Galvanic protection

Tin—2inc coatings (75 wt % tin) have appHcation as a solderable coating for radio, television, and electronic components. They also provide galvanic protection for steel in contact with aluminum. 

Aluminum coatings on steel will perform in a manner similar to zinc coatings. Aluminum has good resistance to many atmospheres in addition, being anodic to steel, it will galvanically protect exposed areas. Aluminum-coated steel products are quite serviceable under high-temperature conditions, for which gooa oxidation resistance is required. 

The required number, n, of anodes can be calculated using Eq. (17-2) from the current requirement, together with the maximum current output 1 of the anodes. The arrangement of the anodes is dealt with in Section 17.3.2.2. Galvanic protection systems are usually designed to give protection for 2-4 years. After this period, a maximum of up to 80% of the anodes should be consumed. 

Galvanizing is a process in which iron is covered with a protective layer of zinc. Galvanized iron is often used to make metal buckets and chain-link fences. Galvanizing protects iron in two ways. First, the zinc acts as a protective layer. If this layer is broken, the iron is exposed to air and water. When this happens, however, the iron is still protected. 

Steel objects, when exposed to humid atmospheres or when immersed in electrolytes, corrode at a rapid rate. For example, abrasively polished, cold-rolled steel panels will show signs of rust within 15 minutes when immersed in dilute chloride solutions with pH in the range of 7-10. One of the methods used to control this rapid corrosion is to coat the metal with a polymeric formulation such as a paint. The role of the paint is to serve primarily as a barrier to environmental constituents such as water, oxygen, sulfur dioxide, and ions and secondarily as a reservoir for corrosion inhibitors. Some formulations contain very high concentrations of metallic zinc or metallic aluminum such that the coating provides galvanic protection as well as barrier protection, but such formulations are not discussed in this paper. 

Another method of providing protective current is to use the heads or water boxes as sacrificial anodes. These water boxes, when made of heavy steel or of cast iron, provide galvanic protection to the tube ends and tube sheet as they corrode. 

This method uses a more active metal than that in the structure to be protected, to supply the current needed to stop corrosion. Metals commonly used to protect iron as sacrificial anodes are magnesium, zinc, aluminum, and their alloys. No current has to be impressed to the system, since this acts as a galvanic pair that generates a current. The protected metal becomes the cathode, and hence it is free of corrosion. Two dissimilar metals in the same environment can lead to accelerated corrosion of the more active metal and protection of the less active one. Galvanic protection is often used in preference to impressed-current technique when the current requirements are low and the electrolyte has relatively low resistivity. It offers an advantage when there is no source of electrical power and when a completely underground system is desired. Probably, it is the most economical method for short life protection. 

Concretes and mortars rapid cure early, high compressive strength even at low temperatures Galvanic protection of bridge decks Filling cable slots Areas where rapid cure and early strength are required... 

What is galvanizing How does galvanizing protect iron from corrosion (21.2)... 

CREVICE TYPE --metal/metal --metal/non-metal --metal/marine growth --galvanically protected... 

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. 

Fig. 15.5 Evans plot for the sacrificial galvanic protection system.

On the other hand, galvanic systems are more favored for small, well-coated, low-current-demand structures or for localized protection. However, in some offshore environments, it has been found to be cost-effective to employ galvanic protection systems to large and uncoated structures where the initial cost of coating applications and maintenance is very high. 

FIGURE 20.66 Sacrificial anode is a simple form of galvanic protection. 

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