Coatings sacrificial

Cadmium also provides a sacrificial coating to steel which gives better protection than zinc in applications where strong acids and alkalis may be encountered and those involving immersion in stagnant or soft neutral waters. It should be used in applications involving bimetallic contact with aluminium and in electrical applications where ease of solderability is important. Cadmium has a low torque resistance and should be used as a coating material in cases where bolted assemblies have to be frequently... 

Various strategies are employed to prevent corrosion. The use of paint as a protective coating is described in our chapter introduction. A metal surface can also be protected by coating it with a thin film of a second metal. When the second metal is easier to oxidize than the first, the process is galvanization. Objects made of iron, including automobile bodies and steel girders, are dipped in molten zinc to provide sacrificial coatings. If a scratch penetrates the zinc film, the iron is still protected because zinc oxidizes preferentially ... 

Metallic coalings arc mnsi often selected lor protective function. Decorative ability is a common secondary function. Most coalings applied by Itoi dipping, such as galvanising or aluminizing, use a film of a more chemically reactive meial over a less reactive material such as iron alloy These are sacrificial coatings because the zinc or aluminum slowly dissolves instead of the underlying steel. 

Sacrificial coatings consist of (i) organic zinc-rich primers (ii) inorganic zinc-rich primers. Corrosion control mechanism by zinc-rich primers may be visualized to occur as shown in Figure 1.65. 

Effective methods of continuous treatment are also available. Addition of a chemical product to the furnish always involves a considerable dilution (as the volume of water used in the industry is vast), but treatment directly onto a surface by spray apphcation can be much more concentrated. For this reason it is often more cost-effective if the problem of deposition is defined to a limited part of the papermaking process. The most common method comprises continuous spraying of a water-soluble cationic polymer onto the surface treated. This cationic polymer will then react with anionic water-soluble macromolecules in the furnish (anionic trash) to form a coating. This coating, which is very thin and flexible, is sacrificial, and anything that deposits on this sacrificial coating will be removed from the surface as the deposit is redissolved. Often the polymer is formulated with other materials (like surfactants) to enhance the performance. For centre press rolls spraying a release wax emulsion seems to be the most successful application. 

The open circuit potential as a function of time is a qualitative method that evaluates sacrificial coating stability. The sacrificial metal removal rate in the corroding solution is controlled by the potential difference of the underlying metal and the sacrificial coating, corroding solution, and sacrificial coating thickness. The open circuit potential changes to the protected metal when the sacrificial metal is completely removed. 

Figure 5.15 shows that the corrosion current is one order of magnitude smaller for the pulse current-deposited (pc) Zn-Ni-Cd (7% Cd) sacrificial coatings when compared to the direct current-deposited (dc) Zn-Ni-Cd with 22% Cd in the alloy [36]. The corrosion rates of pulse current-deposited Zn-Ni-Cd sacrificial coatings that contain 7% and 3% Cd were 0.49 and 1.03 mpy, respectively much lower than the 2.87 mpy observed for the direct current-deposited Zn-Ni-Cd alloy with 22% Cd in the alloy. 

Sacrificial anodes do not require outside power sources ratHer, they provide their own power and need minimum maintenance. Sacrificial coatings are recommended with weU-coated structures with minimum chance of being damaged during the pipeHne s useful life. 

Sacrificial coatings are based on the principle of galvanic corrosion for the protection of metals against corrosion. The substrate is protected by a metal or alloy that is electrochemically more active than the material to be protected. Coatings formulated with metallic zinc powder have been... 

For sacrificial coatings (e.g., zinc, cadmium) and, in certain environments, also aluminum and tin on steel, the direction of galvanic current through the electrolyte is from coating to base metal as a result, the base metal is cathodically protected (Fig. 14.1). As long as adequate current flows and the coating remains in electrical contact, corrosion of the base metal does not occur. The degree of... 

Figure 14.1. sketch of current flow at defects in noble and sacrificial coatings. 

In many aerated hot waters, reversal of polarity between zinc and iron occurs at temperatures of about 60 C (140 °F) or above [19-21], This reversal of polarity leads to zinc having the characteristics of a noble coating instead of a sacrificial coating hence, a galvanized coating under these circumstances can induce pitting of the base steel. 

Tin coatings so thin are, naturally, very porous hence, it is essential that the tin act as a sacrificial coating in order to avoid perforation by pitting of the thin-gage steel on which the tin is applied. This condition usually applies. 

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