Volatile Corrosion Inhibitor Coatings - cortecvci.com Flipbook PDF

14 CORTEC CORP. Supplement to Materials Performance, January 2001 C oatings applied directly to metals normally use con-

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Volatile Corrosion Inhibitor Coatings MICHAEL PRENOSIL

Volatile corrosion inhibitors (VCIs) are organic materials that protect metals against corrosion by emitting vapors. This study shows that VCIs, combined with sulfonates (contact corrosion inhibitors), can retard corrosion in several different coating systems.

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oatings applied directly to metals normally use conventional corrosion inhibitor pigments such as zinc, aluminum, zinc oxide (ZnO), modified ZnO, and calcium ion-exchanged amorphous silica gel. Using corrosion inhibitor pigments has several disadvantages. Some pigments contain metals that are toxic. Several, including metallic zinc, have high densities and settle. A number of pigments react with the resins in the coating. Additional pigmentation also requires added wetting agents that may affect corrosion resistance. Volatile corrosion inhibitors (VCIs) are unique. They are organic compounds that protect metal surfaces by emitting a vapor such as an aminebased compound. The nitrogen on the amine has two electrons that are attracted to the polar metal surface. Once it is attracted to the metal, the rest of the molecule is very hydrophobic and repels water to significantly retard corrosion. B.A. Miksic, et al., 1-3 and Y.I. Kuznetsov, N.N. Andreev, and their

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CORTEC CORP. Supplement to Materials Performance, January 2001

colleagues4-7 studied several amines, their derivatives, and imines used as VCIs. VCIs migrate from the coated area to the unprotected metal area. Metals coated with these VCI paints corrode very little in scribed areas since the films are self healing—the VCI evaporates and redeposits itself where the coating has been scratched. Many VCI coatings also contain other corrosion inhibitors to enforce the corrosion-resistance process. I.L. Rosenfeld, et al., 8 and N.N. Andreev9 measured corrosion inhibition by using saturated vapor pressure. N.N. Andreev also used x-ray photoelectron spectroscopy to show the formation of a protective layer by vaporphase transfer. 4 B.A. Miksic, et al., developed surface analysis methods using electron spectroscopy for chemical analysis (ESCA), x-ray photoelectron spectroscopy (XPS), secondaryion mass spectroscopy (SIMS), and ion scattering (ISS).10 VCIs have been used for years to temporarily protect metals from corrosion in extreme conditions found on automobile underbodies, offshore drilling decks, storage tanks, naval vessels,11-12 and in the petrochemical industry.13 VCIs formulated with standard resins in coatings have largely been ignored for use in industrial maintenance coatings. The reasons VCIs have not been used in coatings include: • Many are temporary films because they can be removed easily. • Some VCI coatings are soft, tacky, or even oily. • VCIs have been used at fairly low levels in conventional industrial coatings. Higher levels may be needed to show the self-healing effect. • Corrosion protection may not be the only requirement in a coating. Color limitation can be a concern, film hardness may be important, or a high gloss may be needed. There are many ways to circumvent these limitations. Even though these coatings can easily be removed, they can be permanent where high abrasion

TABLE 1

WATERBORNE ALKYD FORMULATIONS resistance is not a concern. Modifying typical industrial solvent-based coatings with VCIs is possible. Waterborne coatings can also be modified with VCIs. VCIs are solventand water-soluble compounds that can be incorporated into waterborne coatings by emulsification, adding cosolvents, or simply dissolving them in water. Tack can be eliminated by adding pigments, waxes, hard resins, or curing agents in some cases. Care must be used to maintain good adhesion and compatibility in these situations.

Material EPS-2601 (78%) 12% Mn Active-8 Sag 5440 defoamer Water VCI-M (61%) Totals

Manufacurer EPS Condea R.T Vanderbuilt Union Carbide Cortec

32% VCI 38.9 0.8 0.8 0.7 35.1 100

ASTM B 117. 500-H SALT SPRAY RESISTANCE

VCI latex VCI epoxy VCI petroleum Water-based alkyd 15% VCI-modified water-based alkyd 32% VCI-modified water-based alkyd Solvent-based epoxy Zinc-rich solvent-based epoxy

Film Thickness (mils/µm) 2.2/56 1.7 to 1.9/43 to 48 1.5 to 1.9/38 to 48 1.7 to 1.8/43 to 46 1.9/48 1.6 to 2.1/41 to 53 2.0 to 2.2/51 to 56 1.9 to 2.4/48 to 61

This study examines four temporary VCI coatings, one permanent epoxy VCI coating, two typical epoxies with no VCIs, and a waterborne alkyd with no VCIs. The VCI waterborne alkyd formulations are direct comparisons to (A)ASTM D 1654- 92 Procedure B rating of unscribed areas a standard waterborne alkyd with the 10 = no corrosion, 5 = 11 to 20% corrosion, 0 = 75% and over. (B) ASTM D 1654- 92 Procedure A rating of failure at scribe. same resin, driers, and cosolvent. MATERIALS • VCI petroleum-based coating: A proprietary blend of oxidized petrolatum, calcium salts blended with a low level of petroleum sulfonate, amine carboxylates, and mineral spirits. • VCI latex coating: Acrylic latex, calcium salt of organo sulfonic acid, and amine carboxylates. • VCI solvent-based epoxy: Bisphenol A epoxy with aliphatic amine, a blend of oxidized petrolatum, calcium salts blended with a low level of petroleum sulfonates, amine carboxylates, and mineral spirits. • 15/32% VCI-modified waterbased alkyds: Engineered Polymer Solutions (EPS). An EPS 2601 alkyd, calcium salt of an organo sulfonic acid, and an amino carboxylate. Table 1 shows the formulation. • Water-based alkyd: EPS 2601 (Table 1). • Typical epoxy coating: Bisphenol A, an aliphatic amine with a standard corrosion pigmentation, and other pigments. LX07521 high-solids epoxy primer (Davis-Frost, Inc.).

15% VCI 48 1 1 0.8 38.6 23.7 100

TABLE 2

Type

Experimental Procedure

Control 54 1 1 0.8 43.2 10.6 100

Corrosion Rating(A) 10 9 8 5 8 8 7 0

Scribe Rust(B) 10 9 9 3 10 10 5 (C)

10 = no creepage, 5 = 0.125 to 0.1875 in. (0.32 to 0.48 cm), 0 = 5/8 or more mean average. (C) Entire surface corroded. Creepage could not be evaluated.

VCI TEMPORARY COATINGS • Zinc-rich epoxy primer: BisTwo temporary coatings were phenol A, aliphatic amine with a high level of zinc pigment, 3921 epoxy tested: a VCI latex and a VCI petroleum-based coating. After 500 h of salt (Belzona®). spray testing, both showed very strong SAMPLE PREPARATION VCI effect and no corrosion at the All epoxy coatings were given a 1-h scribe (Table 2 and Figure 2). After induction period after mixing. The 1,000 h of salt spray, the temporary coatings were drawn down using a VCI coatings began to fail (Table 3 and drawdown bar on cold-rolled steel to a Figure 1). This effect is most noticeable dry-film thickness (DFT) of 1.5 to 2.5 with the VCI latex, which shows the mils (38 to 63 µm). They were air-dried greatest change. It went from no corfor 7 days at room temperature before rosion at 500 h to ~50% corrosion at testing them in a salt spray cabinet 1,000 h in the salt spray cabinet. (ASTM B-117) (Tables 2 and 3). All coatThe petroleum-based temporary ings were scribed with a vertical line. coating containing VCI showed only a minor degree of corrosion at 1,000 h All of the VCI-modified coatings are (Table 3 and Figure 1). This coating’s softer than standard non-VCI-modified more hydrophobic nature is the reason coatings. VCI coatings, however, offer for the significantly better results.

Results

better corrosion resistance (Table 2). VCI PERMANENT COATING The VCI solvent-based epoxy and the The VCI solvent-based epoxy VCI petroleum-based coating were still in good condition with