分别采用浸泡和电化学实验方法对Ni-W-Cu-P镀层在常温和高温20%H2SO4溶液中的耐蚀性进行了研究, 用SEM, EDS及XRD对镀层的沉积机制、成分结构进行了分析. 结果表明: 球形Ni-W-Cu-P核心合并生长形成条状组织; 共沉积W和Cu可显著提高Ni-W-Cu-P非晶的热稳定性; 400 ℃热处理非晶的耐蚀性优于镀态非晶和500 ℃热处理纳米晶的; 延长腐蚀时间, 非晶和纳米晶镀层的腐蚀速率和腐蚀电流密度增大, 阻抗则下降; Ni-W-Cu-P非晶和纳米晶镀层的腐蚀机制分别是选择性腐蚀和点腐蚀.
The application of steel in acidic media faces a big challenge due to the corrosion problem. Quaternary Ni-W-Cu-P alloy act as a potential coating material applied to acidic media because of its superior corrosion resistance. However, mechanism of deposition and corrosion of Ni-W-Cu-P coating plated on the surface of steel component is rare in the previous studies. In this work, the Ni-W-Cu-P coatings were deposited onto carbon steel 65Mn substrates via electroless plating. The anti-corrosion properties of the coatings in room and warm acidic solution (20%H2SO4) were evaluated by dipping and electrochemical test, respectively. Their deposition mechanism, composition and structure were investigated using SEM, EDS and XRD, respectively. The results show that the Ni-W-Cu-P coating is composed of spherical and block particles in the early stage of electroless plating, which are gradually transformed into spherical and strip cellular structure with the increasing electroless plating time. With prolonging electroless plating time, the Ni and W contents in the Ni-W-Cu-P coatings increase logarithmically and lineally, respectively. However, the Cu content decreases logarithmically, the P content reaches the maximum value after electroless plating for 60 min and then gradually decreases. The Ni-W-Cu-P coating is amorphous when it is annealed at low temperature, upon increasing the annealing temperature to over 400 ℃, it gradually transforms from amorphous to crystalline. The thermal stability of Ni-W-Cu-P coating can be significantly improved by co-depositing tungsten and copper element. Corrosion resistance of the amorphous coating annealed at 400 ℃ is better than that of amorphous coating as-plated and nanometer crystalline coating annealed at 500 ℃ in both room and warm acid solution. As-plated coatings and those annealed at 400 ℃ are found to corrode selectively, while pitting is observed to be the main corrosion mechanism of coatings annealed at 500 ℃. With increasing the corrosion time, the corrosion rates and corrosion current densities of the Ni-W-Cu-P coatings increase, however, their impedance values decrease.
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