by Alp Manavbasi, Metalast International Inc., Minden, Nev.
Editor's Note: This paper is a peer-reviewed and edited version of a presentation delivered at NASF SUR/FIN 2012 in Las Vegas, Nev., on June 12, 2012.
A novel hexavalent chromate-free conversion coating was developed to improve anti-corrosion and adhesive-bonding characteristics of the magnesium alloys and zinc-nickel (Zn-Ni) plated steel substrates. The corrosion behavior of the coated and uncoated alloys was investigated by neutral salt fog (NSF) and electrochemical corrosion tests. Surface wettability of the pretreated substrates was investigated by static contact angle measurements. Wet-tape adhesion tests verified that there is strong adhesion between the primer and the chem film-treated substrates. The morphology and composition of the coated surfaces were investigated by optical microscopy (OM), scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDX) methods. This trivalent chromium-based surface treatment is a potential hexavalent chromate conversion coating replacement for magnesium alloys and Zn-Ni plated steel.
Chromates (Cr+6) are very effective and extensively used as corrosion inhibitors for ferrous and non-ferrous alloys in aerospace, military and general industry applications. Hexavalent chromate compounds are employed in the conversion coating formulations to provide improved adhesive bonding to the subsequent primer/topcoat and to protect the substrate against the environment and corrosion. In addition to the self-healing characteristics, chromated chemical films exhibit both a corrosion-inhibiting effect and a physical barrier layer to the corrosive media.1,2 Although hexavalent chromate is currently the most effective compound to provide the anti-corrosion properties to the chem films applied on metals, solutions containing Cr+6-based compounds are highly toxic and adversely affect the environment and human health.3 The Environmental Protection Agency (EPA) regulates chromate usage and emissions mainly through Clean Water Act (CWA) and Toxic Substances Control Act (TSCA).4 The Occupational Safety & Health Administration (OSHA) has mandated a Permissible Exposure Limit (PEL) of 5 µg/cu m (in air) of hexavalent chromium, an unrealistic level for the vast majority of metal finishers and manufacturers to obtain. In addition to the European Union (EU) directives and OSHA requirements, the EPA Executive Order 12856 showed the need to eliminate the release of chromates during aircraft coating applications. The cost of preventive maintenance, improved service life requirements of the fleet of airplanes and strict environmental regulations on the use and handling of hexavalent chromates have motivated U.S. Air Force to invest to find a more suitable method of corrosion control of metals used in aerospace applications. There is, therefore, a need for environmentally-green chem films that can provide high corrosion resistance and increase the adhesive bonding strength characteristics of the metal surface. Although there are other commercially available chem film technologies which do not contain hexavalent chromate, their corrosion performance and paint adhesion characteristics are not as effective as the Cr+6-based chem films.
Magnesium alloys and Zn-Ni plated steels have been used in a variety of aerospace and defense sectors. Both magnesium alloys and Zn-Ni platings require a chem film treatment for maximum corrosion protection and to improve the paint adhesion characteristics of the surface. Chromate conversion coatings have been used on magnesium alloys and Zn-Ni platings mainly for temporary corrosion protection and to improve the adhesive bonding characteristics of the surface.
In this work, an eco-friendly trivalent chromium (Cr+3)-based conversion coating technology is studied on magnesium alloys and Zn-Ni plated steel substrates. Neutral salt fog (NSF), potentiodynamic polarization, and open circuit potential (OCP) methods were used to evaluate the corrosion properties of the chem films. Wet-tape adhesion tests were performed on coated and uncoated magnesium and Zn-Ni plated 4130 steel substrates and rated in accordance with ASTM D3359A.5 Comparative performance analysis results for the uncoated, trivalent chromium- and hexavalent chromate-processed magnesium alloys and Zn-Ni plated steel is presented.
The magnesium alloys used were AZ31B-H24 and AZ92A-T6. Test specimens were metallurgically polished to 1200P using SiC wafers (unless otherwise specified), rinsed in DI water, and dried for corrosion experiments and paint adhesion studies. Polished samples were subsequently degreased in a proprietary alkaline magnesium cleaner for 5 min at 140ºF and thoroughly rinsed in deionized water. Rinsed samples were then immediately immersed into the surface activation baths or directly into the conversion coating bath. In addition to surface activation via acid pickling per AMS-M-3171-C specification,6 two new proprietary surface activation agents were used prior to application of the conversion coatings.
The Zn-Ni plated steel samples were degreased with an acetone wipe and activated in diluted nitric acid solution for 30 sec at room temperature prior to the conversion coating treatment. Commercially available hexavalent-chromium-based conversion coatings were applied on magnesium alloys and Zn-Ni plated steel substrates as the benchmark reference in the present work.
Corrosion resistance testing was done on test coupons for each pretreatment process using a neutral salt fog salt spray chamber maintained in accordance with ASTM B117.7 Potentiodynamic polarization and open circuit potential (OCP) measurements were performed by using a G300 Gamry Potentiostat. The experiments were carried out in an aerated 3.5% NaCl (pH 6.5-7.2) electrolyte.
Test samples were primed with non-chromated epoxy primer (MIL-PRF-23377J, Type I, Class N)8 and cured for seven days at room temperature prior to the wet-tape adhesion test. The wet-tape adhesion test was performed in accordance with Federal Test Method Standard 141 (FED-STD-141)9 and rated in accordance with ASTM D3359A.5 Sealed and plated Zn-Ni plated steel test coupons were exposed to salt spray for one week prior to the wet-tape adhesion test. The adhesion rating per ASTM D3359 - Method A is shown in Table 1.
Contact angle measurements were made by using a Rame-Hart model 250-F1 contact angle goniometer. The surface morphology of deposited films was observed using a Hitachi S-4700 scanning electron microscope (SEM/EDX). All coated panels were cured for 24 hr at ambient temperature prior to the measurement of anti-corrosion and paint adhesion properties.
Table 1 - Wet tape adhesion rating per ASTM D3359 - Method A.5 ASTM D3359 - Method A Rating description of coating after tape removal
5No peeling or removal 4Trace peeling or removal along scribes. 3Jagged removal along scribes up to 1/16 in. (1.6 mm) on either side. 2 Jagged removal along most of the scribes up to 1/8 in. (3.2 mm) on either side. 1 Removal from most of the area between the scribes under the tape. 0 Removal beyond the area of the scribes.
Results and discussion
Figure 1 shows the potentiodynamic polarization curves of AZ92A-T6 magnesium alloy samples processed with trivalent chromium, hexavalent chromate, and an uncoated substrate after immersion in 3.5% NaCl for 20 min. Compared with the untreated AZ92A substrate (Ecorr = -1.53 V), Ecorr shifted to more noble potential values after the deposition of trivalent chromium-based conversion coating (Ecorr = -1.38 V) and hexavalent chromate-based chem film (Ecorr = -1.49 V). Similarly, the corrosion current density of the bare AZ92A substrate decreased from 4.48 mA to 18.1 µA for trivalent chromium and to 24.6 µA for hexavalent chromate-coated magnesium. The calculated corrosion rate (mils per year [mpy]) was found to be 682 mpy for uncoated AZ92A, 8.26 mpy for the trivalent chromium-based coating and 11.23 mpy for the hexavalent chromate-based commercial chem film. The above results demonstrate that this novel trivalent chromium-based chem film can effectively improve the corrosion resistance of magnesium alloys by blocking the penetration of aggressive ions.
Scanning electron microscopy (SEM) images along with the energy dispersive x-ray spectroscopy (EDX) studies revealed that the surface of the magnesium alloys was homogeneously covered with the trivalent chromium-based deposits with relatively less mud-cracking. The details will be presented elsewhere.