Metallic corrosion processes

DEZINCIFICATION. A form of electrolytic corrosion observed in sume brasses where the copper-zinc alloy goes into solution with subsequent redepnsition of the copper. The small red copper plugs thus formed in the brass are usually porous and of low strength. In recent years, the term dcziticilicalion lias also been applied in a more general sense to signify any metallic corrosion process that dissolves one of the components from an alloy. 

Oxygen is involved in most metal corrosion processes... 

Radiochemical methods are applied for the study of a wide range of electrochemical surface processes. The most important areas are as follows - adsorption and -> electrosorption occurring on the surface of electrodes the role of electrosorption in -> electrocatalysis -> deposition and dissolution of metals - corrosion processes the formation of surface layers, films on electrodes (e.g., polymer films), and investigation of migration processes in these films study of the dynamics of - electrosorption and - electrode processes under steady-state and equilibrium conditions (exchange and mobility of surface species) electroanalytical methods (e.g., radiopolarog-raphy). 

FIGURE 26.33 Current-voltage Tafel plot for a mass transfer controlled metal corrosion process, where the corrosion current is equal to the limiting current for oxidant reduction. 

Metallic corrosion processes which are chemical in nature (see above) can be written in the following general form... 

The principles which form the basis for understanding metallic corrosion processes in general are described by Strehblow in Chapter 1 of Volume 19 A of this series. The corrosion reaction for black steels in oxygen-containing electrolytes can be written as... 

Since metals have very high conductivities, metal corrosion is usually electrochemical in nature. The tenn electrochemical is meant to imply the presence of an electrode process, i.e. a reaction in which free electrons participate. For metals, electrochemical corrosion can occur by loss of metal atoms tluough anodic dissolution, one of the fiindamental corrosion reactions. As an example, consider a piece of zinc, hereafter referred to as an electrode, inunersed in water. Zinc tends to dissolve in water, setting up a concentration of Zn ions very near the electrode... 

Conserva.tlon, Because the most common conservation problem with metal objects occurs when corrosion processes form a threat to the safety of the object or disfigure its appearance to an unacceptable degree (130,131), many conservation treatments are intended to stabilize the corrosion processes and to remove aesthetically displeasing corrosion cmsts. The latter requkes a great deal of thought and discussion as to when a corrosion layer ceases to be a deskable patina and becomes unacceptable. 

Anodes. Lead—antimony (6—10 wt %) alloys containing 0.5—1.0 wt % arsenic have been used widely as anodes in copper, nickel, and chromium electrowinning and metal plating processes. Lead—antimony anodes have high strength and develop a corrosion-resistant protective layer of lead dioxide during use. Lead—antimony anodes are resistant to passivation when the current is frequendy intermpted. 

Tantalum is not resistant to substances that can react with the protective oxide layer. The most aggressive chemicals are hydrofluoric acid and acidic solutions containing fluoride. Fuming sulfuric acid, concentrated sulfuric acid above 175°C, and hot concentrated aLkaU solutions destroy the oxide layer and, therefore, cause the metal to corrode. In these cases, the corrosion process occurs because the passivating oxide layer is destroyed and the underlying tantalum reacts with even mild oxidising agents present in the system. 

Both iron and aluminum are particulady troublesome because of their abiUty to act as coagulants. Also, their soluble and insoluble hydroxide forms can each cause precipitation of some water treatment chemicals, such as orthophosphate. Airborne contaminants usually consist of clay and dirt particles but can include gases such as hydrogen sulfide, which forms insoluble precipitates with many metal ions. Process leaks introduce a variety of contaminants that accelerate deposition and corrosion. 

Continuous chlorination of a cooling water system often seems most pmdent for microbial slime control. However, it is economically difficult to maintain a continuous free residual in some systems, especially those with process leaks. In some high demand systems it is often impossible to achieve a free residual, and a combined residual must be accepted. In addition, high chlorine feed rates, with or without high residuals, can increase system metal corrosion and tower wood decay. Supplementing with nonoxidizing antimicrobials is preferable to high chlorination rates. 

Biological Corrosion The metabohc activity of microorganisms can either directly or indirectly cause deterioration of a metal by corrosion processes. Such activity can (1) produce a corrosive environment, (2) create electrolytic-concentration cells on the metal surface, (3) alter the resistance of surface films, (4) have an influence on the rate of anodic or cathodic reaction, and (5) alter the environment composition. 

Electrochemical Impedance Spectroscopy (EIS) and AC Impedance Many direct-current test techniques assess the overall corrosion process occurring at a metal surface, but treat the metal/ solution interface as if it were a pure resistor. Problems of accuracy and reproducibility frequently encountered in the application of direct-current methods have led to increasing use of electrochemical impedance spectroscopy (EIS). 

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. 

Perhaps the most important stress factor affecting corrosion fatigue is the frequency of the cyclic stress. Since corrosion is an essential component of the failure mechanism and since corrosion processes typically require time for the interaction between the metal and its environment, the corrosion-fatigue life of a metal depends on the frequency of the cyclic stress. Relatively low-stress frequencies permit adequate time for corrosion to occur high-stress frequencies may not allow sufficient time for the corrosion processes necessary for corrosion... 

The protective coating reforms on the metal surface via a corrosion process. 

The triggering mechanism for the corrosion process was localized depassivation of the weld-metal surface. Depassivation (loss of the thin film of chromium oxides that protect stainless steels) can be caused by deposits or by microbial masses that cover the surface (see Chap. 4, Underdeposit Corrosion and Chap. 6, Biologically Influenced Corrosion ). Once depassivation occurred, the critical features in this case were the continuity, size, and orientation of the noble phase. The massive, uninterrupted network of the second phase (Figs. 15.2 and 15.21), coupled... 

Most galvanic corrosion processes are sensitive to the relatively exposed areas of the noble (cathode) and active (anode) metals. The corrosion rate of the active metal is proportional to the area of exposed noble metal divided by the area of exposed active metal. A favorable area ratio (large anode, small cathode) can permit the coupling of dissimilar metals. An unfavorable area ratio (large cathode, small anode) of the same two metals in the same environment can be costly. 

Coatings of less noble metals than the substrate metal (e.g., Zn on Fe) are only protective if the corrosion product of the metal coating restricts the corrosion process. At the same time, the formation of aeration cells is hindered by the metal coating. No corrosion occurs at defects. Additional cathodic protection to reduce the corrosion of the metal coating can be advantageous. Favorable polarization properties and low protection current requirements are possible but need to be tested in individual cases. The possibility of damage due to blistering and cathodic corrosion must be heeded. 

Nascent atomic hydrogen released at metal surfaces by chemical reactions between the process environment and the metal (corrosion or cathodic protection reactions)... 

Koszman, I., Antifoulant Additive for Steam-Cracking Process, U.S. Patent 3,531,394, Sept. 29, 1970. Hochman, R. F, Fundamentals of the Metal Dusting Reaction, Proceedings, Fourth International Congress on Metallic Corrosion, NACF (1971). 

For most low-pressure air systems, the flow is through sheet metal duct, either circular or rectangular. For very large flows and/or pressures higher than light gage duct work can withstand, the duct is usually made of fahricated metal plate and in some instances may he coated or lined for protection from corrosive process gases. 

Figure 4-419 illustrates the concept of corrosion process under concentration polarization control. Considering hydrogen evolution at the cathode, reduction rate of hydrogen ions is dependent on the rate of diffusion of hydrogen ions to the metal surface. Concentration polarization therefore is a controlling factor when reducible species are in low concentrations (e.g., dilute acids). 

For the corrosion process to proceed, the corrosion cell must contain an anode, a cathode, an electrolyte and an electronic conductor. When a properly prepared and conditioned mud is used, it causes preferential oil wetting on the metal. As the metal is completely enveloped and wet by an oil environment that is electrically nonconductive, corrosion does not occur. This is because the electric circuit of the corrosion cell is interrupted by the absence of an electrolyte. Excess calcium hydroxide [Ca(OH)j] is added as it reacts with hydrogen sulfide and carbon dioxide if they are present. The protective layer of oil film on the metal is not readily removed by the oil-wet solids as the fluid circulates through the hole. 

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