Corrosion Protection of 2024-T351 by a MG-Rich Primer

An organic coating system containing an Mg-pigmented organic primer (MgRP) engineered for corrosion protection of precipitation age hardened Al alloy 2024-T351 used in aerospace applications has been developed as a candidate to replace chromate-containing surface pretreatments and primers. This work addresses the primary mechanisms of protection, analysis of function and performance of the coating system in standard lab accelerated tests compared to field environments. The effects of atmospheric CO2 on degradation of the coating, the mechanisms and valence state of Mg anodic dissolution pertinent to understanding pigment charge capacity and its depletion, as well as the detailed current and potential distribution across a defect or scribe during atmospheric exposure were also examined. Some of the challenges include defining the primary protection mechanisms afforded by the MgRP, identifying remaining life assessment methods and residual protection functions, and understanding the cathodic protection current and potential distributions across a scratch or defect during atmospheric exposure.

This work has further illuminated and verified test methods to assess MgRP degradation that could be used in the laboratory and in the field. These test methods were used to track Mg pigment depletion rate, galvanic protection potential and coating barrier properties throughout exposure in field and laboratory accelerated life environments. The residual barrier properties, after depletion of the Mg primer, were assessed using electrochemical impedance spectroscopy. Open circuit measurements were utilized to assess galvanic couple potentials between the MgRP and 2024-T351 and potentiostatic holds were utilized to asses Mg anodic dissolution charge. Calibrated X-ray diffraction was used to assess the total metallic Mg remaining in the MgRP. Preliminary acceleration factors with respect to pigment depletion and residual barrier properties were developed. Post-mortem characterization with SEM/EDS was conducted to elucidate coating and scribe morphology, corrosion products present, corrosion of the AA2024-T351 substrate, as well as in an attempt to determine the “galvanic throwing power” of the MgRP coating system based on cathodic protection of a scratch exposing bare AA2024-T351. The topcoat was observed to severely mediate the conversion of Mg pigment in the primer to metal oxides and/or carbonates. When a topcoat was absent, high self-corrosion of unprotected Mg pigment in the MgRP occurred. The Mg pigment was depleted monotonically in all environments but the differences in rate of Mg depletion from the coating upon environmental exposure were rationalized to stem from differences in time-of-wetness and in rates of polymer degradation, specifically resistivity, due to certain environmental severity factors such as precipitation, pH, and UV exposure. Mg pigment was observed to deplete the fastest in field and lab environments with lowest pH levels and the barrier properties of the epoxy polymer were shown to severely degrade at sites which included UV radiation. A key result from these studies is that the newer generation MgRP coating formulations display consistent degradation characteristics in lab and field environments, albeit at different rates.

In addition to post-mortem sample evaluation, the galvanic throwing power of the MgRP was studied via finite element analysis of current and potential distributions in conjunction with diagnostic multi-electrode arrays (MEAs), which enable the spatial distribution of cathodic protection to be elucidated. The galvanic protection capabilities of the coating in various full immersion, thin layer, and droplet electrolyte geometries relevant to field service explained long misunderstood field behavior. Current and potential distributions extended across simulated defects when the electrolyte layer was thick, when it was continuous and more conductive (higher concentration) as well as in the absence of a resistive polymer topcoat. Current and potential distributions did not extend across simulated defects when the electrolyte became discontinuous or the ionic path became tortuous due to drying or the addition of a very high resistance polymer coating. Additionally, galvanic protection was shown to intensify during drying and re-wetting over short distances rationalized to be caused by changing solution conductivity, the governing E-i behavior, and electrode area effects. The drying characteristics of individual salts was also shown to have an effect on the evolution of throwing power as MgCl2 (due to its low deliquescence point of ~35% at STP) was shown to be less susceptible to drying at low RH, thus extending the time of which the galvanic couple was active compared to pure NaCl or ASTM Artificial Sea Water.

These results provide a scientific basis for the further development of this promising coating technology.