Coatings and Surface Engineering for Environmental Protection III: Protection from Environmental Degradation, Session II
Sponsored by: TMS Structural Materials Division, TMS Materials Processing and Manufacturing Division, TMS: Surface Engineering Committee, TMS: Corrosion and Environmental Effects Committee
Program Organizers: Arif Mubarok, PPG; Tushar Borkar, Cleveland State University; Rajeev Gupta, North Carolina State University; Mary Lyn Lim, PPG Industries; Raul Rebak, GE Global Research; Brian Okerberg, PPG Industries
Monday 2:00 PM
March 15, 2021
Room: RM 18
Location: TMS2021 Virtual
2:00 PM
Designing Lubricant-impregnated Coatings to Reduce Corrosion and Hydrogen Embrittlement: Sami Khan1; Kripa Varanasi1; 1Massachusetts Institute of Technology
Lubricant-impregnated surfaces (LIS) consisting of thin films of lubricant held stably in micro-textures have been systematically designed and studied for reducing corrosion. We fabricate microposts on silicon using photolithography, varying inter-post spacing systematically from 5μm to 50 μm, and conformally sputter-coat a thin layer of iron to study the corrosion phenomena electrochemically in an aqueous 3.5 wt% sodium chloride solution, and we show that the corrosion rate on LIS is drastically reduced by three orders of magnitude. Using electrochemical impedance spectroscopy, we develop model circuits for various LIS configurations and show that the measured resistances and capacitances agree with a theoretical model, and discuss in detail where deviations occur. We also study the role of lubricant layers towards reducing hydrogen embrittlement, and using a Devanathan-Stachurski permeation cell we show that under accelerated conditions, the effective diffusion coefficient of hydrogen on steel is reduced by an order of magnitude.
2:20 PM
Effects of Heat Treatments, Current Density, and Electroless Ni Layer Thickness on Corrosion Performance of Trivalent Chromium Passivations on ZnNi Coatings: Kevin Foster1; William Fahrenholtz2; Matthew O'Keefe2; James Claypool2; 1Missouri University of Science & Technology; 2Missouri University of Science & Technology
A trivalent chromium based passivation (TCP) and a cobalt-free version of the same passivation (Co-Free) were studied on ZnNi coated, electroless nickel (EN) plated Al 6061-T6 substrates. Each panel had a 5-20 µm EN layer, 150 or 300 A/cm2 current density for plating the ZnNi, and either an 80°C or 191°C heat treatment after passivation. Corrosion performance was evaluated using image analysis of optical micrographs, and electrochemical testing, and ASTM B117 neutral salt spray exposure. Image analysis showed a performance difference between heat treatments, between EN layers with thicknesses >10 µm versus ≤10 µm, and between TCP and Co-Free passivation. Electrochemical testing revealed differences between 80°C and 191°C heat treatments. The results from this study indicate that the corrosion protection mechanisms are more dependent on passivation chemistry than the deposition and post-processing conditions.
2:40 PM
Electronic Structure Mechanisms to Explain the Onset of Cl-induced Localised Corrosion in Al2O3: Aditya Sundar1; Ganlin Chen1; Liang Qi1; 1University of Michigan
We investigate the fundamental electronic mechanisms driving Cl-induced localized pitting corrosion on α-Al2O3 surfaces. In the context of the point defect model, we model the adsorption thermodynamics of molecular HCl and Cl species substitutionally adsorbed on saturated α-Al2O3 surfaces, which have different local chemistries compared to the bulk crystal structure. Structural relaxations and room-temperature ab initio molecular dynamics simulations show that the adsorption energy of Cl is dependent on the local change in the anion electronic structures. By combined Bader charge, bond valence and orbital hybridization analyses, substitutional adsorptions of Cl are shown to be favorable at under-coordinated OH sites. The creation of Al-Cl bonds at these sites shifts the occupied electronic states to lower energies. Finally, we propose mechanisms to inhibit Cl adsorption by introducing alloying elements and stabilizing the electronic structure of the OH molecule at vulnerable sites.
3:00 PM
Galvanic Corrosion Mitigation by Material and Coating Selection for AZ31B bolt-joined with CFRP: Yong Chae Lim1; Jiheon Jun1; Charles Warren1; Zhili Feng1; 1Oak Ridge National Laboratory
Joining of Mg alloys and carbon fiber reinforced polymer (CFRP) is important to enable future lightweight automotive structures. Conventional bolting is a good joining technique to integrate Mg and CFRP into structural applications, but galvanic corrosion between Mg and steel bolt is a concern for the joint reliability. To mitigate galvanic corrosion, this work investigates (1) polytetrafluoroethylene (PTFE) coating on the steel bolts to serve as an insulating/barrier layer and (2) Al alloy bolts as a less cathodic material. All bolted Mg and CFRP joints were tested by NaCl solution immersion and ASTM B117 salt spray exposure, and the galvanic corrosion damage/susceptibility on Mg were assessed by optical profilometry and corrosion potential measurement. The corrosion test results indicate that PTFE coated steel bolt is highly effective to reduce galvanic corrosion. The effect of Al alloy bolts is currently under investigation.
3:20 PM
Using Mechanical and Ion Polishing to Identify Structural and Chemical Defects for the Pitting Corrosion of a Compositionally Complex Steel: Mark Wischhusen1; Carol Glover1; John Scully1; Sean Agnew1; 1University of Virginia
Aqueous corrosion experiments have been performed on a compositionally complex steel with hierarchical microstructure involving coherent Heusler phase (L21) and B2 structured precipitates within a ferritic matrix. The pitting potential of the alloys (denoted FBB8 + Ti) is insensitive to Ti concentration, which ranged from 2 to 6 wt%, but is sensitive to the type of surface preparation. By varying surface preparation between a 0.05 micron colloidal silica and an Ar ion-polished finish, a change in the preferred site of corrosive attack was observed. Matrix attack, presumably at crystal defects, is the norm for mechanically polished samples, while pitting associated with Zr-rich particles occurs in the ion polished samples. A correlation between the passive current density and Ti concentration in the ion-polished samples, unseen with other surface preparations, was also uncovered. An explanation based upon solidification phenomena is proposed for the observed correlation.