Three ways to remove oxide layer on stainless steel fastener surface
Some stainless steel fasteners, after the series of processing, such as welding or annealing, will generate oxide layer on its surface, which will impact the Stainless Steel Fastenersquality. So we must remove the oxide layer. Typically, there are three removal ways, the impregnation method, the salt bath method, as well as salt membrane; they have their own characteristics, which should be chosen according to different situations.
1. Impregnation method
(1) Sulfuric acid - hydrochloric acid and sulfuric acid - nitric acid lotion
The acid lotion corrosion for the stainless steel fasteners substrate is relatively slow, there are much residue left on the surface of the fastener after dipping; dissolution for austenitic stainless steel fasteners is weaker, mainly used for Martensite stainless steel oxide impregnation.
(2) Nitric acid - hydrochloric acid pickling
The pickling has strong dissolvent ability for stainless steel fasteners oxide, fasteners surface is relatively clean after erosion, but the erosion on stainless steel fixings matrix is large, it is easy to cause corrosion if not controlled properly.
(3) Nitric acid - hydrofluoric acid pickling
The pickling has dissolution ability for stainless steel oxide surface, but dissolution for stainless steel fasteners matrix is weak, surface erosion is of good quality, but hydrofluoric acid is toxic, and it needs good ventilation and wastewater treatment conditions.
(4) Nitric acid - hydrofluoric acid - hydrochloric acid (or sulfuric acid) type pickling
The pickling has similar advantages with nitric acid - hydrofluoric acid lotion, fast response, wide scope of work.
2. Salt bath method
Salt treatment is also essentially pretreatment process, it uses different thermal expansion coefficients of the matrix metal and the oxide scale, make the oxidation skin crack by quenching, while enabling cheap oxide present in the outer layer of oxide turn into the acid-soluble higher oxides in strong role of the oxidizing agent, the loose film effect is better than alkaline sodium nitrite solution, but need to use special processing tank, the operating environment is also poor.
Salt bath composition and process: sodium hydroxide 87%; sodium nitrate 13%; temperature is 45-47 °C, operating time: 10min for Martensitic stainless steel fasteners and 20min for austenitic stainless steel fasteners; immediately quench when the fasteners come out of the slot. Treatment effect of the salt bath method is better, but there are some inadequacies. First, salt bath is solid at room temperature, preheating process is energy-intensive, time-consuming is too large; Second, fasteners bring more molten salt when out of the tank, which is a waste and pollutes the environment; Third, harsh operating environment. Therefore, now salt membrane is taken instead of salt-bath method to operate.
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