Weld attack, acid corrosion

Weld attack. Welds are often more susceptible to corrosion than other areas (see Chap. 15, Welds Defects ). Welds may contain porosity, crevices, high residual stresses, and other imperfections that favor attack. Carbon steel welds are usually ditched by acid attack (Fig. 7.10). 

Naphthenic acid corrosion is quite severe in vacuum and visbreaker towers. Crudes with total acid numbers over 2.0 mg KOH/g have corroded type 316 SS in the inlet areas. At times, type 317 (UNS S31700) SS, 904L (UNS N08904), or other corrosion resistant alloys are needed. Matching weld filler metal must be used. In one case, a type 310 (UNS S31000) SS weld filler metal was used on type 316 SS lining. While the strip lining was not attacked, the weld dissolved in one year. 

Oxidation and tarnishing active dissolution in acids anodic oxidation and passivity chemical and electrochemical polishing atmospheric and immersed corrosion in certain cases Crevice corrosion filiform corrosion deposit attack bimetallic corrosion intergranular corrosion weld decay... 

Zirconium resists attack by nitric acid at concentrations up to 70 wt % and up to 250°C. Above concentrations of 70 wt %, zirconium is susceptible to stress-corrosion cracking in welds and points of high sustained tensile stress (29). Otherwise, zirconium is resistant to nitric acid concentrations of 70—98 wt % up to the boiling point. 

In general, the higher the residual or applied metal stress, the more severe the corrosion at a given acidic pH. This explains why many heat exchanger tube ends are often attacked so severely (Fig. 7.1). Tube ends that have been rolled or welded often contain high residual stress. Further, crevices are sometimes present in which acidic species may concentrate (see Chap. 2, Crevice Corrosion ). Screens, rolled sheet metal, and other highly worked metals (not stress relieved) are also prone to attack. 

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). 

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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