Causes Of Corrosion Cracking Of Stainless Steel Pipe Welded Joints

Stainless steel pipes are mainly divided into stainless steel seamless pipes and stainless steel welded pipes. The welded joints of welded pipes are often damaged by corrosion and leakage, and most of the corrosion occurs in the weld or heat affected zone. The most common types of corrosion are intergranular corrosion and stress corrosion. And pitting.

1. Intergranular corrosion of welded joints, stainless steel tubes have good resistance to intergranular corrosion in the state of supply (1050-1100 °C). After the steel pipe is welded and used in corrosive medium, it can be found that there is corrosion along the grain boundary near the weld, which is mainly related to the precipitation of chromium carbide, because the carbon content of the welding material is high or increased during welding. The carbon phenomenon, which is also affected by the sensitization temperature sensitive to carbide precipitation after welding, has a tendency to intergranular corrosion. Therefore, in addition to the selection of high-quality welding consumables for 304 stainless steel pipe welding, the weld design should avoid cross-shaped, T-shaped welds, welds after contact with corroded media, etc. from the perspective of preventing intergranular corrosion.

In order to prevent the intergranular corrosion of the spiral submerged arc pipe, the carbon content is generally reduced as much as possible, and the elemental welding material with stronger affinity of niobium, titanium and carbon is added to prevent the precipitation of chromium carbide, and the performance of the weld to resist intergranular corrosion is remarkably improved.

A small amount of ferrite in the austenitic stainless steel weld is not only beneficial for preventing and eliminating hot cracks, but also beneficial for improving the resistance to intergranular corrosion. Ferrite exists between austenite grains and eliminates The orientation of the columnar crystals and dendrites of the austenite, and the continuous network carbides of the austenite grain boundaries are separated to prevent intergranular corrosion. Therefore, in addition to chromium, the addition of ferrite forming elements Mo/Nb, Ti, etc. in the weld is advantageous for improving the corrosion resistance of the weld.

2. Weld joints are stress corrosion cracked. The stress corrosion of stainless steel pipes is generated under the action of static tensile stress and in specific media such as chloride, hydroxide, sulfide, etc. The welded joints have a large residual at the joints without heat treatment after welding. Stress, sometimes stress corrosion cracking occurs after the yield limit is reached. The commonly used 304/316L stainless steel tube is sensitive in stress corrosion environment. The 18-8 type welding consumables with Ti and Nb are selected and then post-weld stress relief treatment.

For example, stainless steel welded pipe and pipeline after welding need to be stabilized at 940 °C. In the austenitic stainless steel tube, as the nickel content increases, the transgranular stress corrosion in the stress corrosion resistance of many media can be improved. Molybdenum has no direct effect on the prevention of stress corrosion in austenitic stainless steel, but it can improve the pitting resistance, which is often the origin of stress corrosion. Therefore, the molybdenum-containing steel improves the pitting corrosion caused by C1 and improves the chloride stress corrosion resistance. Therefore, high nickel and high molybdenum stainless steel are better resistant to stress corrosion than ordinary austenitic stainless steel.

3. Stainless steel pipe welded joint pitting. In the medium of C1.Br. ions, pitting corrosion often occurs on the inner and outer surfaces of steel pipes. The passivation film is mainly weak in metal inclusions, precipitated phase, and grain outcrop. The weld bead is large, and the metallurgical reaction of the flux and the coating during loose welding is more sensitive than the base material. At the same time, the surface passivation film of the steel pipe is destroyed due to the flying smash and the surface oxidation color during welding. Pitting corrosion is caused in the C1 ion solution. To prevent pitting corrosion of the welding head, firstly improve the purity of the weld metal, reduce the weld impurities and carefully clean the surface of the weld and its base material after welding and then passivate.

A worker from a steel pipe company, which is located in China.