Historically, paints designed to protect steel and other metals have been formulated with slightly soluble, lead- and chromate-based corrosion-inhibiting pigments. Some of the most important pigments of this type are zinc chromate, zinc tetroxy chromate, strontium chromate, red lead, and basic lead silicochromate. Although these pigments are highly effective corrosion inhibitors in various coating applications, their use has been declining in response to concern about the toxicity of hexavalent lead and chromium.
Government regulations concerning the protection of both the environment and industrial workers have prompted paint manufacturers and end users to desire replacements for these pigments. The U.S. Environmental Protection Agency (EPA) has established requirements on the maximum leachable concentration of certain metals that may be present in industrial waste materials (e.g., spent abrasive and coating debris from blasting operations to remove old coatings). Materials that do not meet these limitations must be disposed of as hazardous waste. In the workplace, employers must take measures to keep airborne concentrations of hazardous materials below permissible exposure levels as established by the Occupational Safety and Health Administration (OSHA). The use of lead- and chromate-free pigments can eliminate the need for many such measures. Anticorrosive molybdate pigments, for example, are regulated only as a nuisance dust material, as are other nontoxic materials such as calcium carbonate.
Corrosion is an electrochemical process involving the oxidation
of a metal (the anodic reaction) and the corresponding reduction of
another material (the cathodic reaction). Molybdates' ability to
effectively inhibit the corrosion process has been recognized for
many years. Molybdates are classified as anodic or passivating
inhibitors. When a coating film that contains molybdate pigments is
exposed to water, a small amount of molybdate ions is released into the
coating film. When these ions contact the metal substrate, they promote
the formation of a protective, passive oxide layer on the metal that
prevents corrosion of the underlying substrate. This process can
be demonstrated using a Pourbaix diagram for iron in contact with water
(Figure 1). The presence of the molybdate ion raises the electrode
potential and moves iron into a region of passivation. The interaction
of molybdates with other metal substrates such as aluminum, although
studied less extensively, is expected to be similar to that with iron.
Chromates inhibit corrosion in a similar fashion.
Click here to see Figure 1
Three main kinds of nontoxic corrosion inhibitors are used commercially: borates, phosphates, and molybdate pigment products. Molybdates have been used commercially for more than 20 years. Considerable testing has demonstrated that the anticorrosion capabilities of molybdates are comparable to, and in some cases exceed, those of chromate- and lead-based pigments. Additionally, because molybdates are white pigments, they can be used in white or tintable primers, top coats, and direct-to-metal systems.
Three main kinds of molybdate pigments are commercially available: zinc molybdates, calcium molybdates, and phosphomolybdates. Zinc molybdates are primarily used in solvent-based coatings, particularly alkyd and epoxy systems. Calcium molybdates have been used more widely in water-based coatings, including latex emulsions and water-reducible systems. Calcium molybdates are considered more compatible and stable in water-based coatings because their solubility is lower than that of zinc molybdates.
Phosphomolybdates, the newest of the three molybdate pigments, incorporate a synergistic combination of phosphate and molybdate compounds. They have exhibited cost and performance advantages over their calcium and zinc molybdate counterparts. Products of phosphomolybdates include zinc phosphomolybdate, which is generally recommended for solvent-based coatings, and calcium zinc phosphomolybdate, which is effective in both water- and solvent-based systems. The market for phosphomolybdates is the fastest growing of those for all kinds of molybdate pigments.
U.S. federal and military specifications for paints (such as MIL-P-28577A, MIL-P-85658, TT-P-645B, and TT-P-2756) include several examples of coating systems formulations using molybdates. Detailed information for these formulations can be obtained through the General Services Administration. The U.S. Navy recently patented a number of corrosion-inhibiting coating systems for steel and aluminum that use molybdate inhibitors (Table 1). These systems, developed to replace existing chromate- and lead-based coatings, are self-priming, thus eliminating the need for a two-step process of primer and top-coat application.
| U.S. Patent number | Date | Title |
|---|---|---|
| 5 100 942 | 31 Mar. 1992 | Corrosion-resistant acrylic coatings |
| 5 089 551 | 18 Feb. 1992 | Corrosion-resistant alkyd coatings |
| 5 059 640 | 22 Oct. 1991 | Epoxy corrosion-resistant coatings |
| 5 043 373 | 27 Aug. 1991 | High-gloss, corrosion-resistant coatings |
| 4 885 324 | 5 Dec. 1989 | Combination primer and top-coat coatings |
Based on independent studies reported by the Steel Structures Painting Council (2) and the Cleveland Society of Coatings Technology (3), the Prohesion/QUV test mimics outdoor exposure conditions more closely than other new methods proposed to industry (Table 2). In both studies, standard salt-spray testing had a negative correlation with outdoor exposure. The ASTM has recently approved and published a standard procedure for Prohesion/QUV testing, designated ASTM D5894, "Standard Practice for Cyclic Salt Fog/UV Exposure of Painted Metal."
| Test Method | Marine exposure site | Average of sites |
|---|---|---|
| ASTM B117 salt spray | -0.110 | -0.107 |
| KTA Envirotest type 1 | 0.485 | ND | KTA Envirotest type 2 (with UV) | 0.481 | 0.213 |
| KTA Envirotest type 3 | -0.043 | ND |
| KTA Envirotest type 4 (with UV) | 0.619 | ND |
| Cyclic salt spray | ND | 0.046 |
| Prohesion | 0.065 | -0.091 |
| Prohesion/QUV | 0.699 | 0.459 |
To determine the statistical correlation of Prohesion/QUV and outdoor exposure -- specifically, in the evaluation of molybdate and other types of anticorrosive pigments -- we prepared 14 high-solids alkyd primers (each formulated using a different inhibitor, but otherwise identical) and applied them to steel panels. One set of panels was exposed for two years at a marine testing site, and another set was exposed to the Prohesion/QUV test conditions for 1008 h. After testing, correlation plots were prepared for scribe undercutting and general surface rust (Figures 2 and 3). The correlation between the Prohesion/QUV test and two years of outdoor exposure was reasonably good.
Click here to see Figure 2
Click here to see Figure 3To better understand the statistical significance of the data, the rankings of the panels after the two exposure conditions were then compared using a statistical Spearman ranking procedure. Correlation coefficients (scribe undercutting, 0.88; surface rust, 0.92) showed that the test was quite useful in providing meaningful indicators of exterior performance.
These test results also demonstrated the performance advantages of the more recently developed grades of phosphomolybdate pigment. In Figures 2 and 3, it is clear that the zinc phosphomolybdate and calcium zinc phosphomolybdate formulations provide better resistance to surface rust and scribe line corrosion than the zinc molybdate formulation does.
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