Corrosion Performance Materials

Corrosion Performance Materials

(Parte 1 de 4)

CHAPTER 35 CORROSION

Milton G. Wille, Ph.D., P.E. Professor of Mechanical Engineering

Brigham Young University Provo,Utah

35.1INTRODUCTION / 35.1 35.2CORROSION RATES / 35.2 35.3METAL ATTACK MECHANISMS / 35.2 35.4CORROSION DATA FOR MATERIALS SELECTION / 35.28 REFERENCES / 35.28

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 Engineeringby 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 associated 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 simplification 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

Guideby 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.

Source: STANDARD HANDBOOK OF MACHINE DESIGN

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35.2CORROSION 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).Multiplympyby 0.0254 to obtain millimeters per year (m/yr).To convert to mass-loss rate,multiply the surface regression rate by surface area and material density,using consistent units.

Polymer attacktypically 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 surface texture.Certain plastics are degraded by ultraviolet light,which limits their usefulness in sunlight unless they are pigmented with an opaque substance such as lamp-black carbon.

35.3METAL 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 reaction 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.

35.2PERFORMANCE OF ENGINEERING MATERIALS CORROSION

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35.3.2Galvanic Corrosion and Protection

When two dissimilar metals are electrically connected and both are exposed to the same environment,the more active metal will be attacked at a faster rate than if there had been no electrical connection between the two.Similarly,the less active metal will be protected or suffer less attack because the surface areas of both metals can be used to dissipate the electrons generated by oxidation of the more active metal.The net flow of electrons from the more active to the less active metal increases the attack rate of the more active metal and decreases that of the less active metal.

Anadverse area ratiois characterized by having a larger surface area of less active metal than that of the more active metal.Cracks in a barrier protective coating (i.e.,polymers) applied to the more active metal in a galvanic-couple situation can create an extremely adverse area ratio,resulting in rapid localized attack in the cracks.The standard electromotive force (emf) series of metals (Table 35.1) lists

CORROSION 35.3

TABLE 35.1Standard EMF Series of Metals

metals in order of increasing activity,starting with gold (Au),which is the least active.If two of the metals listed were joined in a galvanic couple,the more active one would be attacked and plating or deposition of the less active one would occur. This is based on the fact that solutions contain only unit activity (concentration) of ions of each of the two metals.

The standard EMF series is valid only for pure metals at 25°C and in equilibrium with a solution containing unit activity (concentration) of its own ions.If ion concentrations are greater than unit activity,the potentials are more positive;if less,the opposite is true.

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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 corrosion 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.

35.4PERFORMANCE OF ENGINEERING MATERIALS

TABLE 35.2Galvanic 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 mentioned also exhibit passivity,but to a lesser extent.

CORROSION 35.5

FIGURE 35.1Corrosion 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 potentiostatis utilized to electrochemically maintain a passive condition and hence a low rate of corrosion.However,accelerated corrosion 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 condition,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 polarizationcurves 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.

35.3.4Crevice 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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(Parte 1 de 4)

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