Corrosion resistance pure zinc coating

In the automotive industry, coatings of pure zinc have been more and more replaced by zinc alloy coatings [43, 44]. The latter offer better corrosion resistance at lower coating thickness. They are also used as replacements for... 

Compared with pure zinc coatings the zinc-iron alloy coatings provide increased corrosion resistance in acid atmospheres but slightly reduced corrosion resistance in alkaline atmospheres. 

The external layer of pure zinc is of primary importance with regard to the corrosion resistance of the bars. If galvanized steel is exposed to a neutral environment, such as the atmosphere, the duration of protection is primarily dependent on the thickness of the zinc coating, and its composition and microstructure has a negligible effect. Similarly, for galvanized-steel bars embedded in concrete, the protective properties of zinc coatings are due for the most part to the external layer of pure zinc, which can form a passive film if it has a sufficient thickness [36-39]. In fact, a loss of thickness of 5-10 pm is required prior to passivation, while, if the thickness is insufficient, the underlying layers of Zn-Fe alloy passivate with more difficulty. 

Zinc-nickel Zn-Ni alloys with 5 to 15 wt% Ni offer excellent corrosion resistance and are mainly used in the automotive, aerospace, and electronics industries. Above 15% Ni, the alloy coating becomes more noble than steel, and the corrosion-protection mechanism changes from a sacrificial to a pure physical one (comparable to pure Ni coatings, see Sect. 5.5.4.2.2). They can be electrode-posited from acid or alkaline baths. The acid baths are usually based on sulfate, chloride, sulfate-chloride, pyrophosphate, or acetate (Table 15). The system shows anomalous codeposition (see Sect. 5.5.1.2), which has been explained by a hydroxide suppression mechanism [47]. As in the case of Ni-Fe, the alkaline baths must contain complexing agents (see Sect. 5.5.4.6.2). The alloys electroplated from add haths contain approximately 10 to 14% Ni, whereas the alkaline Zn-Ni... 

Currently, more than 100,000 tons of steel is plated weekly in zinc-nickel alloy baths, almost all of it continuous strip plating for car production (Geduld, 1988). Coated strip is used under paint to give higher corrosion resistance than pure zinc, but the degree of improvement may be less in practice than is shown by accelerated tests. The coating is also applied to... 

Zinc-4% aluminum alloy castings produced according to recognized specifications behave much like pure zinc. They are resistant to atmospheric corrosion, and usually more zinc can be lost without affecting performance than is the case with zinc coatings. 

All of the zinc-aluminum alloys with an 8-29% aluminum content corrode less than 99.99% zinc. ZA-4 (zinc-4% aluminum) castings resist atmospheric corrosion much in the same manner as pure zinc. One advantage is that more zinc may be lost than with zinc coatings without affecting the performance. 

Traditionally, metallic coatings serve only one or two functions. For example, zinc has excellent corrosion resistance and functions as a sacrificial anode (Tsura, 2005). Zinc galvanizing provides sacrificial cathodic protection and acts as a barrier (Jones, 1996) but does not nsnally supply inhibitor ions. The release of zinc ions during the sacrificial protection of galvanized steels (Tsuru, 2005 Pourbaix, 1974) only provides a small additional benefit compared with galvanic protection provided by the potential driving force. Metallic coatings used to protect Al alloys (Reddy et al., 2000 Walton et al, 1953) consist of a thin layer of nearly pure Al mechanically bonded to standard precipitation age... 

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