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CHAPTER 35
CORROSION
Milton G. Wille, Ph.D., P.E.
Professor of Mechanical Engineering Brigham Young University Provo, Utah
35.1 INTRODUCTION / 35. 35.2 CORROSION RATES / 35. 35.3 METAL ATTACK MECHANISMS / 35. 35.4 CORROSION DATA FOR MATERIALS SELECTION / 35. REFERENCES / 35.
35.1 INTRODUCTION
Corrosion removal deals with the taking away of mass from the surface of materials by their environment and other forms of environmental attack that weaken or otherwise degrade material properties. The complex nature of corrosion suggests that the designer who is seriously concerned about corrosion review a good readable text such as Corrosion Engineering by Fontana and Greene [35.1]. Included in this chapter are many corrosion data for selected environments and materials. It is always hazardous to select one material in preference to another based only on published data because of inconsistencies in measuring corrosion, lack of completeness in documenting environments, variations in test methods, and possible publishing errors. These data do not generally indicate how small variations in temperature or corrosive concentrations might drastically increase or decrease corrosion rates. Furthermore, they do not account for the influence of other associ- ated materials or how combinations of attack mechanisms may drastically alter a given material’s behavior. Stray electric currents should be considered along with the various attack mechanisms included in this chapter. Brevity has required simpli- fication and the exclusion of some phenomena and data which may be important in some applications. The data included in this chapter are but a fraction of those available. Corrosion Guide by Rabald [35.2] can be a valuable resource because of its extensive coverage of environments and materials. Again, all corrosion data included in this chapter or published elsewhere should be used only as a guide for weeding out unsuitable materials or selecting potentially acceptable candidates. Verification of suitability should be based on actual experience or laboratory experimentation. The inclusion or exclusion of data in this chapter should not be interpreted as an endorsement or rejection of any material.
35.
Source: STANDARD HANDBOOK OF MACHINE DESIGN
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35.2 CORROSION RATES
The vast majority of metal corrosion data in the United States are expressed in terms of surface regression rate mpy (mils, or thousands of an inch, per year). Multi- ply mpy by 0.0254 to obtain millimeters per year (mm/yr). To convert to mass-loss rate, multiply the surface regression rate by surface area and material density, using consistent units. Polymer attack typically involves volume changes, usually increases, caused by liquid absorption; reductions in mechanical properties such as yield strength, tensile strength, flexure strength, and tensile modulus; discoloration; and/or changes in sur- face texture. Certain plastics are degraded by ultraviolet light, which limits their use- fulness in sunlight unless they are pigmented with an opaque substance such as lamp-black carbon.
35.3 METAL ATTACK MECHANISMS
The attack on metals involves oxidation of neutral metal atoms to form positively charged ions which either enter into solution or become part of an oxide layer. This process generates electrons, which must be consumed by other atoms, reducing them, or making them more negatively charged. Conservation of electrons requires that the rate of metal oxidation (corrosion) equal the rate of reduction (absorption of electrons by other atoms).
35.3.1 General Attack
In general attack, oxidation and reduction occur on the same metal surface, with a fairly uniform distribution. Most of the corrosion data in this chapter are for selected materials subject to uniform attack in a given environment. Once a suitable material is selected, further control of uniform attack can be achieved by coatings, sacrificial anodes (see Galvanic Corrosion), anodic protection (see Passivation), and inhibitors. Coatings are many times multilayered, involving both metallic and polymer layers. Inhibitors are additions to liquid environments that remove corrosives from solution, coat metal surfaces to decrease surface reac- tion rates, or otherwise alter the aggressiveness of the environment. Chemically protective metallic coatings for steels are usually zinc (galvanized) or aluminum (aluminized). Aluminized steel is best for elevated temperatures up to 675 °C and for severe industrial atmospheres. Both may be deposited by hot dipping, electrochemistry, or arc spraying. Common barrier-type metallic platings are those of chromium and nickel. The Environmental Protection Agency has severely limited or prohibited the use of lead-bearing and cadmium platings and cyanide plating solutions. Polymer coatings (such as paints) shield metal surfaces from electron-receiving elements, such as oxygen, reducing corrosion attack rates. Under mild conditions, even “decorative paints” can be effective. Under more severe conditions, thicker and tougher films are used which resist the effects of moisture, heat, chlorides, and/or other undesirable chemicals. Acrylics, alkyds, silicones, and silicone-modified alkyds are the most commonly used finishes for industrial equipment, including farm equipment. The silicones have higher heat resistance, making them useful for heaters, engines, boilers, dryers, furnaces, etc.
35.2 PERFORMANCE OF ENGINEERING MATERIALS
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The galvanic series (Table 35.2) shows a similar relationship, except that impure metals such as alloys are also included and the medium is seawater. Other media, other concentrations, and other temperatures can further alter the order of the list. Therefore, care should be exercised in applying these data to a given galvanic corro- sion situation except as a general, loose guide.
35.3.3 Passivation
Certain common engineering materials, such as iron, nickel, chromium, titanium, and silicon as well as their alloys (i.e., stainless steels), exhibit a characteristic of being able to behave both as a more active and as a less active (passive) material.
35.4 PERFORMANCE OF ENGINEERING MATERIALS
TABLE 35.2 Galvanic Series of Some Commercial Metals and Alloys in Seawater
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Note in the galvanic series (Table 35.2) that several stainless steels are listed twice, once as passive and once as active. Some common metals other than those men- tioned also exhibit passivity, but to a lesser extent. A graphical representation of passivity is shown in Fig. 35.1. The three regions— active, passive, and transpassive—help to explain seemingly inconsistent behavior of active-passive materials under various degrees of attack severity.
CORROSION 35.
FIGURE 35.1 Corrosion characteristics of an active-passive metal.
There are both advantages and disadvantages to be gained or suffered because of active-passive behavior. In very aggressive environments, a method called anodic protection can be used whereby a potentiostat is utilized to electrochemically main- tain a passive condition and hence a low rate of corrosion. However, accelerated cor- rosion test results may be useless because increasing the corrosion power of the medium may cause a shift from a high active corrosion rate to a low passive condi- tion, producing the invalid conclusion that corrosion is not a problem. Another example involves inhibitors which function by maintaining a passive condition. If the concentration of these inhibitors were allowed to decrease, high corrosion could result by passing from a passive to an active condition. Active-passive materials have a unique advantage in the area of corrosion testing and corrosion rate prediction. Potentiodynamic polarization curves can be generated in a matter of hours, which can provide good quantitative insights into corrosion behavior and prediction of corrosion rates in a particular environment. Most other corrosion testing involves months or years of testing to obtain useful results.
35.3.4 Crevice Corrosion and Pitting
Crevice corrosion is related to active-passive materials which are configured such that crevices exist. Mated screw threads, gaskets, packings, and bolted or lapped joints
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35.3.8 Hydrogen Embrittlement
In any electrochemical process where hydrogen ions are reduced, monatomic hydro- gen atoms are created prior to their joining in pairs to form diatomic hydrogen gas (H 2 ). Monatomic hydrogen, being small, can diffuse into metals, causing embrittle- ment. Corrosion of metals by acids, including cleaning by pickling, can produce hydrogen embrittlement. Heating can drive out monatomic hydrogen, reversing the process. If monatomic hydrogen diffuses into voids in a metal, high-pressure pockets of H 2 gas are formed which are not eliminated by heating, but rather may form hydrogen blisters.
35.3.9 Intergranular Corrosion
In some alloys, frequently related to prior heating, grain boundaries can experience localized variations in composition that can result in corrosion attack along or imme- diately adjacent to grain boundaries.The 18-8 stainless steels (such as type 304), when heated in the approximate range of 500 to 790°C, experience the precipitation of chromium carbides in grain boundaries, removing chromium from the regions adja- cent to grain boundaries. This process is called sensitization. It is theorized that inter- granular attack proceeds in the chromium-depleted regions of the grain boundaries, since these lack the protection provided by chromium alloying. When this class of stainless steels is welded, regions a bit removed from the weld axis are heated suffi- ciently to become sensitized and hence become subject to subsequent intergranular (continued on page 35.28)
CORROSION 35.
TABLE 35.3 Environments That May Cause Stress Corrosion of Metals and Alloys
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TABLE 35.
Corrosion Data by Environment and Material
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