Zinc coatings corrosion resistance

Uses Corrosion inhibitor, pigment in metal protective coatings, corrosion-resist. metal primers mfg. of zinc hydrosulfide precipitating agent reducing agent in lubricants, chemicals Features Unslaked lime minimizes formation of hydrogen gas in org. 

Chem. Desaip. Zinc dust CAS 7440-66-6 EINECS/ELINCS 231-175-3 Uses Corrosion inhibitor, pigment in thin-film metal protective coatings, corrosion-resist, metal coatings, aerosol paints in lubricants, chemicals Features For use with either org. or inorg. binders Reguiatory ASTM Spec. D-520 Type I compliance Properties Superfine powd. 100 /. thru 325 mesh 4.5 p median particle size sp.gr. 7.0 dens. 58.4 Ib/solid gal 99.3% total zinc, 96.5% metallic zinc, 3.1 zinc oxide ZCA Zinc Dust 64L [Zinc Corp. of Am.]... 

The corrosion resistance of zinc is so important that about half the world s annual consumption of zinc is used to protect steel from rust. A vast amount of information now exists on the resistance of zinc (including its alloys and their use as a coating) in a wide variety of corrosive conditions and is summarized herein. The present volume draws upon these earlier works, but also contains much valuable data from previously unpublished and untranslated works. Zinc Its Corrosion Resistance was originally compiled in 1970 by the Battelle Memorial Institute, Columbus, Ohio, which had been commissioned by the International Lead Zinc Research Organization, Inc. (ILZRO) to collate the available information on the corrosion resistance of zinc and zinc-coated steel. The 1983 edition included information that had become available since 1970 plus one new chapter dealing with the performance of zinc as a sacrificial anode. 

The noble metals, i.e. gold, platinum, palladium, and rhodium, and the corrosion-resistant metals, i.e. chromium, nickel, tin, tin-lead solder, and titanium, require no finish other than cleaning. Soldered joints should be protected with a moisture-proofing compound or coating. Corrosion-resistant (or treated to resist corrosion) minor devices (fasteners, etc.) should be used. Fasteners should be treated with zinc chromate, zinc chromate paste, or graphite-free dry-film anti-seize compound. 

Ziac foil coated with a conductive, pressure-sensitive adhesive is used for repair of other ziac coatings or for imparting corrosion resistance at field sites. The 0.08-mm ziac tape or sheet has a 0.025-mm coaductive adhesive. The laminate is cut to size and pressed tightly to activate the adhesive. Conductive tape can be wrapped around pipe, especially around welds or connections. The corrosion resistance of this material is iatermediate between galvanized or thermally sprayed coatings and zinc-filled paints (21,50). 

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). 

Zinc—Iron. The Zn—Ee aHoy is plated from an alkaline bath. Deposits are 0.3—0.8% iron and can be given attractive, resistant, black, sHver-free chromate coatings. Corrosion protection requires the heavier, darker chromates. Zinc—iron baths are the most economical of the 2inc aHoys. 

The resistance of a metal to erosion-corrosion is based principally on the tenacity of the coating of corrosion products it forms in the environment to which it is exposed. Zinc (brasses), aluminum (aluminum brass), and nickel (cupronickel) alloyed with copper increase the coating s tenacity. An addition of V2 to 1)4% iron to cupronickel can greatly increase its erosion-corrosion resistance for the same reason. Similarly, chromium added to iron-base alloys and molybdenum added to austenitic stainless steels will increase resistance to erosion-corrosion. 

Galvanized steel A zinc-coated steel sheet or plate with good corrosion resistance properties used for ductwork and other applications. 

Zinc silicate This material has good corrosion resistance and can withstand temperatures up to 540°C, particularly when over-coated with silicone-based aluminum. The zinc silicate requires a high standard of surface preparation before application. 

The excellent resistance of zinc to corrosion under natural conditions is largely responsible for the many and varied applications of the metal. In fact nearly half the world consumption of zinc is in the form of coatings for the prevention of corrosion of steel fabrications exposed to the atmosphere and to water. For its varied applications zinc is obtainable in a number of grades. Ordinary commercial (G.O.B.) zinc contains up to about I -5% total of lead, cadmium and iron. Electrolytic zinc has a minimum zinc content of 99-95% and contains small amounts of the same impurities. Special high-purity zinc has a minimum of 99-99% zinc. Even purer zincs are commercially available. 

Temperatures well in excess of 400°C can be used for processing in this case much deeper coatings are obtained, but the iron content of the surface alloy is higher and the diffusion layer is very brittle and less corrosion-resistant. This effect is easily explained when it is remembered that the rate of interdiffusion is far more rapid when the temperature is above the melting point of zinc (420°C). 

The corrosion rate of a bare sprayed coating is comparable to that of solid zinc or aluminium, although the greater surface area exposed may cause apparent corrosion rates to be a few per cent higher. For most uses, however, the sprayed coating is sealed or painted and achieves the much higher corrosion resistance associated with duplex coatings. The extra life depends on the sealer or on the thickness and type of paint used, and on the environment. 

In atmospheric exposure to industrial environments its corrosion rate is only about one-third that of zinc and the corrosion reaction is stifled by the tenacious oxide which is produced nevertheless it can frequently function as an anodic coating both for steel and for the less corrosion-resistant aluminium alloys. 

Full chromate passivation (Section 15.3) improves the corrosion resistance of both zinc and cadmium towards all environments and is applied for a wide range of applications. Clear and olive-coloured chromate coatings can also be applied for certain purposes. The highest degrees of corrosion protection... 

It is estimated that approximately 40% of the world production of zinc is consumed in hot-dip galvanising of iron and steel, and this adequately demonstrates the world-wide use of zinc as a protective coating. The success of zinc can be largely attributed to ease of application, low cost and high corrosion resistance. 

The corrosion resistance of zinc is discussed in Section 4.7, and it is only necessary here to say that zinc is protected against further attack by a film of corrosion products. It is remarkably resistant to atmospheric corrosion except perhaps in the most heavily contaminated industrial areas, and even there its use as a protective coating is still a sound practical and economic proposition. The value of zinc coatings as a basis for painting under very aggressive conditions has been clearly demonstrated. 

The natural corrosion resistance of zinc is, therefore, its most important property in relation to zinc coatings. The electrochemical property becomes important when the zinc coating is damaged in any way to expose the steel, when sacrificial corrosion of the zinc occurs and the steel is thereby protected. Moreover, the corrosion product of the zinc normally fills the break in the coating and prevents or retards further corrosion of the exposed steel. 

As the protective value of the zinc coating depends largely on the corrosion resistance of zinc, the life of a coating is governed almost entirely by its thickness and by the severity of the corrosive conditions to which it is exposed. Extensive tests and field trials which have been carried out have shown that the life of a zinc coating is roughly proportional to its thickness in any particular environment and is independent of the method of application. 

---