Schweitzer P.A. Fundamentals of corrosion. Mechanisms, causes, and preventative methods

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Corrosion Mechanisms 19

2.5 Measuring Polarization

While polarization always leads to lower rates of corrosion, identifying the

effects of the environment on polarization of the corrosion circuit is use-

ful in predicting corrosion behavior. Measurement of the corrosion circuit

while the corrosion potential is varied is possible with the apparatus shown

in Figure 2.9.

Turning to the example of iron corroding in hydrochloric acid solution,

if the iron sample is maintained at the natural corrosion potential of −0.2V,

no current will ow through the auxiliary electrode. The plot of this data

point in the study would equate to that of A or C in Figure 2.10. As the

potential is raised, anodically polarized, the current ow will increase and

curve AB will approximate the behavior of the true anodic polarization

curve. Alternatively, if the potential were lowered below −0.2V, the current

measurements would result in the curve CD and approximate the nature

of the cathodic polarization curve. By using the straight-line portion of the

Tafel regions of these curves, an approximation of the corrosion current

can be made.

Most often it is the anodic polarization behavior that is useful in under-

standing alloy systems in various environments. Anodic polarization tests

can be conducted with relatively simple equipment and the scans themselves

can be done in a short time. They are extremely useful in studying the active-

passive behavior that many materials exhibit. As the name suggests, these

Concentration

polarization

Activation polarization

Log i

–

FigurE 2.8

Combined curve of activation and concentration polarizations for reduction process.

20 Fundamentals of Corrosion

Reference

electrode

(Calomel)

Corrodent

Salt bridge

Potentiostat

Te st specimen

Auxiliary

electrode (Pt)

FigurE 2.9

Anode polarization measurement apparatus.

Tafel

region

True

Current, Log i

Measured

Potential

FigurE 2.10

Anodic and cathodic polarization curve.

Corrosion Mechanisms 21

materials can exhibit both a highly corrosion-resistant behavior and that of

a material that corrodes actively, while in the same corrodent. Metals that

commonly exhibit this type of behavior include iron, titanium, aluminum,

chromium, and nickel. Alloys of these materials are also subject to this type

of behavior.

Active-passive behavior depends on the material–corrodent combination

and is a function of the anodic or cathodic polarization effects that occur in

that specic combination. In most situations where active-passive behavior

occurs, there is a thin layer at the metal surface that is more resistant to the

media than the underlying metal. In stainless steels, this layer is composed

of various chromium and/or nickel oxides that exhibit substantially differ-

ent electrochemical characteristics than the underlying alloy. If this resistant,

or passive layer is damaged while in aggressive media, active corrosion of

the freshly exposed surface will occur. The damage to this layer can be either

mechanical or electrochemical in nature.

The behavior of iron in nitric acid underscores the importance of recog-

nizing the nature of passivity. Iron is resistant to corrosion in nitric acid at

concentrations around 70%. Once passivated under these conditions, it can

also exhibit low rates of corrosion as the nitric acid is diluted. However, if this

passive lm is disturbed, rapid corrosion will begin and repassivation will not

be possible until the nitric acid concentration is raised to a sufcient level.

2.5.1 anodic Polarization

Active-passive behavior is schematically represented by the anodic polar-

ization curve as shown in Figure 2.11. Starting at the base of the plot, the

curve starts out with a gradually increasing current as expected. However,

Transpassive

Passive

Active

crit

Log i

pass

Volts

FigurE 2.11

Anodic polarization curve for material exhibiting active-passive behavior.

22 Fundamentals of Corrosion

at point A there is a dramatic polarizing effect that drops the current to a

point where corrosion is essentially halted. As this potential is increased

further, there is little change in current ow until the next critical stage B,

where a breakdown of the passive lm occurs and the current again begins

to rise.

Even with an established anodic polarization behavior, the performance of

a material can vary greatly with relatively minor changes in the corrodent.

This is also illustrated in Figure 2.12. Frame 1 illustrates the case where the

anodic and cathodic polarization curves intersect much the same as in mate-

rials with no active-passive behavior. The anode is actively corroding at a

high but predetermined rate.

Frame 2 represents the condition often found perplexing when using

materials that exhibit active-passive behavior. With relatively minor

changes within the system, the corrosion current could be very low when

the material is in the passive state or very high when active corrosion

begins.

Frame 3 in Figure 2.12 typies the condition sought after when using mate-

rials in the passive state. In this example, the cathodic polarization curve

intersects only in the passive region, resulting in a stable and low corrosion

current. This type of system can tolerate moderate upset conditions without

the onset of accelerated corrosion.

The anodic polarization technique is also useful in studying the effects of

variations in the environment and the benets of alloy additions. As illus-

trated in Figure 2.13, temperature increases cause a shift of the curve to

higher currents. Increasing chromium content in steel expands the passive

region signicantly, and adding molybdenum raises the potential required

for the initiation of pitting-type attack. The presence of chloride or other

strong oxidizing ions will shrink the passive region.

Frame 3Frame 2

Frame 1

FigurE 2.12

Schematic representation of a material with active-passive behavior in different corrosive

environments.

Corrosion Mechanisms 23

2.6 Other Factors Affecting Corrosion

Temperature can have a signicant inuence on the corrosion process. This

is not unexpected because it is an electrochemical reaction and reaction rates

do increase with increasing temperature. There are additional inuences on

corrosion other than the corrodent itself.

The relative velocities between the component and the media can have a

direct effect on the corrosion rate. In some instances, increasing the veloc-

ity of the corrodent over the sufface of the metal will increase the corrosion

rate. When concentration polarization occurs, the increased velocity of the

media will disperse the concentrating species. However, with passive mate-

rials, increasing the velocity can actually result in lower corrosion rates. This

occurs because the increased velocity shifts the cathodic polarization curve

such that it no longer intersects the anodic polarization curve in the active

corrosion region, as shown in Figure 2.14.

Increasing

temperature

Increasing chloride

Increasing

chromium

Increasing

molybdenum

Log iLog i

FigurE 2.13

Effects of environment and alloy content on anodic polarization behavior.

24 Fundamentals of Corrosion

The surface nish of the component also has an impact on the mode and

severity of the corrosion that can occur. Rough surfaces or tight crevices can

facilitate the formation of concentration cells. Surface cleanliness can also be

an issue, with deposits or lms acting as initiation sites. Biological growths

can behave as deposits or change the underlying surface chemistry to pro-

mote corrosion.

Other variations within the metal surface on a microscopic level inu-

ence the corrosion process. Microstructural differences such as secondary

phases or grain orientation will affect the way corrosion manifests itself.

For corrosive environments where grain boundaries are attacked, the grain

size of the material plays a signicant role in how rapidly the material’s

properties deteriorate. Chemistry variations in the matrix of weld deposits

are also a factor.

Radiation can have an effect on a material’s properties. The effect on metal-

lic materials is very gradual and not very pronounced. Stresses, either resid-

ual or applied, impact the mode of corrosion and lower the energy needed

for corrosion to begin. Stress is a requirement for stress corrosion cracking

(which will be discussed later) or corrosion fatigue, but can also inuence the

rate of general corrosion.

Finally, time is a factor in determining the severity of corrosion. Corrosion

rates are expressed using a time dimension. Some corrosion processes are

violent and rapid while most are so slow as to be imperceptable on a day-to-

day basis. Equipment is planned to provide a useful service life. A chief goal

in understanding corrosion is the proper selection of materials, equipment,

processes, or controls to optimize our natural and nancial resources.

Decreasing

concentration

polarization

Log i

FigurE 2.14

Increased corrodent velocity can shift the cathodic polarization curve such that passive behav-

ior can be induced.

Corrosion Mechanisms 25

2.7 Corrosion Rate Measurement

Measurement of corrosion is essential for the purpose of material selec-

tion. The compatibility of a metal with its environment is a prime require-

ment for its reliable performance. Corrosion rate measurement may

become necessary for the evaluation and selection of materials for a spe-

cic environment, a given denite application, or for the evaluation of

a new or old metal or alloys to determine the environments in which

they are suitable. Often, the corrosive environment is treated to make it

less aggressive, and corrosion rate measurements of a specic material

in untreated environments will reect the efcacy of the treatment. In

addition, corrosion rate measurement is also essential in the study of the

mechanisms of corrosion.

2.7.1 Corrosion rate Expressions

Corrosion involves dissolution of metal, as a result of which the metal-

lic part loses its mass (or weight) and becomes thinner. Corrosion rate

expressions are therefore based on either weight loss or penetration into

the metal.

The most widely used weight expression, based on weight loss, is mg/

/day (mdd) and the rate expression on penetration is inch penetration/

year (ipy) and mils penetration/year (mpy). One mil is one thousandth of

an inch. The last expression is very convenient because it does not involve a

decimal point or zeroes. Thus, 0.002 ipy is simply expressed as 2 mpy. The

expression is readily calculated from the weight loss of the metal specimen

during the corrosion test by the empirical formula:

mpy

534W

DAt

(2.18)

where W is weight loss (mg), D is density of metal (g/cm

), A is area of speci-

men (in.

), t is exposure time (h), and mdd and mpy are convertible through

the following multiplying factors:

mpy

1.44 mdd

density

(2.19)

mdd=mpy(0.696) (density)

(2.20)

Conversion factors for the above equations are given in Table 2.2.

26 Fundamentals of Corrosion

TabLE 2.2

Conversion Factors from Inches per Year (ipy) to

Milligrams per Square Decimeter per Day (mdd)

Metal

Density

(g/cm

)

0.00144

Density

(× 10

-3

)

696 ×

Density

Aluminum 2.72 0.529 1890

Brass (red) 8.75 0.164 6100

Brass (yellow) 8.47 0.170 5880

Cadmium 8.65 0.167 6020

Columbium 8.4 0.171 5850

Copper 8.92 0.161 6210

Copper-nickel (70/30) 8.95 0.161 6210

Iron 7.87 0.183 5480

Duriron 7.0 0.205 4870

Lead (chemical) 11.35 0.127 7900

Magnesium 1.74 0.826 1210

Nickel 8.89 0.162 6180

Monel 8.84 0.163 6140

Silver 10.50 0.137 7300

Tantalum 16.6 0.0868 11550

Titanium 4.54 0.317 3160

Tin 7.29 0.198 5070

Zinc 7.14 0.202 4970

Zirconium 6.45 0.223 4490

Note: Multiply ipy by (696 × density) to obtain mdd. Multiply

mdd by (0.00144/density) to obtain ipy.

Forms of Metallic Corrosion

The mechanisms of metallic corrosion were discussed in Chapter 2. The

various mechanisms can result in different forms or types of corrosion, as

discussed in this chapter. The primary forms of corrosion are as follows:

General (uniform) corrosion•

Intergranular corrosion•

Galvanic corrosion•

Crevice corrosion•

Pitting corrosion•

Erosion corrosion•

Stress corrosion cracking•

Biological corrosion•

Selective leaching•

Hydrogen damage•

Liquid metal attack•

Exfoliation•

Corrosion fatigue•

Filiform corrosion•

Each form of corrosion and the conditions responsible for its initiation

are discussed.

3.1 General (Uniform) Corrosion

Although other forms of attack must be considered in special circumstances,

uniform attack is one form most commonly confronting the user of metals

and alloys. Uniform (or general) corrosion, which is the simplest form of cor-

rosion, is an even rate of metal loss over the exposed surface. It is generally

thought of as metal loss due to chemical attack or dissolution of the metallic

component into metallic ions. In high-temperature situations, uniform metal

loss is usually preceded by its combination with another element rather than

28 Fundamentals of Corrosion

its oxidation to a metallic ion. Combination with oxygen to form metallic

oxides, or scale, results in the loss of material in its useful engineering form;

scale ultimately akes off to return to nature.

A metal resists corrosion by forming a passive lm on the surface. This

lm is formed naturally when the metal is exposed to the air for a period

of time. It can also be formed more quickly by chemical treatment. For

example, nitric acid, if applied to austenitic stainless steel, will form this

protective lm. Such a lm is actually corrosion but once formed, it prevents

further degradation of the metal, provided that the lm remains intact. It

does not provide an overall resistance to corrosion because it may be sub-

ject to chemical attack. The immunity of the lm to attack is a function of

the lm composition, temperature, and the aggressiveness of the chemical.

Examples of such lms are the patina formed on copper, the rusting of iron,

the tarnishing of silver, the fogging of nickel, and the high-temperature oxi-

dation of metals.

There are two theories regarding the formation of this lm. The rst theory

states that the lm formed is a metal oxide or other reaction compound. This is

known as the oxide lm theory. The second theory states that oxygen is adsorbed

on the surface, forming a chemisorbed lm. However, all chemisorbed lms

react over a period of time with the underlying metal to form metal oxides.

Oxide lms are formed at room temperature. Metal oxides can be classied as

network formers, intermediates, or modiers. This division can be related to

thin oxide lms on metals. The metals that fall into network-forming or inter-

mediate classes tend to grow protective oxides that support anion or mixed

anion/cation movement. The network formers are noncrystalline, whereas the

intermediates tend to be macrocrystalline at low temperatures.

3.1.1 Passive Film on iron

Iron in iron oxides can assume a valence of two or three. The former acts as

a modier and the latter as a network former. The iron is protected from the

corrosion environment by a thin oxide lm 1 to 4 mm in thickness with a com-

position of

Fe O/Fe O

23 34

This is the same type of lm formed by the reac-

tion of clean iron with oxygen or dry air. The

Fe O

layer is responsible for

the passivity, while the Fe

provides the basis for the formation of a higher

oxidation state. Iron is more difcult to passivate than nickel because with iron

it is not possible to go directly to the passivation species

Fe O

. Instead, a

lower oxidation state of Fe

is required and the lm is highly susceptible to

chemical dissolution. The

Fe O

layer will not form until the Fe

phase

has existed on the surface for a reasonable period of time. During this time, the

layer continues to form.