Coatings, protective sacrificial

Galvanized rebar is used successfully in structures where carbonation is a risk such as cladding panels. Galvanizing can easily be carried out in most countries although the quality and composition of the coating can affect its durability. It suffers from fewer problems when handled roughly because the coating is sacrificial and protects bare areas. 

Sacrificial metal coatings protect iron and steel by two or three protective abilities such as ... 

Metallic Coating An electroplated or melted coating that acts as a corrosion resistant coating or sacrificial coating for cathodic protection. 

Because the shaft is also exposed to friction and wear in the region of the stuffing box, hardened coatings and sacrificial sleeves can be used to protect the shaft. In high speed or abrasive applications, protection of the shaft may be very important. Besides the maintenance cost of replacing a mixer shaft, a sudden failure may occur if shaft wear is sufficient to reduce the cross-sectional area and create a location of high stresses. 

The coating has an that is negative to the material to be protected and the layer serves as a sacrificial anode. 

The most significant chemical property of zinc is its high reduction potential. Zinc, which is above iron in the electromotive series, displaces iron ions from solution and prevents dissolution of the iron. For this reason, zinc is used extensively in coating steel, eg, by galvanizing and in zinc dust paints, and as a sacrificial anode in protecting pipelines, ship hulls, etc. 

Cathodic protection using sacrificial anodes or applied current can retard or eliminate tuberculation. However, costs can be high and technical installation can be very difficult. Costs are markedly reduced if surfaces are coated (see Material substitution below). 

Sometimes there is no practical, economical way to reduce deposition, protect surfaces cathodically, change design and operation, or treat existing systems chemically. A material change is required. The most economical solution usually is to coat existing structures with water-impermeable, sacrificial, or corrosion-resistant materials. 

CoiTosion prevention is achieved by correct choice of material of construction, by physical means (e.g. paints or metallic, porcelain, plastic or enamel linings or coatings) or by chemical means (e.g. alloying or coating). Some metals, e.g. aluminium, are rendered passive by the formation of an inert protective film. Alternatively a metal to be protected may be linked electrically to a more easily corroded metal, e.g. magnesium, to serve as a sacrificial anode. 

The modern procedure to minimise corrosion losses on underground structures is to use protective coatings between the metal and soil and to apply cathodic protection to the metal structure (see Chapter 11). In this situation, soils influence the operation in a somewhat different manner than is the case with unprotected bare metal. A soil with moderately high salts content (low resistivity) is desirable for the location of the anodes. If the impressed potential is from a sacrificial metal, the effective potential and current available will depend upon soil properties such as pH, soluble salts and moisture present. When rectifiers are used as the source of the cathodic potential, soils of low electrical resistance are desirable for the location of the anode beds. A protective coating free from holidays and of uniformly high insulation value causes the electrical conducting properties of the soil to become of less significance in relation to corrosion rates (Section 15.8). 

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