Basic forms, of corrosion

Corrosion of metallic materials can take on many forms. Understanding and recognizing the basic forms of corrosion are necessary for developing a strategy for mitigation. These concepts can also be utilized in assessing non-metallic materials. Fig. 1 illustrates these basic corrosion mechanisms. 

The term "form" of corrosion is generally well known from Fontana s popular corrosion engineering textbook [6]. The different forms of corrosion represent corrosion phenomena categorized according to their visual appearance. Dillon [7] considered Fontana s eight basic forms of corrosion and divided them into three groups, based on their ease of identification ... 

There are nine basic forms of corrosion that metallic materials may be subject to ... 

The main forms of corrosion are shown in Fig. 5.1, together with the respective group categories. In this figure, the number of forms has been expanded somewhat from Fontana s original grouping of eight basic forms. A description and an example of each basic form of corrosion follows. 

Basic forms of corrosion and their prevention methods are discussed below for consideration during the design stages and to help the parties involved to analyze the project with respect to corrosion. 

The selection of materials to be used in design dictates a basic understanding of the behavior of materials and the principles that govern such behavior. If proper design of suitable materials of construction is incorporated, the eqiiipment should deteriorate at a uniform and anticipated gradual rate, which will allow scheduled maintenance or replacement at regular inteivals. If localized forms of corrosion are characteristic of the combination of materials and environment, the materials engineer should still be able to predict the probable life of equipment, or devise an appropriate inspection schedule to preclude unexpected failures. The concepts of predictive, or at least preventive, maintenance are minimum requirements to proper materials selection. This approach to maintenance is certainly intended to minimize the possibility of unscheduled production shutdowns because of corrosion failures, with their attendant possible financial losses, hazard to personnel and equipment, and resultant environmental pollution. 

In dry air the stability of zinc is remarkable. Once the protective layer of zinc oxide formed initially is complete, the attack ceases. Even under under normal urban conditions, such as those in London, zinc sheet 0 -8 mm thick has been found to have an effective life of 40 years or more when used as a roof covering and no repair has been needed except for mechanical damage. The presence of water does, of course, increase the rate of corrosion when water is present the initial corrosion product is zinc hydroxide, which is then converted by the action of carbon dioxide to a basic zinc carbonate, probably of composition similar to ZnCOj 3Zn(OH)2 . In very damp conditions unprotected zinc sometimes forms a loose and more conspicuous form of corrosion product known as wet storage stain or white rust (see p. 4.171). 

It will be noted from the preceding discussion that impurities may induce more than one form of corrosion. The particular types of corrosion and the influence of these impurities work in a developing chain of cause and effect, depending in large part on the specific operating circumstances in question. Such factors include the boiler system basic configurational design, the localized areas of stress, and the temperature of various metal surfaces. 

Although each form of concentration cell may be considered a discrete form of corrosion, in practice, more than one type may occur simultaneously. These forms of corrosion are all characterized by localized differences in concentration of hydrogen, oxygen, chloride, sulfate, and other minerals, but especially oxygen (producing the so-called differential oxygen concentration cell, or differential-aeration cell). The basic mechanisms surrounding each of these specific forms of concentration cell corrosion are discussed next. 

The two basic types of corrosion discussed above form the general background to the subject. How, and to what extent, any particular object or structure corrodes also depends on other factors, in particular, on whether corrosion is uniform or not and on the effects of mechanical strain. These factors are interactive and in combination, their individual effects can be enhanced. 

Poor basic controls and process contamination permitted slime growth. The development of algal and bacterial slimes, SRBs, etc. encouraged various forms of corrosion, especially pitting corrosion. 

Six forms of corrosion can be identified based on the apparent morphology of corrosion, the basic factor influencing the mechanism of corrosion in every form. The six forms are given in Table 6.1. 

Corrosion is basically a combination of technical and economic problems. To understand the economics of corrosion, it is necessary that one is proficient in both the science of corrosion and the fundamental principles of economics. There are many forms of corrosion, which can be deleterious in a variety of ways. It is logical to discuss the various forms of corrosion of metallic structures occurring in different corrosive environments. 

Corrosion occurs in several forms. A few of these are shown in Figure 9.1. Almost all are seen on various components of oil and gas industries. Perhaps the most well known is uniform corrosion, which basically means the uniform dissolution of materials from the surface, thereby losing the load-bearing capability of the structural component. Usually, this form of corrosion is taken care of... 

The corrosion coupons are 31.75 mm diameter discs 3.175 mm thick and with a hole in the centre to fit over an insulated stainless steel rod. Plastic insulators separate individual coupons. The coupons are not preoxidized at elevated temperature in water but have the normal air formed oxide. This condition should make them more sensitive to any water chemistry changes. Four basic types of corrosion coupon are being tested in the L basin and RBOF water environments ... 

In many industrial applications, MIC of different metallic constructional materials is a serious problem. It does not represent a novel form of corrosion but is based on the modification of the electrochemical processes by microorganisms. Thus, it is necessary to briefly recall the basic electrochemical mechanisms of corrosion. 

These corrosion processes follow the basic laws of thermodynamics. Corrosion is an electrochemical process. Under controlled conditions it can be measured, repeated, and predicted. Since it is governed by reactions on an atomic level, corrosion processes can act on isolated regions, uniform surface areas, or result in subsurface microscopic damage. Complicate these forms of corrosion with further subdivisions, add just basic enviromnental variables such as pH, temperature, and stress, and the predictability of corrosion begins to suffer rapidly. 

The basic ingredients of corrosion products are oxides CU2O and CuO which are highly protective. They react with the environmental pollutants to form salts. 

Although emphasis in this Chapter has been placed on irons and steels, the electrochemical corrosion basics and the forms of corrosion described are applicable to all metallic materials. For more detailed information on the corrosion resistance of various metals and their alloys, the reader should consult the selected references listed at the conclusion of this Chapter, as well as Corrosion, Vol 13, of the ASM Handbook or Corrosion Understanding the Basics, published by ASM International in 2000. 

Forms of Corrosion Recognition and Prevention, Corrosion Understanding the Basics, J.R. Davis, Ed., ASM International, 2000, p 99-192... 

Additionally, a number of basic technical measures can be taken to minimize corrosion damage in its various forms. A brief summary of generally accepted methods for controlling the various forms of corrosion follows. 

The basic principle of cathodic protection (CP) is a simple one. Through the application of a cathodic current onto a protected structure, anodic dissolution is minimized. Cathodic protection is often applied to coated structures, with the coating providing the primary form of corrosion protection. The CP current requirements tend to be excessive for uncoated systems. The first application of CP dates back... 

Chemical control of hydrogen embrittlement is usually difficult because environmental alterations will affect only one of the four basic conditions leading to this form of corrosion. 

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