Knife-line corrosion

Knife-line corrosion occurs at the solder joints at chloride contents of > 1 mol/m. The solder L-Ag 40 Cd is more sensitive to corrosion than the other two solders. 

In the case of rubber-sealed joints, the amount of chlorides that can be extracted by water must not exceed 0.05% otherwise there is a risk of pitting or crevice corrosion. Brazed joints, independent of the braze, are no longer permitted for stainless steels because of attack by knife-line corrosion [32]. 

Other parts of the weldment (Ref 2,3). A very selective knife-line corrosive attack can result fixxm this immersed service. 

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. 

The selective corrosion of cast iron (graphitization), the preferential corrosion of the steel welding (grooving corrosion), sensitization and knife line attack of welded stainless steels are typical examples of corrosion influenced by metallurgical parameters. 

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). 

V. Cihal, Metallurgical Aspects of Knife-Line Attack on Stabilized Stainless Steels, Localized Corrosion NACE 3, R.W. Staehle, B.F. 

A special form of intergranular corrosion is knife-line attack, which can occur in Nb/Ti-stabilized austenitic stainless steels after incorrect heat treatment [7.1]. 

HASTELLOY alloy B-2—An improved wrought version of HASTELLOY alloy B. Alloy B-2 has the same excellent corrosion resistance as alloy B, but with improved resistance to knife-line and heat-affected zone attack This alloy resists the formation of grain-boundary carbide precipitates in the weld heat-affected zone, thus making it suitable for most chemical process applications in the as-welded condition. Alloy B-2 also has excellent resistance to pitting and stress-corrosion cracking it is particularly well suited for equipment handling hydrogen chloride gas. and hydrochloric, sulfuric, acetic, and phosphoric acids. 

This phenomenon is restricted to the stabilized grades of steel, such as 321 and 347. The knife-line attack occurs immediately adjacent to the weld and shows as a thin line of intergranular corrosion. It results from intergranular corrosion like the weld decay. It may be noted that weld decay develops at some distance away fi"om the weld. The following is the mechanism suggested for the knife-line attack of stabilized steels. 

Knife-line attack Intergranular corrosion of an austenitic steel (stabilized grade) extending only a few grains away from the fusion line of a weld during sensitization of steel. 

Knife-line attack, immediately adjacent to the weld metal, is a special form of sensitization in stabilized austenitic stainless steels. Stabilizing elements (notably Ti and Nb) are added to stainless steels to prevent intergranular corrosion by restricting the formation of Cr-rich grain boundary precipitates. Basically, these elements form carbides in preference to Cr in the austenitic alloys. However, at the high temperatures experienced immediately adjacent to the weld fusion zone, the stabilizer carbides dissolve and remain in solution during the subsequent rapid... 

Two steel tanks were removed from the same site and at the same time as the composite tank. There are no records to show how long these steel tanks were in service. One of the steel tanks was dusted with white metal oxide corrosion, while the other had signs of corrosion at the weld line. Rust had weakened the joint so much that the weld line could be scraped away with a pocket knife, revealing holes. Corrosive attack on an underground gasoline tank comes from both the inside and outside. An inside problem is moisture, which is often present in the fuel and settles at the bottom. Depending on various conditions, the moisture can create severe internal corrosion. But an inspection of the 25 year old composite tank showed no internal corrosion, even though there was moisture with the fuel on the bottom. 

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