Abstract
Organic corrosion inhibitors have become competent alternatives to hazardous chrome conversion coatings due to their rapid adsorption over metal surfaces in corrosive environments. Literature suggests a wide range of organic corrosion inhibitors with high inhibition efficiency, barrier properties, and adsorption mechanisms. However, the long-term durability and protectiveness of an organic inhibitor film need to be understood with in-depth insights on its interaction with heterogenous alloy surfaces like AA6xxx, reduction of galvanic activities and time-resolved degradation due to ionic diffusion. The present article is focused on the time-resolved adsorption and degradation of 2-mercaptobenzimidazole (2-MBI)-induced inhibitor layer/film over AA6061 in 0.1 M NaCl solution. Electrochemical and surface analysis data indicate that the presence of 2-MBI drives the rapid formation of a 20–30 nm thick protective film comprised of constitutional elements of C, S, and N from 2-MBI upon the surface of AA6061 substrate. This film mitigated the corrosion cells associated with nano- and micro-sized Fe and Si-rich intermetallic particles (IMPs) in AA6061. XPS reveals two distinguished bonding states of S and N in the inhibitor film and chemical interactions between 2-MBI and the surface of AA6061. The protective film maintained 65% inhibiting efficiency after 1 day, which progressively degraded due to electrolyte ingress and eventually with a drop in inhibition efficiency down to 21% after 14 days. Inhibitor-induced film over AA6061 reduced the corrosion susceptibility of Fe, and Si-rich IMPs up to 1 day given the subsequent adsorption by S and N heteroatoms. However, this film became thick and defective after 1 day, which undermined its barrier properties against ingress of aggressive ions and facilitated water adsorption.Graphical abstract
Highlights
Organic film-forming corrosion inhibitors are popular with coating technologies for their rapid adsorption over metal surfaces
With the increment of exposure time, there was the shift of Open circuit potential (OCP) to more negative values i.e. 7 and 14 days, and the instability of OCP is evident from 10 min to the rest of the exposure time
This study elucidates the role of 2-MBI molecules in forming a protective layer over AA6061, identifying barrier properties of 2-MBI against electrolyte diffusion and the durability of inhibition over an exposure period of 14 days
Summary
Organic film-forming corrosion inhibitors are popular with coating technologies for their rapid adsorption over metal surfaces. A limited number of studies reported the inhibitory efficiency of corrosion inhibitors on high-strength Al alloys 6xxx (AA6xxx) series [6–11], and the adsorption and durability of organic corrosion inhibitors in corroding environments need to be well understood. It is critical to understand inhibitor interactions with an oxide layer and other available surface species, the breakdown mechanism of established inhibitor film and the role of ionic diffusion therein, and the changing coverage IMPs of inhibitors and the impact of such coverage on micro galvanic or nano galvanic couplings [12, 13]. Systematic studies on the long-term behavior of such inhibitor systems in terms of film stability, durability, and breakdown should address existing knowledge gaps and develop sound systems with inhibitor layers promoting sustained inhibition efficiency
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