Products of iron metal corrosion

In Chapter 10 the corrosion of iron is explained by the removal of a metal ion from an anodic site with the production of electrons. The corrosion if iron is described by Equation 10.2. 

All refined metals have a tendency to revert to a thermodynamically more stable form such as those in which they occur naturally on earth. Thus one of the corrosion products of iron is iron oxide (Fea03(s)) which is one form of iron ore. Almost all types of corrosion can be explained in terms of electrochemistry (oxidation-reduction reactions) for this reason we will consider corrosion as an example of the application of redox chemistry or electrochemistry to a practical situation. We will not present a detailed quantitative analysis of corrosion and the design of corrosion-control systems. Other texts should be consulted for this type of information. 

In the realization of a structure or of a plant, we can make use of different metallic materials that, although not in contact and therefore not in galvanic coupling, must cross each other or, in any case, be positioned one above the other. In the case in which the products of the atmospheric corrosion of a material may constitute cathodic reagents for the dissolution of a material onto which they can leach out, the relative positions of the materials (i.e., above or below) are not equivalent from the corrosion point of view this is the case, for example, for the products of the atmospheric corrosion of copper, which can act as a cathodic reagent for the dissolution of iron and especially of zinc. It is therefore preferable that zinc and iron components are located above the copper ones. 

The driving force that causes metals to corrode is a natural consequence of their temporary existence in metallic form. In order to produce metals starting from naturally occurring minerals and ores, it is necessary to provide a certain amount of energy. It is therefore only natural that when these metals are exposed to their environments they would revert back to the original state in which they were found. A typical cycle is illustrated by iron. The primary corrosion product of iron, for example, is Fe(OH)2 (or more likely FeO-nH O), but the action of oxygen and water can yield other products having different colors ... 

The most common form of corrosion is uniform corrosion, in which the entire metal surface degrades at a near uniform rate (1 3). Often the surface is covered by the corrosion products. The msting of iron (qv) in a humid atmosphere or the tarnishing of copper (qv) or silver alloys in sulfur-containing environments are examples (see also SiLVERAND SILVER ALLOYS). High temperature, or dry, oxidation, is also usually uniform in character. Uniform corrosion, the most visible form of corrosion, is the least insidious because the weight lost by metal dissolution can be monitored and predicted. 

Sulfides are intermixed with iron oxides and hydroxides on carbon steels and cast irons. The oxides are also produced in the corrosion process (Reaction 6.6). Although theoretical stoichiometry of 1 to 3 is often suggested between sulfide and ferrous hydroxide, empirically the ratio of iron sulfide to ferrous hydroxide is highly variable. Sulfide decomposes spontaneously upon exposure to moist air. Additionally, corrosion-product stratification is marked, with sulfide concentration being highest near metal surfaces. 

The resistance of a metal to erosion-corrosion is based principally on the tenacity of the coating of corrosion products it forms in the environment to which it is exposed. Zinc (brasses), aluminum (aluminum brass), and nickel (cupronickel) alloyed with copper increase the coating s tenacity. An addition of V2 to 1)4% iron to cupronickel can greatly increase its erosion-corrosion resistance for the same reason. Similarly, chromium added to iron-base alloys and molybdenum added to austenitic stainless steels will increase resistance to erosion-corrosion. 

Homogeneous galvanic corrosion may also occur on the surface of steel components that are covered or partially covered with mill scale (magnetite, Fe304) or iron sulfide corrosion products. Both mill scale and iron sulfide are noble with respect to steel. Significant galvanic corrosion can occur where breaks or holidays in these corrosion products expose unprotected metal. 

The pump has experienced graphitic corrosion. Figures 17.10, 17.12, and 17.14 illustrate typical appearances of graphitically corroded cast iron. In addition, cavitation damage (see Chap. 12) has produced severe metal loss in specific areas (see Fig. 17.13). The soft, friable corrosion products produced by graphitic corrosion are susceptible to cavitation damage at relatively low levels of cavitation intensity. 

Gaseous corrosion is a general form of corrosion whereby a metal is exposed to a gas (usually at elevated temperatures). Direct oxidation of a metal in air is the most common cause. Cast iron growth is a specific form of gaseous corrosion in which corrosion products accumulate onto the metal surface (and particularly at grain boundaries) to the extent that they cause visible thickening of the metal. The entire metal thickness may succumb to this before loss of strength causes failure. 

Sodium arsenite can be used to detect the presence of iron sulfide on the metal surface. Iron sulfide is the corrosion product of the reaction between hydrogen sulfide in drilling fluid and iron in the drillpipe. An acid solution of sodium arsenite reacts with the sulfide to form a bright yellow precipitate. 

The metal lost from the inside of pumps, reaction vessels, pipework, etc. usually contaminates the product. The implications of this depend upon the product. Ppb levels of iron can discolor white plastics, though at this level the effect is purely cosmetic. Ppm levels of iron and other metals affect the taste of beer. Products sold to compositional requirements (such as reagent-grade acids) can be spoiled by metal pick-up. Pharmaceutical products for human use are often white tablets or powders and are easily discolored by slight contamination by corrosion products. 

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