The processing or design cracks in the welding of stainless steel pipe fittings

Crevice corrosion is a form of localized corrosion crack in the surface sediments or shielded area surface passivation layer destruction. 



The processing or design cracks in the welding of stainless steel pipe fittings mouth at the other junction. In effective corrosion places, the cracks must be wide enough to allow corrosion electrolyte into the stagnation zone. Corrosion crack width reach several meters wide, but not absolute, and the critical crack size cause corrosion occurred. 



Especially very reactive with the negatively charged chloride ions, therefore, if there is crack external bulk solution, it is easy then positively charged metal ions attracted into the narrow cracks, the crack in the formed metal chlorides (typically chromium) reacts with water to form hydrochloric acid, reducing the pH value to the depassivation pH, then the cracks in the passivation film is destroyed, pitting will further. Event to passivation, corrosion will be fast and be able to self-sustaining, especially in the case of bulk solution has provided chlorine ions. 



Elements to avoid crevice corrosion in stainless steel seamless pipe The chromium, nickel, support the stability of the passivation layer can promote the stability of the stainless steel passivation layer. Although chromium metal ions is more supportive than the ion or nickel anode reaction in the crevice corrosion, improve the chromium content of the alloy is more resistant to crevice corrosion. For specific chromium content steel, austenitic stainless steel seems more tolerant than low nickel ferritic steel erosion. Molybdenum and nitrogen is very significant for improving the corrosion resistance. Generally, stainless steel, such as 6% Mo austenitic stainless steel, to provide the best corrosion resistance to crack. Ordinary stainless steel, has a good effect to reduce the resistance to crack corrosive. The cracks can not implemented completely infiltrated the place of welding beads generated in the welding manufacturing process. 



Completely root dissolved permeability is essential, as round and smooth weld bead, in the internal bead to the metal region of the substrate is not the bottom groove.

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