A mechanistic investigation of corrosion-driven organic coating failure on magnesium and its alloys

Abstract

The failure mechanism of model organic coatings from Mg alloy surfaces is characterised by a combination in-situ scanning Kelvin probe analysis and time lapse photography. Initiation of underfilm corrosion by application of group I chloride salts to a penetrative coating defect produces an apparent cathodic-driven coating delamination, where the disbondment distance increases linearly with time at high relative humidity, although filiform corrosion (FFC) is also observed in the vicinity of the defect. The disbondment process occurs both in the presence and absence of oxygen, indicating that hydrogen evolution comprises the predominant underfilm cathodic reaction. Post-corrosion elemental analysis of the delaminated region shows an abundance of group I cation, but no chloride. When magnesium chloride or HCl are used to initiate corrosion, then only FFC is produced. The mechanism is discussed in terms of net anodic dissolution at the defect coupled with underfilm cathodic hydrogen evolution, producing organic coating disbondment under conditions where cations are able to transport ionic current within a zone of increased pH.

Hydrogen embrittlement through the formation of low-energy dislocation nanostructures in nanoprecipitation-strengthened steels

“…These are, however, difficult to realize; for instance, the offshore environment of wind turbines remains fixed and lubricant decomposition would require substantial alteration of its chemistry (14,15). Coatings applied to rolling elements in wind turbines may alter their rolling contact performance, and their wear has to be controlled (16,17). It follows that a common solution is altering the component bulk microstructure, which can be achieved through the addition of certain elements, which, in solid solution in the matrix, may alter HE susceptibility, e.g., Al in TWIP steels (18) or Cr in precipitation hardening steels (19).…”

The Influence of Chloride Ion Concentration on the Localized Corrosion of E717 Magnesium Alloy

“…Chloride ion penetration into the inner MgO-base layer was detected at high concentrations of ≥ 1% w/v by using a combination of SEM and STEM, but not after immersion in more dilute concentration of 0.01 % w/v 20 , although the authors report a different growth mechanism within the dense MgO base layer. In addition, a recently published investigation of organic coated E717 surfaces demonstrated that filiform-like features propagated underneath the polymer layer when corrosion was initiated at a penetrative coating defect by the application of MgCl2 or HCl (aq) 21 . However, the work also identified an apparent cathodically-driven coating disbondment mechanism which occurred when underfilm corrosion was initiated using group I chloride salts 21 .…”

“…In addition, a recently published investigation of organic coated E717 surfaces demonstrated that filiform-like features propagated underneath the polymer layer when corrosion was initiated at a penetrative coating defect by the application of MgCl2 or HCl (aq) 21 . However, the work also identified an apparent cathodically-driven coating disbondment mechanism which occurred when underfilm corrosion was initiated using group I chloride salts 21 .…”

High rate oxygen reduction reaction during corrosion of ultra-high-purity magnesium

“…), where oxygen reduction reaction (ORR) is the main cathodic process. 1,[4][5][6][7][8] A convincing explanation for the predominance of HER is the highly negative corrosion potential exhibited by Mg. 9 HER : 2H 2 O þ 2e À ! H 2 " þ2OH À (1)…”