Study On Electrodeposition Of Cracks-free Fe-Cr Alloy Coating And Its High Temperature Oxidation Resistance

Low-alloying steels,because of their good comprehensive mechanical property and low prices,have been extensively applied to thermal power plants and petrochemical industries as metallic structural materials in service at high temperatures.However,when exposed to the environmental temperatures normally below 850?,the low Cr contents of low-alloying steels are far less than a critical amount(close to 20mass%)required for the formation a protective scale of Cr₂O₃.To increase the resistance of high temperature oxidation of low-alloying steels,adding a Cr₂O₃-forming coating is conventionally a preferred choice.Cr₂O₃-forming Fe-Cr alloy should be an ideal type of protective coating,because it can highly prevent the degradation of the mechanical property of the underlying low-alloying steel by interdiffusion.So far,there are few reports on the development of the oxidation-resistant Fe-Cr alloy coatings by a simple electrodeposition.The responsible reason is that this technique is difficult to deposit a cracking-free Fe-Cr alloy coating with a desired thickness and a high Cr content.To solve this problem,an environmentally friendly bath using Cr(?)instead of toxic Cr(?)was used to co-deposit Fe-Cr alloy coatings on a low-alloying T91 alloy.A method to break through the bottleneck of electroplating a cracking-free thick Fe-Cr alloy containing Cr content as high as 35mass%has been successfully acquired,through modifying the chemical compositions of the basic bath and optimizing the processing parameters,based on the dependence of Fe-Cr co-deposition on the variation of the deposition potential of Cr and Fe in the bath.The main results are presented below.(1)An urea-sodium citrate-sodium formate composite complexing agent bath with special additives,in which has a close precipitation potential of Fe and Cr,was explored the Fe-Cr alloy coating as desired.The plating solution is composed of Cr Cl₃0.6 mol·L^-1,Fe Cl₂ 0.05 mol·L^-1,urea 2 mol·L^-1,sodium citrate 0.3 mol·L^-1,sodium formate 0.2 mol·L^-1 NH₄Cl 1 mol·L^-1,H₃BO₃ 0.6 mol·L^-1,Na F 10 g·L^-1,Na Br 10 g·L^-1,ascorbic acid 5g·L^-1,sodium dodecyl sulfate 0.1g·L^-1.(2)The Cr content of the as-deposited Fe-Cr alloy is increased with increasing the current density for electrodeposition.When the Cr content reaches 35 mass%,the as-deposited Fe-Cr alloy experiences a transformation from a crystalline structure to an amorphous one.(3)Fe-Cr alloy coating with 30 mass%Cr significantly improves the oxidation resistance of a T91 steel in air at 850?.The 18?m-thick Fe-30Cr alloy deposition converts the T91 steel from a non-protective oxide former(main oxide phase:Fe₂O₃,Fe₃O₄;secondary phase:Fe O)to an oxidation-resistant Cr₂O₃ former,leading to an order of magnitude smaller in weight gain of T91 during100 h oxidation.