Effects of Coatings on Intergranular Corrosion and Stress-Corrosion Cracking of Al-Mg Alloys at Macro-Defects
Abstract
The opportunity exists to mitigate intergranular corrosion (IGC) and IGSCC of Al-Mg alloys through the use of coatings but the scientific foundations and coating attributes that can optimize protection have not been elucidated. The objective is to establish the scientific foundations which define coating attributes that maximize IGC and IGSCC mitigation in sensitized Al-Mg alloys across macro-defects. IGC and IGSCC incubation time and rates will be established in laboratory controlled conditions as a function of sacrificial coatings that can control the potential across macro-defects. Local potential and pH will be mapped across macro-defects as a function of coating type, defect geometry, alloy orientation, stress intensity level, degree of sensitization, and environment. Scientific factors controlling potential driven corrosion mitigation will be identified that can guide coating development to mitigate IGC and IGSCC of Al-Mg alloys under broad conditions. The initial program will test coated 5XXX alloys in full immersion in concentrated and dilute NaCl with a variety of coatings and machined controlled macro coating defect widths. Both IGC and IGSCC testing will be undertaken for a range of coatings, environments, and macro coating defect dimensions. The coatings anticipated include an Al-rich primer under development at Navair, a Mg rich primer and a zinc rich primer as well as possibly a metal spray coating. A severe highly susceptible metallurgical condition will be tested first: high DOS and L orientation from a T-S surface for IGC and loading in the S with cracking in the L-direction for IGSCC (termed S-L). IGC testing will be repeated in ASTM B-117 or other thin electrolyte environment. Unsuccessful coatings with respect to mitigation may be retested at various (lower) DOS levels until the critical DOS level and scratch width can be determined. Further details are below. First, Measurement of IGC kinetics for each material at one DoS in one severe propagation directions (L with reference to the plate) will be undertaken in the presence of several coatings with various width macro-defects geometries exposing a T-S surface of various widths. Both the dependence of incubation time and rate on coating type, location in scratch, and local potential across the macro-defect will be determined. Second, potential and current distributions will be mapped and modeled across a scratch width for the macro-defect geometry. From the IGSCC perspective, interaction of a single crack tip with a spatially variable potential (given by the far field potential of the coating) creates interpretation challenges as crack tip electrochemistry may also be time dependent and stochastically variable. However, new tools (e.g., the Scanning Kelvin Probe) have become available that allow measurements of electrochemical conditions (i.e., the electrochemical potential) in thin electrolyte layers. MicropH electrodes and ion sensitive electrodes can be used to characterize the chemical conditions at least at the mouth of the crack. Electrode arrays and flush mounted reference electrodes may be used to experimentally assess throwing power from the edge of a coating. Combining the results from these tools with measurements of IGC and IGSCC kinetics will enable elucidation of the controlling mechanisms in mitigation by coatings. The results will be used to assess to what extent the IGC and IGSCC kinetics can be mitigated.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Aug 12, 2016
- Source ID
- N000141512491
Entities
People
- John R. Scully
Organizations
- Office of Naval Research
- United States Navy
- University of Virginia