Knifeline attack

Selective corrosion in the heat-affected zone of a weld occurs most commonly when unstabilised stainless steels are used in certain environments. The obvious answer is to use an extra-low-carbon grade of stainless steel, e.g. types 304L, 316L or a stabilised grade of steel, e.g. types 321 and 347. Knifeline attack at the edge of a weld is not commonly encountered and is seldom predictable, and it must be hoped that it is revealed during preliminary corrosion testing. 

For both heat-affected zone corrosion (intergranular attack) and knifeline attack the heat flux during welding and the time at temperature can critically affect the severity of the attack. Both these factors may vary from one welder to another, and when preparing pieces for corrosion testing not only should fabrication welding conditions be accurately reproduced, but the work of more than one welder should be evaluated. 

The corrosion of stainless steel welds has probably been studied more fully than any other form of joint corrosion and the field has been well reviewed by Pinnow and Moskowitz , whilst extensive interest is currently being shown by workers at The Welding Institute. Satisfactory corrosion resistance for a well-defined application is not impossible when the austenitic and other types of stainless steels are fusion or resistance welded in fact, tolerable properties are more regularly obtained than might be envisaged. The main problems that might be encountered are weld decay, knifeline attack and stress-corrosion cracking (Fig. 9.29). 

Fig. 9.29 Corrosion sites in stainless steel welds. The typical peak temperatures attained during welding (°C) are given at the foot of the diagram. Note that knifeline attack has the appearance of a sharply defined line adjacent to the fusion zone...

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). 

Jarman and Cihal have made a study of the related IGA problem of knifeline attack (KLA) also associated with the HAZs of welded stainless steels. They considered that there are two manifestations of KLA of similar appearance. One is classical IGA which they refer to as KLA, whilst... 

What is the major difference between knifeline attack and weld decay as far as the location of the attack with respect to the weld is concerned ... 

Knifeline attack (KLA) is similar to weld decay and they both result from intergranular corrosion and are associated with welding. The three major differences are ... 

The main problems that might be encountered in stainless steel are weld decay, knifeline attack... 

Represented by HasteUoy AUoys B and B-2, nickel-molybdenum aUoys have been primarUy used for their resistance to corrosion in non-oxidizing environments such as HCl. HasteUoy AUoy B has been used since about 1929 and has suffered from one significant limitation weld decay. The welded structure has shown high susceptibility to knifeline attack adjacent to the weld metal and to HAZ attack at some distance from the weld. The former has been attributed to the precipitation of molybdenum carbide (MojC) the latter, to the formation of MgC-type carhides. This necessitated postweld annealing, a serious shortcoming when large structures are involved. Many approaches to this problem... 

The addition of 1% V to an Alloy B-type composition was first patented in 1959. The resultant commercial alloys, Corronel 220 and Hastelloy AUoy B-282 were found to be superior to Alloy B in resisting knifeline attack, but were not immune to it In fact, it was demonstrated that the addition of 2% V decreased the corrosion resistance of the base metal in HCl solntions. During this time, improvements in melting techniques led to the development of a low-carbon, low-iron version of Alloy B called Alloy B-2. This alloy did not exhibit any propensity to knifeline attack. 

Alloy B-2 has improved resistance to knifeline and heat-affected zone attack. It also resists formation of grain-boimdary precipitates in weld-heat-affected zone. 

Segregation of molybdenum in weld metal can be detrimental to corrosion resistance in some environments. In the case of boiling HCl solutions, the weld metal does not corrode preferentially. However, in H2SO4 -1- HCl and H2SO4 -1- H3PO4 acid mixtures, preferential corrosion of as-welded Alloy B-2 has been observed. No knifeline or HAZ attack was noted in these tests. During solidification, the initial solid is poorer in molybdenum and therefore can corrode preferentially. In such cases, postweld annealing at 1120 °C (2050 °F) will be beneficial. 

---