EFFECT OF CHLORIDES ON THE ELECTROCHEMICALBEHAVIOUR OF THERMALLY SPRAYED ALUMINIUMPROTECTIVE COATINGS

Abstract

Sacrificial metallic coatings have been used in the past, for protecting steel inindustrial and urban developments. It has been suggested that thermally sprayed aluminiumcoatings, immersed in marine environments, are also capable of protecting steel byproviding galvanic protection. Researchers have also speculated that the growth of inertoxide products, on top of the coating as well as inside pores and cracks, could enhance evenfurther its protective "shielding" properties. In addition, the self-healing abilities of thecoatings, in case of mechanically induce damage, are still a matter of debate. This researchproject takes a critical look at these assumptions, focusing on the electrochemical responseof aluminium metal sprayed (AMS) coatings immersed in 0.6 M NaCl and 0.6 M Na2SO4solutions, in order to investigate how the presence of chlorides can alter the corrosionbehaviour of these coatings.For such, a thin aluminium protective coating was deposited over two differentpanels (aluminium 1050 and low carbon steel), by the steel making company Fairfield-Mabey, using electric arc thermal spraying (TS). Subsequently, the coated panels weresectioned into smaller specimens, and with the aid of a SEM/EDS, and XRD, observationsand analysis were conducted in their surfaces and cross sections, in order to determine themorphology, quality of deposition, and chemical composition of the coatings. Internalporosity/oxide growth after immersion, was studied using 3-D x-ray tomography scanning.Furthermore, the electrochemical behaviour of the coatings (intact and scribed) was alsoinvestigated, initially by analyzing the evolution of the open circuit potential in time, whenimmersed in chloride rich and chloride free environments. Additionally, the anodicpolarization behaviour of the substrate and coatings were simultaneously analyzed, byconnecting a zero resistance ammeter (ZRA) with the W.E. terminal of a potentiostat. Toconclude, EIS and Rp vs. time plots were made in order to corroborate the data obtainedfrom other tests.Results show that although the coating can generate superficial and internalcorrosion products, these oxides are not isolating in nature nor will enhance its protectiveproperties. On the other hand, the corrosion potential behaviour of the coatings revealed thatAMS coatings have a more active than the substrate; regardless of the environment in whichthey were immersed or their substrate. Nevertheless, it was also observed that AMScoatings deposited onto steel will corrode faster than their substrates, and for that mattercapable of offering corrosion protection, exclusively if chlorides are present in the solution.

Date of Award	1 Aug 2013
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Stuart Lyon (Supervisor)