Zinc layer

On galvanized steel, tubercles may develop rapidly at breaches in the zinc layer. Attack is frequently highly localized if aggressive ions such as chloride or sulfate concentrate beneath deposits (Fig. 4.9). 

Zinc is susceptible to attack from oiQ gen concentration cells. Shielded areas or areas depleted in oxygen concentration tend to corrode, forming voluminous, white, friable corrosion products. Once the zinc layer is breached, the underlying steel becomes susceptible to attack and is severely wasted locally (Figs. 5.12 and 5.13). 

If scratches and breaks occur in the zinc layers by accidental damage - which is certain to occur when the sheets are erected - then the zinc will cathodically protect the iron (see Fig. 24.4) in exactly the way that pipelines are protected using zinc anodes. This explains the long postponement of rusting. But the coating is only about 0.15 mm thick, so after about 30 years most of the zinc has gone, rusting suddenly becomes chronic, and the roof fails. 

At first sight, the answer would seem to be to increase the thickness of the zinc layer. This is not easily done, however, because the hot dipping process used for galvanising is not sufficiently adjustable and electroplating the zinc onto the steel sheet increases the production cost considerably. Painting the sheet (for example, with a bituminous paint) helps to reduce the loss of zinc considerably, but at the same time should vastly decrease the area available for the cathodic protection of the steel and if a scratch penetrates both the paint and the zinc, the exposed steel may corrode through much more quickly than before. 

Additional metal layers can create bimetallic corrosion cells if discontinuities appear in service. The layer of copper beneath cadmium plate on aluminium (using a zincate plus cuprocyanide deposit technique) can cause corrosion troubles. When aluminium is plated with nickel and chromium, rapid service corrosion in the zinc layer causes exfoliation. 

FIGURE 18.14 A layer of zinc protects iron from oxidation, even when the zinc layer becomes scratched. The zinc (anode), iron (cathode), and water droplet (electrolyte) constitute a tiny galvanic cell. Oxygen is reduced at the cathode, and zinc is oxidized at the anode, thus protecting the iron from oxidation. 

Yet another positive aspect of the diffusion phenomenon is the creation of alloys by first depositing alternate layers of different coatings and then creating an alloy by heating to promote diffusion to produce an alloy. Specifically, brass deposits may be produced by first depositing copper and zinc layers alternately. Subsequent heating produces the required brass. This type of approach obviates the undesired direct method of brass deposition via cyanide process. 

Galvanoaluminum layers thicker than 12 pm are extraordinarily corrosion resistant to exposure to SO2, in which they surpass the performance of cadmium and zinc layers [31]. After 20 rounds of the SFW 2.0 L, DIN 50018 Kesternich test, in contrast to the ca. 12 pm thick, yellow chromated galvanoaluminum layers, the zinc and cadmium coatings were already heavily corroded. The SO2 resistance of anodized galvanoaluminum can also be considered excellent in comparison to cadmium and zinc coatings. 

The plated composite was sectioned and examined for elemental composition. This analysis showed that the zinc coating on the fiber surface was uniform throughout the length and over the cross-section of the composite. The plating process was simple to operate and control, which suggests that the zinc layer could be regenerated in situ on a spent composite by the application of current while flowing zinc chloride solution. 

Surprisingly, it is better to coat steel with another metal that does corrode. Trash cans, for example, are made of zinc-coated steel. This coating does not stop corrosion. But the zinc corrodes first, making the steel underneath last much longer than it would without the zinc layer. 

Dent and Kokes consider that wurtzite derives from isotropically expanded, hexagonal close-packed layers of oxide ions, with correspondingly expanded zinc layers in which zinc ions occupy one half of the tetrahedral holes between oxide layers. This expansion increases the radius of the trigonal holes in the oxide layers such that, at 0.058 nm, they can almost accommodate a zinc ion. The structure is quite open and consists of straight channels of octahedral sites, each 0.20 nm in diameter, separated by trigonal squeeze points , 0.12 nm in diameter. 

Thin coatings not only are used for mechanical protection and lubrication, but are widely applied to protect against corrosion and chemical reaction [47-49]. The steel body of a car is first covered by a zinc layer, which is further treated in a phosphate bath, to promote the adhesion of the paint. Damaging this sandwich structure, by breaking one of these layers, results in quick corrosion of the car body rust. The improvement in the quality of this protective coating during the last 20 years is shown by the increase of the duration of the corrosion warranty proposed by car manufacturers (from 2 years less than 15 years ago, to 8 years at the present time). 

Galvanization protects iron in two ways. As long as the zinc layer is intact, water and oxygen cannot reach the irons surface. Inevitably, the zinc coating cracks. When this happens, zinc protects iron from rapid corrosion by becoming the anode of the voltaic cell set up when water and oxygen contact iron and zinc at the same time. Figure 20.18 illustrates how these two forms of corrosion protection work. 

Growth of Zinc Layers The morphology of the Zn layer is strongly influenced by the structure of the underlying Sn layer. Although excellent Zn films could be grown on Cu layers, nucleation of Zn on Sn layers was found to be very uneven. 

For the electroplating of pure zinc layers, there are mainly three types of zinc electrolytes alkaline cyanide, al-kahne noncyanide, and acid. The acid electrolytes, which exhibit current efficiencies >95% (cf. Fig. 5), can be subdivided into weak acid-containing ammonium, weak acid ammonium free, and moderate acid (see Table 1). They contain zinc as chloride and/or sulfate. 

The most important treatment is the conversion coating (see Chapter 5.3 of the same volume). This type of treatment is typically used for zinc or cadmium layers or on bulk metals like aluminum or magnesium. The classical conversion coating is chromating, the formation of a metal oxide/chromium oxide film. We will discuss the process for the example of zinc layers. 

In galvanized steel constructions, blasting is also necessary to provide a coatable surface. The sweep-blasting method is used in which the zinc surface is roughened without removing a significant amount of the zinc layer. When constructions made of aluminum alloys are coated, their surfaces must be blasted with iron-free blasting materials. 

Table 8.1 Average Percent Decrease in and j o with Zinc Layers vs. Bare Iron...

D.H. Coleman, G. Zheng, B.N. Popov, R.E. White, The effects of multiple electroplated zinc layers on the inhibition of hydrogen permeation through an iron membrane, J. Electrochem. Soc. 143 (1996) 1871-1874. 

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