DEVELOPMENT OF ELECTRODEPOSITED NICKEL-COBALT ALLOY COATINGS FOR CARBON DIOXIDE CORROSION PROTECTION

Abstract

The protection effectiveness of previous studied CO2 corrosion mitigation methods, including FeCO3 scale and corrosion inhibitors, cannot satisfy the long-term requirements of oil and gas industry. Ni-Co alloy coatings have shown high hardness, wear-resistance, corrosion resistance, good adhesion and thermal stability. The entire scope of this Ph.D. research is to develop the Ni-Co alloys coatings on carbon steel by economic and scalable electroplating process and investigate the protection effectiveness of these coatings in CO2 corrosive environment. This research is divided into three main sections: microstructural influences, electroplating manufacturing process and corrosion behaviour in varied CO2 corrosion conditions. To investigate the microstructural influences on CO2 corrosion resistance, two types of coatings having different micro-structures, but similar cobalt contents were fabricated. Experimental results show that the fibre-like Ni-Co micro-structured coating offers a corrosion resistance that is 2 orders of magnitude greater than the cone-like coating. This strong effect of microstructure is believed to arise from the larger surface/solution contact area, the higher density of grain boundaries and active defects, and the lower protective oxide content on surface of the cone-like Ni-Co coating. The variables during electroplating manufacturing process were researched. Within the studied scope, optimal CO2 corrosion resistance of Ni-Co coating is obtained with Watt-type bath with 4 % Co2+ concentration, deposition at 60 oC with 20 mA/cm2 current density. For thick coatings (200 µm and higher), new Ni anodes and electroplating solution are recommended after 8-hour plating period than continued use of the same anode and solution. These electroplating conditions provide consistent and defect free Ni-Co alloy coatings with surface cobalt content 31 % ~ 34 % that show low corrosion rates under CO2 conditions at a pH of 4 and operating at 60 oC. The corrosion behaviours of Ni-Co alloy coatings are further investigated in CO2 corrosion with enlarged parametric envelope, including different temperature/pH, mechanical damage in CO2 corrosion. By increasing temperature from 60 oC to 80 oC at pH 4.0, the CO2 corrosion is accelerated by an order of magnitude. High pH (6.8) promoted the rapid deposition of corrosion products and improved corrosion resistance by ~4 times within 24 hours. No localised corrosion occurred at the mechanical damage point. Besides CO2 internal corrosion, Ni-Co coatings also show great potential in external corrosion protection where O2 is dissolved.

Date of Award	9 Nov 2020
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Philip Withers (Co Supervisor) & Ping Xiao (Main Supervisor)