NCER Assistance Agreement Final Report Executive Summary

 

Date of Final Report: January 31, 2009

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

Research Category: Congressionally Mandated Center

Project Period:  September 1, 2006 – October 30, 2008

 

Description and Objective of Project:

The main objective of this work is to understand specifically the role of magnesium particles as sacrificial anodes in magnesium rich primers (MRPs) of different formulations (organic and inorganic) on 2024-T3 and 7075-T6 in different corrosive environments.  The corrosion protection of MRPs received from North Dakota State University (NDSU) were evaluated using: open circuit potential (OCP), potentiodynamic polarization analysis, and electrochemical impedance spectroscopy (EIS).  The changes in electrochemistry are correlated to the chemical and physical changes of the MRPs using: thickness measurements, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and wavelength dispersive spectroscopy (WDS).

 

1. MRPs Tested

Three different primer formulations on two different aluminum panel types were investigated.  The first formulation is an epoxy polyamide magnesium rich primer (CPM1040) formulated at 45% pigment volume concentration (PVC) on Al-2024-T3 and Al-7075-T6 panels.  CPM1040 is an in-house MRP formulated by NDSU.  The formulation consisted of Mg particles with an average diameter of 30–40 μm.  The coating was spray applied to an optimal thickness of 150 ± 40 mm. 

The second formulation is a commercial epoxy polyamide magnesium rich primer (Akzo) formulated at 45 % PVC on Al-2024-T3 and Al-7075-T6 panels.  The primer also contains additional additives to aid in the dispersion of Mg particles.  The coating was spray applied to a nominal thickness of 47 ± 12 mm.  The magnesium rich primer was prepared using Mg particles, with an average diameter of 30–40 μm, manufactured by Ecka Granules, Salzburg, Austria.

The third formulation is a silane-based inorganic magnesium rich primer with plasticizer (SMT30) formulated at 30 % PVC on Al-2024-T3.  The coating was spray applied to a nominal thickness of 112 ± 6 mm.  r of 30–40 μm, manufactured by Ecka Granules, Salzburg, Austria.

 

2. Exposure Conditions

      Samples were analyzed during and after either A) constant immersion in dilute HarrisonÕs solution (DHS-CI) or B) 90 ± 6.5 % relative humidity (90 % RH).  DHS-CI consisted of 0.35 wt% (NH4)2SO4 and 0.05 wt% NaCl in distilled water (pH =5.4).  The 90% RH exposure consisted of a saturated salt solution containing equal amounts of (NH4)2SO4 and NaCl in distilled water (pH =3.9). For 90 % RH exposure; a set of samples was exposed without a defect and another set was scribed.  For the scribed samples an X was scribed though each primer with a razor blade prior to exposure.  The data from each exposure type were compared to as-received samples.

Each bare aluminum panel type and respective primers was exposed to both the DHS-CI and 90% RH exposures and samples were evaluated using electrochemical techniques, SEM, XPS, EDS, and WDS during exposure times ranging from the initial start of exposure to over 6 months.

 

Summary of Findings:

 

1.   Open Circuit Potential (OCP)

      Each MRP system provided initial cathodic protection to the aluminum substrates, with OCP values ranging (-1.0 V to -1.5 V) versus. saturated calomel electrode (SCE).  When the OCP for an MRP reaches the OCP of bare aluminum (baseline), ~ -0.5 to -0.6 V versus SCE, the magnesium particles in the MRP are no longer providing cathodic protection.  A compiled list of when each primer formulation has reached the baseline according to exposure type is provided in Table 1.  With respect to OCP measurements, it is apparent that there is a distinct difference in the behavior of the coating with respect to the exposure method.  OCP curves for the 90%RH samples indicate that the samples have some passivating/barrier layer protection whereas the DHS-CI samples show no indication of a barrier layer.

Table 1. Time to Reach Baseline (Days) for Eeach Primer Formulation and Exposure Type

Formulation

Time to reach baseline (days) DHS-CI

Time to reach baseline (days) scribed 90 % RH

Time to reach baseline (days) non-scribed 90 % RH

CPM1040 2024

25

1

45

CPM1040 7075

37

1

85

Akzo 2024

10

7

85+

Akzo 7075

18

100

85+

SMT30 2024

8

6

85

 

2.   Potentiodynamic Polarization Curves

      Potentiodynamic scans provide information about 1) the rate of corrosion, i.e. reaction of the Mg particles through current density, and 2) cathodic protection, through corrosion potential.  A high current density with a low potential indicates that Mg particles are abundant and available to provide sacrificial protection.  Therefore, samples that show high current density and low potential in theory should provide substantial cathodic protection.  For MRPs on 2024-T3, CPM1040 provided the best cathodic protection (lowest potential), followed by Akzo and lastly SMT30.  CPM1040 has the highest current density, therefore it is reacting the fastest with DHS-CI due to the high amount of available Mg on the primer surface.  The abundance of Mg particles was later confirmed by XPS, SEM, EDS and WDS.  Akzo and SMT30 have lower current densities than CPM1040, indicating not as much available Mg at the primer surface, confirmed as well by XPS, SEM, EDS and WDS.

      For MRPs on 7075-T6, Akzo provides the best initial cathodic protection, followed by CPM1040.  The difference in potential is not much, however CPM1040 displays a much higher (one order of magnitude) current density, indicating more available Mg at the primer surface compared to Akzo confirmed by XPS, SEM, EDS and WDS.

 

3.   Thickness Measurements

DHS-CI exposure resulted in a decrease in primer thickness with increasing exposure time for all MRP formulations due to Mg particles dissolving out of the coating.  The loss of Mg particles was confirmed by SEM, EDS and WDS.  90 % RH exposure showed an increase in thickness with increasing exposure time for all MRP formulations due to the lack of Mg dissolution and the build up of corrosion products surrounding the Mg particles confirmed by SEM and EDS. 

 

4.   X-ray Photoelectron Spectroscopy (XPS)

XPS was used to evaluate the as-received primers and corrosion products present after exposure.  CPM1040 on 2024-T3 and 7075-T3 as-received displayed 60 % atomic concentration of MgO available at the sample surface.  Akzo on 2024-T3 and 7075-T6 as-received displayed only ~ 5 % atomic concentration of available MgO at the sample surface.  SMT30 on 2024-T3 as-received displayed 10 % atomic concentration of MgO available at the sample surface.  DHS-CI caused the formation of Mg(OH)2 for all MRP formulations.  However DHS-CI did not cause the formation of MgCO3 for any MRP formulation, due to the lack of available CO2 during exposure.  After 24 hours of DHS-CI the atomic concentration of MgO dropped below 10 % at the sample surface for CPM1040, below 2 % for Akzo and SMT30, indicating Mg dissolution.  This is confirmed by SEM, EDS and WDS. 

90 % RH exposure allowed for the formation of MgCO3 due to the availability of CO2.  The formation of MgCO3 varied for each MRP.  CPM1040 on 2024-T3 and 7075-T6 displayed MgCO3 formation after the first hour of exposure.  Akzo on 2024-T3 and 7075-T6 displayed MgCO3 formation after 600 hours of 90 % RH exposure.  SMT30 on 2024-T3 displayed MgCO3 formation after 48 hours of 90 % RH exposure.  The formation of carbonates during 90 % RH exposure is important due to their passivating properties providing an additional form of corrosion protection.  Of importance is that samples under DHS-CI exposure do not show the formation of carbonates due to the lack of CO2.  Therefore, the environment used for accelerated testing is an integral part of determining a more accurate depiction of how the coating will behave in the field.

 

5.   Scanning Electron Microscopy (SEM)

      SEM was used to determine two specific characteristics associated with the primers.  First, we were able to look at particle connectivity in the as-received samples.  CPM1040 displayed the best particle connectivity and distribution.  Akzo did not show as high degree of particle connectivity compared to CPM1040.  SMT30 displayed the lowest amount of particle connectivity and also had several voids throughout the primer. 

      Second, we were able to monitor the dissolution of Mg particles from the coatings with respect to exposure condition.  DHS-CI exposure caused the Mg particles to dissolve out of the MRPs, leaving behind little to no corrosion products on the sample surface, a depleted Mg layer, and a porous structure that facilitated faster diffusion of corrosive species to the aluminum substrate.  Alternatively, magnesium particles did not dissolve out of the MRPs when using our 90 % RH exposure.  Corrosion products from magnesium oxidation were able to build up on the surface as indicated from XPS analysis.  In this case, 90 % RH can be considered a more realistic exposure condition than DHS-CI or any other constant immersion technique.  It is more likely that aircraft will encounter high humidity with the presence of salt, as in marine environments, rather than pooled water or immersion as emulated by the DHS-CI conditions.

 

6.   Energy Dispersive Spectroscopy (EDS) & Wavelength Dispersive Spectroscopy (WDS)

      EDS and WDS results further reinforce the variation in the MRPs response to different exposure types.  Samples exposed to DHS-CI displayed a thick oxide layer surrounding the Mg particles that remained in the primer.  By comparing EDS and WDS maps of samples as-received, 24 hours and 480 hours DHS-CI exposure, Mg particles are seen to develop a thick oxide layer, shrink in size and eventually dissolve into solution. 

90 % RH did not cause the Mg particles to dissolve out of the primer, confirmed by EDS and WDS.  During 90 % RH exposure the initial oxidation of the Mg particles at the sample surface is seen.  Increased exposure time causes the oxide layer to grow surrounding the Mg particles and penetrate deeper into the primer eventually creating an oxide layer that surrounds a majority of the particles in the primer.  MRP SMT30 was the only primer to show an Mg based oxide layer that re-deposited on the sample surface after 480 hours exposure.  The formation of carbonates at the sample surface was not detected by EDS and WDS.

Even though OCP results may indicate that an MRP has reached the Al baseline, Mg particles may still remain in the primer according to XPS, SEM, EDS, and WDS.  This result is further reinforced by CPM1040 samples that were scribed after 900+ hours of DHS-CI.  Upon scribing, the OCP of the primer dropped from the baseline to below -1.0 V.  This proves that Mg particles still remain in the primer, however their activity is not detected by OCP measurements.  Unless a mechanical defect exposes the Mg particles, they are essentially inactive.

 

Conclusions:

All of the MRPs provided initial cathodic protection.  Due to the complexity of these systems, it is not possible to say one MRP provided the best overall protection regardless of exposure environment.  However, it is apparent that the environment to which MRPs were exposed played a distinct role on the primersÕ responses.

Electrochemical analysis using OCP indicated rapid deterioration of the cathodic protection of the DHS-CI samples with no passivating layer forming to provide extra protection.  In contrast, OCP of 90% RH samples indicated the formation of a passivating layer, which was later confirmed by XPS to be a carbonate species.

XPS, EDS and WDS were used to monitor the particle connectivity and dissolution with respect to exposure condition.  If we look at the amount of Mg remaining in the primer systems after 480 hours of exposure to DHS-CI the MRPs are ranked as follows: 1) CPM1040 on 7075-T6, 2) CPM1040 on 2024-T3, 3) SMT30 on 2024-T3, 4) Akzo on 7075-T6, 5) Akzo on 2024-T3.  This ranking almost directly correlates to OCP results with the exception of SMT30 on 2024-T3. 

The primers exposed to 90 % RH did not show Mg dissolution.  However, all three formulations showed MgCO3 formation when exposed to 90 % RH.  The time required for the formation of MgCO3 varied with respect to the type of primer.  Carbonates formed the fastest on CPM1040, followed by SMT30 and lastly Akzo.  Carbonate formation due to 90 % RH may be the reason why the OCP trends are essentially opposite to DHS-CI exposure.  The only formulation to show an Mg based layer that re-deposited on the primer surface was SMT30 on 2024-T3.

 

Publications/Presentations:

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

 

I. Szabo and R.L. DeRosa, ÒSurface Behavior of Magnesium Rich CoatingsÓ, poster presented at FSCT, 2007 FutureCoat, Toronto, Ont., October 4, 2007.

 

Supplemental Key Words: corrosion, metal rich coating, magnesium rich primer, 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

http://ceer.alfred.edu