Steel knife-line attack

A somewhat similar phenomenon is knife-line attack which may be observed after welding titanium or niobium stabilised austenitic stainless steels. In this case there is a very narrow band of severe intergranular attack along the interface between the parent metal and the fusion zone. During welding, the parent metal immediately adjacent to the fusion zone is heated to just below the melting point and both chromium carbides and niobium or titanium carbides dissolve completely. On cooling rapidly, the conditions are such that when relatively thin sections are welded, neither chromium carbide nor niobium or titanium carbide have time to precipitate. If the weld is now... 

Knife-line Attack severe highly localised attack (resembling a sharp cut into the metal) extending only a few grains away from the fusion line of a weld in a stabilised austenitic stainless steel, which occurs when the metal comes into contact with hot nitric acid and is due to the precipitation of chromium carbides. 

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. 

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

The phenomenon of knife-line attack within weld HAZs describes susceptibility to ICC and IGSCC in stabilized grades of austenitic stainless steels [61, 68]. Stabilization is a term used to describe depletion of solid solution carbon due to niobium and titanium alloying. These elements produce carbides in the temperature range from 870 to 1150 °C in austenitic stainless steels such as AISI 347 [61]. Little carbon remains in solid solution to be precipitated as (Fe,Cr)23C6. Normally, the initial get-tering of carbon above 870 C eliminates sensitization by Cr-carbide formation that normally occurs over the range from 425 to 815 °C in austenitic stainless steels. 

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]. 

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 in stainless steels is caused by one of the following ... 

Why stainless steels which are stabilized undergo knife-line attack If the zone on which knife-line attack has occurred is reheated between 950-1450°C, would you expect the steel to be sensitive to intergranular attack ... 

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... 

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]. 

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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