Salt spray resistance

Synthetic Applications. Oxazolines, which ate synthesized as indicated above, have been utilized in many different appHcations (25). When used in resin formulations, AMP, AEPD, and TRIS AMINO can incorporate the oxazoline stmeture into the polymer stmeture (26). Because they ate polyols, both AEPD and TRIS AMINO can be used in polyester resin modification. Oxazoline alkyd films ate characterized by improved performance, particularly salt-spray resistance and gloss (see Alkyd resins Coatings, special purpose, high performance). 

Sa.lt Spray Tests. One of the older accelerated corrosion tests is the salt spray test (40). Several modifications of this imperfect test have been proposed, some of which are even specified for particular appHcations. The neutral salt spray test persists, however, especially for coatings that are anodic to the substrate and for coatings that are dissolved or attacked by neutral salt fog. For cathodic coatings, such as nickel on steel, the test becomes a porosity test, because nickel is not attacked by neutral salt fog. Production specifications that call for 1000 hours salt spray resistance are not practical for quahty acceptance tests. In these cases, the neutral salt spray does not qualify as an accelerated test, and faster results from different test methods should be sought. 

Zinc—Nickel. Steel has the best salt spray resistance when the nickel is 12—13% of the alloy. At increasing nickel contents, the deposit becomes more difficult to chromate and more noble, eventually becoming cathodic to steel. At those levels and above, corrosion resistance usually decreases and is dependent on a complete lack of porosity for protection of the steel. In efforts to replace cadmium and nickel—ca dmium diffused coatings in the aircraft industry, 2inc—nickel has insufficient wear properties for some appHcation, but is under study as an undercoat to various electroless nickel top coats (153). 

Jaudon tested phosphate coatings with and without paint and found the salt-spray resistance, as judged by the first appearance of rust, to be as follows ... 

Table 15.14 Salt-spray resistance of phosphate coatings under various finishes...

ASTM B 117, 1973, Salt Spray Resistance Test, American Society for Testing and Materials, Philadelphia, PA. 

Zinc—Cobalt. Alloys of Zn—Co usually contain 0.3—0.8% cobalt. Higher cobalt alloys, from 4—8%, have shown better salt spray resistance (156), but the commonly plated alloy is 0.3—0.8%. One automotive company specifies 0.3—1.0%. Cobalt is expensive, and economics favor the lower alloys. Costs have been quoted for zinc—cobalt at 1.2 times the cost of chloride zinc, with zinc—nickel alloys at 1.5—1.6 times the chloride zinc. Deposits can be very bright, but the improved corrosion resistance advantage requires yellow or bronze chromates. Alkaline baths give fewer problems in plating components with lapped, spot-welded seams. 

Post-Treatments. Although many post-treatments have been used over plated metals, chromate conversion coatings remain as the most popular. Chromates are used to improve corrosion resistance, provide good paint and adhesive base properties, or to produce brighter or colored finishes. Formulations are usually proprietary, and variations are marketed for use on zinc, zinc alloys, cadmium, copper and copper alloys, and silver (157). Chromates are also used on aluminum and magnesium alloys (158,159). More recently, chromate passivation has been used to extend salt spray resistance of autocatalytic nickel plated parts. 

For zinc deposits, various chromates can produce very thin, blue-bright coatings to get up to about 24 hours salt spray protection before the zinc corrodes (white corrosion salts), thicker yellow iridescent films for up to about 100 hours, and thickest, olive-drab coatings that may give upward of 300 hours (160). A modification of the olive-drab chromate solution with a silver salt produces a black finish, but salt spray resistance drops back to 48—72 hours. Topcoats of waxes are often used on such blacks. Higher salt spray resistance is being claimed for chromates on the zinc alloys eg, 250 hours for... 

Exp. no. E-1007/ HMMM weight ratio Mechanical properties Methyl ethyl ketone resistance ASTM B117-6U salt-spray resistance Residual OH content of cured films, meq/100 g... 

A binder of this type (no. U in Table III), in combination with small amounts of melamine or phenolic resins as crosslinkers, showed good ED behaviour and good mechanical film properties. Its salt-spray resistance was only slightly lower than that of a CARDURA-free analogue (no. 2 in Table III). CE 10 seems to have a lesser detrimental effect on the salt-spray resistance than natural fatty acids, possibly because its hydrocarbon chain is much shorter. 

With melamine or phenolic resins as crosslinkers binder no. 5 shows an attractive combination of good ED behaviour, good mechanical film properties and high bath stability. Its salt-spray resistance is comparable to that of a commercial LMPB system (no. 

In view of these encouraging results, we decided to submit binder no. 5 to customers under the code LR-2052. Customer reactions so far have been quite positive and our views regarding the high salt-spray resistance of this binder on non-phosphated steel were fully confirmed. 

With a view to lowering raw material costs (the DMPA used in binder LR-2052 is rather expensive), we later developed another OH-rich binder, designated LR-2053. This system is based on a linear backbone containing a liquid epoxy resin, EPIKOTE 828, diphenylolpropane, adipic acid and CE 10 (molar ratio 3/2/2/2) and again is rendered water-soluble with a TMA/CE 10 combination (2.5 moles TMA and 2.6 moles CE 10 per mole of backbone). This binder performs as well as LR-2052, with an only marginally lower salt-spray resistance. 

Film thickness, um Film appearance Pendulum hardness (Konig), s Adhesion (crosshatch, DIN 53151) Erichsen impact, reverse, mm Conical mandrel bend (ASTM D522-60) Salt-spray resistance (ASTM B117-6U 10 d) 22-28 very good 195 Gt 0 >3 passed 9-9-2 22-28 good 170 Gt 0 >3 passed 85... 

From this work we can conclude that the salt-spray resistance of cationic ED coatings also appears to benefit from a high level of hydroxyl groups in the cured films. 

After the successful introduction to appliances, the automotive industry began a major conversion to cationic electrocoating. Advantages of the system are good chemical resistance, excellent salt spray resistance, hardness of 4H+, flexibility, and low dissolution of phosphate at the anode (33). 

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