Scientific Investigation of the Corrosion Performance of Magnesium and Magnesium Oxide Primers on Al Alloy 2024-T351 in Field Exposures

• Published in: Corrosion (Houston, Tex.) Tex.), 2019-05, Vol.75 (5), p.440-456
• Main Authors: Santucci, R.J., Kannan, B., Abbott, W., Scully, J.R.
• Format: Article
• Language: English
• Subjects: Aerospace industry; Alloys; Aluminium; Aluminum; Aluminum base alloys; Cathodic protection; Corrosion; Corrosion mechanisms; Corrosion prevention; Corrosion products; Crystal defects; Defects; Depletion; Diagnostic systems; Electrochemistry; Electrolytes; Exposure; Magnesium; Magnesium oxide; Nondestructive testing; Pigments; Precipitates; Primers; Primers (coatings); Protection; Protective coatings; Substrates; Superconductors (materials); X-ray diffraction
• ISSN: 0010-9312; 1938-159X
• DOI: 10.5006/2879

The corrosion protection attributes of a magnesium oxide-rich primer (MgORP) on an AA2024-T351 substrate were evaluated after 2.5 y of coastal marine field exposure using several diagnostic electrochemical methods and nondestructive characterization techniques. The extent of protection afforded by Mg-converted and MgO pigments to exposed AA2024-T351 in the underlying substrate and within scribed coating defects was investigated. X-ray diffraction results showed that elemental Mg pigment was not entirely depleted after 4.25 y of exposure and that Mg(OH)2 was a prevalent corrosion product for magnesium-rich primer (MgRP), whereas there was limited indication of MgO pigment present in the MgORP after only 2.5 y of exposure and no evidence of crystalline corrosion product formation within the primer. Mg2+ deposited in scribes where bare AA2024-T351 was exposed in both the MgRP and the MgORP. The amount of Mg2+ that repartitions from the primer onto the surface of an artificial scribe increased throughout field exposure testing for MgRP. Thus, Mg(OH)2 precipitates at defects and scratches, providing a mechanism of chemical corrosion inhibition. The mechanisms associated with this process are assessed. The performance of MgORP coating was compared/contrasted with field-exposed (MgRP) to understand how a field-exposed and spent MgRP may continue to provide protection to the substrate after Mg depletion. Observations support the viability of MgRP for long-term exposure corrosion protection of susceptible aluminum-based assets based on this chemical protection mechanism.