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The fasteners to be blackened are placed in the solution tank (145±5 °C) of sodium hydroxide (NaOH) and sodium nitrite (NaNO2) oxidant to heat and oxidize, so that the surface of the metal fastener generates a magnetic ferric tetroxide (Fe3O4) film, the thickness is generally 0.6-0.8μm, black or blue-black. The HG/20613-2009 and HG/T20634-2009 standards for fasteners for pressure vessels require bluing treatment.

Phosphating is a process of chemical and electrochemical reaction to form a phosphate chemical conversion film, and the phosphate conversion film formed is called a phosphate conversion film. The main purpose of phosphating is to provide protection for the base metal and prevent the metal from being corroded to a certain extent, to improve the adhesion and anti-corrosion ability of the paint film layer before painting, and to reduce friction and lubrication in the metal cold working process. Large-diameter double-ended studs for pressure vessels are required to be phosphated.

Hot-dip galvanizing is to immerse the derusted steel components into the molten zinc melted at a high temperature of about 600 °C, so that the surface of the steel components is attached to the zinc layer, and the thickness of the zinc layer shall not be less than 65μm for thin plates below 5mm, and not less than 86μm for thick plates of 5mm and above. So as to play the purpose of anti-corrosion.

Dacromet is the transliteration and abbreviation of DACROMET, referred to as Dacromet, Dacromet, Dicron. Named zinc-chromium coating in China, it is a new type of anticorrosive coating with zinc powder, aluminum powder, chromic acid and deionized water as the main components. There is no hydrogen embrittlement problem and the torque-preload consistency is good. Regardless of the environmental concerns of hexavalent chromium, it is actually the most suitable for high-strength fasteners with high corrosion protection requirements.