Mechanistic Study Of The Corrosion Resistance Of Zinc-based Alloy Coatings

With the development and progress of modern science and technology,the requirements for the corrosion resistance of steel are becoming higher and higher.Pure zinc plating has long been unable to meet the needs of the steel industry."Thin plating,high corrosion resistance"is gradually becoming the standard for plating.Zinc-iron alloys have great academic research value and great practical significance because of their low cost,high corrosion resistance,simple process,low hydrogen embrittlement and machinability and other excellent properties.In this paper,we address a series of problems that arise in the production process of door and window hardware enterprises,such as insufficient protection time of galvanised layers for steel parts in atmosp Heric environment,poor decorative appearance of the plating layer,high cost of plating solution（existing plating process）and environmental pollution hazards of cyanide plating solution,etc.We use the existing plating process equipment of enterprises to improve the research and study a high corrosion resistance,low hydrogen embrittlement,green and environmentally friendly Zn-Fe alloy coating to replace galvanic,cadmium and cyanide plating systems and to meet the company’s requirements for corrosion resistance and decorative properties.In this study,Sur Tec and DE-98 were used as the main additives to compare and select the optimum additives for the preparation of Zn-Fe alloy coatings.Using Zn O and ferrous chloride as the main salts,the optimum plating solution parameters were first determined by DC plating,and then the Zn-Fe alloy plating was electrodeposited using pulse plating power supply equipment in comparison to DC power supply equipment.The effect of pulse plating on the electrochemical behaviour and crystallisation process of the Zn-Fe alloy plating was investigated in comparison to DC plating.The effects of plating process parameters and plating solution additives on the morp Hology,decorative properties,crystalline structure and corrosion resistance of the plated layers were investigated.An electrochemical workstation was used to analyse the corrosion resistance of zinc-iron alloy plating in a corrosive system of3.5 wt%Na Cl solution,the crystalline structure of the plating was analysed by XRD,the surface microscopic morp Hology of the plating was observed by scanning electron microscopy and the elemental content of the plating was analysed by EDS fluorescence spectrometry.The experimental results show that a Zn-Fe alloy coating with a matt colour appearance and excellent corrosion resistance can be obtained under the conditions of the optimised process,where the elemental content of iron is 0.5%,and the coating has a flat and detailed surface microstructure.In the neutral salt spray corrosion resistance test,the plating was red-embroidered for 300 h.The following conclusions were finally reached:（1）After paving the way for the DC galvanic iron alloy experiments,the optimum process parameters for monopulse electro-galvanic iron alloy were determined:Zn O at 10 g/L,Na OH at 160 g/L,Na KC₄H₄O₆-4H₂O at 60 g/L,Fe Cl₂at 6 g/L,vanillin at 0.04 g/L,EDTA at0.8 g/L,Sur Tec softener at 2 ml/L,and a current density of 1.5 A/cm².（2）The plated layer obtained under single pulse power supply has better leveling properties than the Zn-Fe alloy plated layer prepared by DC plating,with fine and neatly arranged grains,less surface defects and significantly improved corrosion resistance.A decorative appearance with a matt surface was obtained and the iron content in the plated layer was 0.5%.The electrochemical impedance curve was used to analyse the corrosion resistance of the plated layer.The impedance radius of the single pulse plated layer was much larger than that of the DC plated layer,and its resistance（Rct）value was 10 times higher than that of the DC plated layer.（3）With the addition of reverse current,the surface morp Hology of the plated layer can be effectively improved and the porosity of the plated layer reduced.The amount of additive Sur Tec used was reduced from 2 ml/L to 1.4 ml/L,which reduced the production cost and made the surface of the plated layer smoother and denser while ensuring the performance of the plated layer.The parameter settings for double pulse plating were determined:forward energising time:100ms,reverse energising time:50ms,pulse forward current density:1.6A/cm²and pulse reverse current density:0.2A/cm².