducing bacteria on alloy 625 and austenitic stainless steel weldments. Corrosion, Vol. 52, No. 11, pp. 831-842, 1996. 

Enos DG, Taylor SR (1996) Influence of suffate-reducing bacteria on Alloy 625 and austenitic stainless steel weldments. CORROSION 52(ll) 831-842... 

For purposes other than weight loss from a single metal or alloy, (e.g. bimetallic corrosion, weldment corrosion, stress corrosion cracking), novel coupon designs wfll be required, appropriate to the corrosion phenomenon being evaluated. 

Galvanic probes comprising parent metal, weld metal, and heat affected zone combinations may be used to assess the potential for preferential weldment corrosion within the process streams. 

Investigators have shown that in almost all cases the environment causing the damage was natural, essentially untreated water containing one or more culprit species of microbiological organisms. In the case of austenitic stainless steel weldments, corrosion generated by bacteria takes a distinctive form, that is, sub-surface cavities with only minute pinhole penetration at the surface. 

Weldments subjected to corrosive attack over a period of time may crack adjacent to the weld seams if the residual stresses are not removed. Gas—tungsten arc welding and gas—metal arc welding ate recommended for joining magnesium, the former for thinner materials and the latter for thicker materials. Maintaining a protective atmosphere is a critical issue in welding these alloys. 

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. 

Pakhomova, N. A. and Levin, I. A., Effect of Grain Size on the Inter-crystalline Corrosion of Stainless Steel Type 18-8 , Zashch. Metal, 9, 676 (1973) C.A., 80, 86207j Brown, M. H. and Kirchner, R. W., Corrosion of High Alloy Weldments 1. Sensitisation of Wrought High Nickel Alloys , Corrosion, 29, 470 (1973)... 

Under certain conditions it is possible for a weldment to suffer corrosive attack which has the form of a fusion line crack emanating from the toe of the weld this is termed knifeline attack. It is occasionally experienced in welded stabilised steels after exposure to hot strong nitric acids. The niobium-stabilised steels are more resistant than the titanium-stabilised types by virtue of the higher solution temperature of NbC, but the risk may be minimised by limiting the carbon content of a steel to 0-06 Vo maximum (ELC steel). 

Palladium Advantages have been claimed for new baths (e.g. using chelated complexes ). Antler summarised the use of palladium as coatings, inlays and weldments in electronic connectors . Crosby noted that palladium deposits are of two kinds (1) soft but continuous or (2) hard but porous or cracked. To resist wear and substrate corrosion on contacts, he proposed the application of type 1 (from a bath with tetranitropalladium(ii) anion) over type 2 (from solution containing tetramminepalladium(ii) cation) . 

Henthorne , in considering the corrosion testing of weldments, points out that the test will also give high rates due to (a) end-grain attack, which is particularly prevalent in resulphurised or heavily cold-worked material and (d) dissolution of Ti(C, N) such as occurs in Type S2J weldments and leads to knife-line attack. Since most service conditions do not cause attack on the alloy in these conditions the test can be misleading. 

NACE International Standard RP0472-95, Methods and Controls to Prevent In-Service Environmental Cracking of Carbon Steel Weldments in Corrosive Petroleum Refining Environments. 

Some attractive features of radiography are access to interior difficult-to-reach portion, such as valves and pipes, the objects or sample covered with insulation can be tested, reference radiographs can be accessed, the nature and severity of the attack can be assessed with ease, in principle, pit depths can be obtained although laser profilometry is the preferred method in this regard. The main drawback of the method is the whole sample needs to be exposed. Some applications of the method are determination of flaws in weldments and castings to identify sites of initiation of crevice corrosion, valve... 

Carbon steels. The corrosion behavior of carbon steel weldments produced by fusion welding can be due to metallurgical effects, such as preferential corrosion of the heat-affected zone (HAZ) or weld metal, or it can be associated with geometric aspects, such as stress concentration at the weld toe, or creation of crevices due to joint design. 

Unmixed zones. All methods of welding stainless steel with a filler metal produce a weld fusion boundary consisting of base metal that has been melted, but not mechanically mixed with filler metal and a partially melted zone in the base metal. An unmixed zone has the composition of base metal, but the microstructure of an autogenous weld. The microsegregation and precipitation phenomena characteristic of autogenous weldments decrease the corrosion resistance of an unmixed zone relative to the parent metal. Unmixed zones bordering welds made from overalloyed filler metals can be preferentially attacked when exposed on the weldment surface.14,61... 

Localized biological corrosion of stainless steels. There are three general sets of conditions under which localized biological corrosion of austenitic stainless steel occurs (Figure 6.29). These conditions should be examined for metals that show active-passive corrosion behavior. Microbiological corrosion in austenitic steel weldments has been documented. (Wahid)61, (Krysiak)14... 

Corrosion resistance of aluminum alloys. Researchers have shown that aluminum alloys, both welded and unwelded, have good resistance to uninhibited HN03 (both red and white) up to 50°C. Above this temperature, most aluminum alloys exhibit knife-line attack (a very thin region of corrosion) adjacent to the welds. In inhibited fuming HNO3 containing at least 0.1% hydrofluoric acid (HF), no knife-line attack was observed for any commercial aluminum alloy or weldment even at 70°C (160°F). 

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