NCER Assistance Agreement Annual Project Summary

 

Date of Report: January 13, 2008

EPA Agreement Number: X-83254101-1

Center: Center for Environmental and Energy Research (CEER)

Project Title: Magnesium Rich Coatings for Corrosion Control of Reactive Metal Alloys

Investigator(s): Rebecca DeRosa

Institution(s) of PI(s): Alfred University, Inamori School of Engineering

Research Category: Congressionally Mandated Center

Project Period: January 1, 2007 – August 2008

 

Objective of Research: We are testing magnesium rich coating (MRC) systems as possible chromate replacement coatings for corrosion control of reactive metal alloys. The hypothesis is that the Mg particles embedded in an inorganic or organic matrix coating will sacrificially corrode and provide extended corrosion protection to the underlying metal substrate. Our goal is to understand the role of the Mg particles as sacrificial anodes in MRC’s on aluminum 2024 and 7075 in a corrosive environment. We are currently using electrochemical techniques including electrochemical impedance spectroscopy (EIS), d.c. polarization, and open circuit potential (OCP) monitoring to determine the reliability of the coatings as corrosion inhibitors. In addition, we are also using scanning electron microscopy (SEM), electron probe x-ray microanalysis (EPMA), and x-ray photoelectron spectroscopy (XPS) to determine the state of the magnesium particles.

 

Progress Summary/Accomplishments: We have received six different primer systems to date from North Dakota State University (NDSU): two separate organic coatings (CPM 1040 & Akzo) on Al-2024 and Al-7075 panels, and one inorganic coating (SMT 30) on Al-2024 and Al-7075 panels.

Panels from each system were cut into 1.5”x 2.5” sections. A glass cylinder was clamped to each section exposing a surface area of 7.06 cm². The glass cylinder was then filled with dilute Harrison’s solution (DHS), consisting of 0.35wt% (NH₄)₂SO₄ and 0.05wt% NaCl in distilled water (ph=5.4). DHS was chosen because it mimics acid rain which has shown to cause pitting corrosion when exposed to bare aluminum. A saturated calomel electrode was used as the reference electrode and a platinum wire was used as the counter electrode. A Solartron 1260 potentiostat/galvanostat with a 1287 frequency response analyzer and dedicated EIS Zplot and Zview software were used to collect and analyze electrochemical impedance and OCP data. The impedance spectra were collected at a rate of 10 points per decade using the frequency range of 10 kHz to 0.1 Hz. The applied AC sinusoidal amplitude was 10 mV applied at the open circuit potential for all systems. Electrochemical impedance analysis consisted of measuring the initial OCP for the system followed by EIS analysis versus exposure time over a period of 25+ days. Samples were removed from exposure based on changes in impedance modulus intensity.

Coating thickness was determined using an Elcometer 345 under non-ferrous conditions. At least 10 points were measured for each sample area (exposed versus unexposed regions).

The elemental composition of the coating surfaces was determined using XPS. Measurements were carried out at 13 μPa with a PHI Quantera SXM equipped with an Al K_α (1486.6 eV) source. The XPS was calibrated to + 0.1 eV using Ag (3d) line at 368.3 eV. The system linearity was calibrated within + 0.1 eV of the difference between Cu (2p) at 932.7 eV and Au (4f_7/2) at 84.0 eV. Charging of the surface was prevented using an electron neutralizer at 1 V and 20 μA. A take-off angle of 45° was used in all scans. High resolution scans for C(1s), O(1s), Mg(2p), Al(2p), Na(2p), Cl(2p), and Cu(2p) were collected using a 200 μm beam size at 41 W and 15 kV. An additional high resolution scan of Si(2p) was also performed on the inorganic systems. Sputter depth profiling was accomplished through the following schedule using Ar⁺: 1 minute at 3 kV 3x3 mm for one cycle and 2 minutes at 5 kV 3x3 mm for 5 cycles. Compositional identification and atomic concentrations were determined with MultiPak 8 (Version 8.0, Ulvac-Phi, Inc.).

For SEM and EPMA imaging, a cross section of the panel was mounted in a jet set two part epoxy. After cure the sample was leveled using 400 grit followed by 600 grit SiC sandpaper. The sample was inspected optically between each step to check for coating pullout and even grinding. Hand fine polishing was carried out using 9 μm, 3 μm, and 1 μm diamond paste followed by 0.05 μm Al₂O₃ until mirror finish. Images were taken using an FEI Co. Mod. Quanta 200F environmental scanning electron microscope (ESEM) equipped with field emission gun. The electron source is a tungsten filament. The chamber was held in high vacuum mode at 13.3 mPa. The accelerating voltage is at 20 keV. Backscattered images were taken at a working distance of 9.8 mm.

 

Publications/Presentations:

R.L. DeRosa, I. Szabo, D. Battocchi, and G.P. Bierwagen, “Assessing the Role of Magnesium in Magnesium Rich Coatings”, proceedings of the FSCT, 2007 FutureCoat, Toronto, Ont., Oct 3-5 2007.

I. Szabo and R.L. DeRosa, “Surface Behavior of Magnesium Rich Coatings”, poster presented at FSCT, 2007 FutureCoat, Toronto, Ont., October 4, 2007.

 

Future Activities: Complete crosshatched exposures in 90% relative humidity for all systems. Fully evaluate crosshatched samples and compare results to DHS exposures. Experiment with plasma pretreatment and compare results to previous exposures.

 

Supplemental Key Words: corrosion, metal rich coating, magnesium rich coating, aluminum alloy, magnesium alloy

 

Relevant Web Sites:

http://www.estcp.org/Technology/WP-0731-FS.cfm

http://www.ndsu.nodak.edu/research/article.php?article_number=75