Czichos H., Saito T., Smith L.E. (Eds.) Handbook of Metrology and Testing

Подождите немного. Документ загружается.

698 Part D Materials Performance Testing

Current (μA)

400030002500200015001000 3500500

Time (s)

1% NaCl, inhibitor-free

Addition NaNO

to 2×10

–3

mol/l Addition NaNO

to 4.2×10

–2

mol/l

Fig. 12.36 Current noise plot of mild steel in air saturated 1 mass %

NaCl solution. Effect of NaNO

concentration

Similar results were obtained with Admirality brass,

which generally showed only localized corrosion but

with a higher initiation rate of pits in cooling water 2.

These and other results reported in the literature

allow one to conclude that ENA can be used for cor-

rosion monitoring with the possibility of distinguishing

between uniform and local corrosion.

However, it has been shown [12.65–67] that mea-

suring only potential noise can be misleading in cases

where small ﬂuctuations in mass-transport control can

produce large changes in the corrosion potential U.

Such is the case for metals and alloys that are immune

to corrosion such as Pt or are passive, such as stain-

less steel in neutral, aerated aqueous solutions; e.g. it

was found that potential noise plots, as analyzed with

the use of power-spectral density (PSD), were simi-

lar for Pt and for an Al/SiC metal matrix composite,

which pits severely in aerated 0.5N NaCl. However,

the current-noise PSD plots were very different for the

two materials. The explanation is that mass transport of

oxygen can play a large role in potential noise.

For corrosion monitoring, speciﬁcally large-scale

corrosion monitoring and ﬁeld testing, the statistical

analysis of noise data resulting in RMS values has many

advantages over spectral analysis by FFT or MEM

which requires expensive equipment and/or compli-

cated analysis programs. It was shown [12.65] that the

noise resistance R

and the polarization resistance R

have similar values and trends; therefore theoretical

relationships between R

and R

are currently investi-

gated. Thus for cases where only qualitative results are

needed in a fast, simple and inexpensive way, collection

and evaluation of R

data is sufﬁcient. However, a quan-

titative understanding of electrochemical noise and its

relationship to corrosion, speciﬁcally to localized corro-

sion phenomena, can only be achieved through spectral

analysis of noise data, leading to the determination of

the spectral noise resistance R

which shows very

strong relations between R

and R

[12.66, 67]. The

pitting index (PI)deﬁnedas

PI =δI/I

mean

, (12.74)

where δI is the RMS current noise and I

mean

the mean

coupling current [12.63], appeared not to be applicable

for all corrosion systems for the determination of the na-

ture of the corrosion mechanism, speciﬁcally localized

corrosion; e.g. for the system steel/aqueous salt solu-

tion the PI gave misleading results [12.66,67]. This can

be understood considering that I

mean

in the above deﬁ-

nition equation should be close to zero for all cases of

uniform corrosion. PI can therefore show large ﬂuctua-

tions independent of the actual corrosion mechanism.

ENA seems to be very useful for investigation

of corrosion-inhibitor performance. The great advan-

tage is that only two metal electrodes are necessary

to measure a current signal, which indicates high or

low general corrosion rates and susceptibility to pit-

ting and other localized corrosion. Figure 12.36 gives

an example how in a very short time the critical con-

centration of a dangerous anodic inhibitor (NaNO

)for

steel corrosion in aerated 1% NaCl solution can be

evaluated. In this example two identical mild-steel elec-

trodes were placed in the inhibitor free brine. In the ﬁrst

20 min considerable current ﬂuctuation indicates typi-

cal oxygen corrosion of the steel electrodes. Addition

of 2 × 10

−3

mol/lNaNO

reduces the noise, however,

the noise level indicates insufﬁcient inhibition. After an-

other 20 min more NaNO

isaddedtomakeaNaNO

concentration of 4.2×10

mol/l, which is above the

critical concentration of NaNO

. Consequently the cur-

rent noise ceases nearly completely, indicating complete

inhibition. ENA technology today is advanced enough

to be used as a monitoring device for critical inhibitor

concentration and the start of localized corrosion.

ENA can also be applied in media with low conduc-

tivity and also in multiphase media because, in contrast

to other electrochemical methods, it needs no reference

electrode which could give measuring problems due to

high IR drops and – speciﬁcally in oil/water mixtures

– plugging or sealing of the diaphragm. In technical

systems the life of the reference electrode is also deter-

mined by the tendency of the test media to contaminate

the electrolyte in the electrode.

Part D 12.3

Corrosion 12.5 Corrosion Without Mechanical Loading 699

12.4 Exposure and On-Site Testing

There is a considerable number of sources of material

testing data that have been accumulated over the years

and are available to process plant designers to assess the

most suitable materials selection for a particular plant

and operating conditions. However, many of the fac-

tors involved in the selection of an optimum choice are

subject to variability, and this makes the choices more

complex.

The ranking of the most valuable corrosion data for

plant designers is generally understood to be as follows.

1. Operating experience on full-scale equipment under

the actual process environment.

2. Operating experience on a pilot plant with similar

feedstock and operating conditions.

3. Sample tests in the ﬁeld, coupons, electrical resis-

tance (ER) probes, or stressed samples exposed to

the process environment.

4. Laboratory evaluations on actual plant ﬂuids, or

simulated environments.

5. Materials and corrosion database.

It should also be recognized that the economics of the

various material selection choices is not constant. A de-

cision that makes economic sense at the design time, or

at the materials selection time, may not make sense by

the time the plant is actually built or operating. In ad-

dition, the economics of plant shutdowns due to failure

can depend on the market for the product being gen-

erated by the plant, although these costs are generally

always upward.

The balance between choice of a more-expensive

corrosion-resistant alloy versus the cost of subsequent

chemical treatment costs to inhibit corrosion is also

subject to change, due to changes in the raw mater-

ial costs of chromium, nickel, and other constituents in

corrosion-resistant alloys. Sometimes a less-expensive

material that would otherwise be unacceptable due to

low corrosion resistance can still be the best choice

when used with chemical corrosion inhibitors.

Corrosion testing may be conducted in the labora-

tory, pilot plant, or ﬁeld. Systems that are either in the

design phase or operating beneﬁt from laboratory and

pilot plant testing. In situ testing is useful for pilot plant

testing and for providing information for systems that

are currently operating.

Laboratory tests should simulate the water chem-

istry and operating conditions of the freshwater system.

A sample of water from the system may be used if it is

available. Immersion testing and electrochemical test-

ing (Sect. 12.1) are commonly used for laboratory tests.

In situ testing is commonly conducted in pilot

plants. This testing is useful in evaluating or monitor-

ing a water-treatment system or the effects of varying

operating parameters such as ﬂow rate and temperature.

In addition, the performance of different alloys in a sys-

tem may be studied by in situ testing. In situ testing

provides information on uniform corrosion rate, types

of corrosion, and pitting tendencies (Sect. 12.4).

In situ testing is also performed in the ﬁeld to de-

termine the effectiveness of a water treatment system,

the performance of difference alloys, and the effects of

varying operating parameters such as ﬂow rate and tem-

perature. Uniform corrosion rates, types of corrosion,

and pitting tendencies are identiﬁed with in situ testing.

Another form of testing the corrosion behavior es-

pecially for coatings is atmospheric exposure under

various climatic conditions. This type of testing yields

more reliable information on the corrosion behavior

than testing in climatic chambers. A disadvantage is the

long duration of such tests. For reasons of practicabil-

ity a mixture of laboratory testing and exposure under

natural environmental conditions is recommended.

12.5 Corrosion Without Mechanical Loading

By taking into consideration the different causes of

corrosion and their mechanisms, as well as the ap-

pearance of the attack, one can differentiate between

several types of corrosion. A common approach is

to distinguish between corrosion systems with or

without mechanical loading. In the latter case the re-

sult of corrosion is the initiation and propagation of

cracks. If only corrosion takes place and there is

no conjoint action of mechanical loading the vari-

ous forms of corrosion occurring in such systems can

either be described as local or uniform. Also cor-

rosion testing is different if mechanical loads have

to be included in the test set up. Therefore this

section is speciﬁcally dedicated to types of corro-

sion where mechanical loading is unavailable in the

system.

Part D 12.5

700 Part D Materials Performance Testing

Table 12.2 Corrosion rate conversion

mA/cm

mm year

−1

mpy gm

−2

day

−1

mA/cm

1 3.28 M/nd 129 M/nd 8.95 M/n

mm year

−1

0.306 nd/M 1 39.4 2.74 d

mpy 0.00777 nd/M 0.0254 1 0.0694 d

−2

day

−1

0.112 n/M 0.365/d 14.4/d 1

where: mpy = milliinch per year

n = number of electrons freed by the corrosion reaction

M = atomic mass

d = density

Note: you should read the table from left to right, i. e.:

1mA/cm

=(3.28M/nd)mmy

−1

=(129 M/nd)mpy= (8.95M/n)gm

−2

day

−1

For example, if the metal is steel or iron (Fe), n = 2, M =55.85 g and d =7.88gcm

−3

and the table of conversion becomes

mA/cm

mm year

−1

mpy gm

−2

day

−1

mA/cm

1 11.6 456 249

mm year

−1

0.0863 1 39.4 21.6

mpy 0.00219 0.0254 1 0.547

−2

day

−1

0.00401 0.0463 1.83 1

Note: you should read the table from left to right, i. e.: 1 mA/cm

=11.6mmy

−1

=456 mpy =249gm

−2

day

−1

12.5.1 Uniform Corrosion

Uniform corrosion is given when a metal is corroding

with the same rate over the whole surface of the metal

exposed to the corrosive environment. The extent can

be given by the weight loss per unit area or the aver-

age penetration, which is the average of the corrosion

depth. This can be determined by direct measurement

or by calculation from the weight loss per unit area

when the density of the material is known. Uniform

corrosion takes place as a rule via the effect of cor-

rosion cells without clearly deﬁned anode and cathode

surfaces. A prominent case for uniform corrosion is the

rusting of railways due to atmospheric exposure. From

a technical standpoint uniform corrosion is quite easy to

handle because the design engineer must only include

an additional thickness to the material which equals the

loss due to corrosion over the life time of the structure.

Uniform corrosion rates may be determined by

weight loss or electrochemical methods. Both methods

average a specimen’s corrosion rate over its sur-

face area. Corrosion rates calculated from weight-loss

data also average the loss over the exposure period.

Electrochemical methods may yield instantaneous or

time-averaged corrosion rates. Weight-loss testing is

performed by immersing a coupon in a test solution

either in the laboratory or in situ. Another method is

exposing a metal or a coated specimen to a speciﬁc

environment (atmosphere) or a climatic chamber and

measure the corrosion loss over a certain period of time.

It should be noted that measurements carried out in cli-

matic chambers very rarely correspond to the behavior

of a material under practical conditions. Therefore in-

vestigations in the laboratory are mostly advisable for

comparing different materials and give a ranking on

their sensitivity to corrosion in a laboratory environment

than to get reliable information on their performance in

service.

There is a correlation between the electrical param-

eters and the weight loss of the material. Table 12.2

yields information how the values correspond. The fol-

lowing charts provide a simple way to convert data

between the most common corrosion units in usage, i. e.

corrosion current (mA/cm

), mass loss (g/(m

day))

and penetration rates (mm/y or mpy) for all metals or

for steel.

Another important factor for determining uniform

corrosion and its rate of corrosion is the removal of

corrosion products from the surface to measure the cor-

rosion loss in weight units.

There are a large number of chemical solutions

available for the different materials used in practice to

remove oxide layers from surfaces. Table 12.3 yields

information on typical solutions for various materials.

Part D 12.5

Corrosion 12.5 Corrosion Without Mechanical Loading 701

Table 12.3 Some solutions used to remove corrosion products from metals

Metal Corrosion products Reagents and temperature

Iron and steel FeO; Fe

×H

O; 1. HCl (20–25%) +0.8–1% PB-5 inhibitor

×H

O 2. 0.15 l H

(speciﬁc gravity 1.84)+0.85 lH

O+1% PB-5 or

PB-8 inhibitor

3. 10% ammonium tartrate + NH

OH, 25–70

◦

4. 10% ammonium citrate +NH

OH, 25–70

◦

5. 10% H

+ 1% As

,25

◦

6. 5% NaOH + Zn (granulated or turnings), 80–90

◦

C, 30–40 min

Zinc-coated iron, Zn(OH)

;ZnCO

; 1. 10% ammonium persulfate

zinc ZnO; ZnCO

×2ZnO×3H

O; 2. Saturated CH

COO(NH

)

ZnCO

× 3Zn(OH)

Lead PbO; PbSO

; 1. Saturated CH

COO(NH

)

Pb(OH)

2. 1% CH

COOH, boiling

2PbCO

× Pb(OH)

3. 80 g/lNaOH+ 50 g/l mannite + 0.62 g/l hydrazine sulfate

Tin SnO

;SnO 1. 5% HCl

2. 15% solution of neutral sodium phosphate

Copper and its alloys Cu

O; CuO; Cu(OH)

1. 5% solution of H

CuSO

× 3Cu(OH)

; 2. 18% HCl

CuCO

× Cu(OH)

Aluminum and its alloys Al

; Al(OH)

1. 5% HNO

2. 5% HNO

(speciﬁc gravity 1.4) +1% H

3. 65% HNO

4. 20% orthophosphoric acid +8% CrO

Magnesium MgO; MgCO

1. 20% CrO

+ 1% AgNO

, boiling solution, 1 min

12.5.2 Nonuniform and Localized Corrosion

In contrast to uniform corrosion nonuniform or local-

ized corrosion is corrosion attack that is preferentially

concentrated on discrete sites of the metal surface ex-

posed to the corrosive environment. Localized corrosion

can result in, for example, pits, cracks, grooves or scars.

In any case it is the occurrence of a local nonuniform

attack due to the build up of corrosion cells over the

metal, which means a local and uneven distribution of

anodic and cathodic sites over the metal surface. What

makes the case worse is the comparatively high corro-

sion rate at the anodic sites and, especially in pitting

or crevice corrosion, self-accelerating processes occur

in the course of the corrosion process. Therefore pit-

ting and crevice corrosion are the most prominent and

dangerous forms of corrosion attack as well as inter-

crystalline corrosion which can be encountered along

or in the vicinity of grain boundaries.

Pitting

Pitting is localized corrosion which results in pits in the

metal surface (Fig. 12.37). This type of corrosion gener-

ally takes place in corrosion cells with clearly separated

anode and cathode surfaces. The anode is situated in the

pit and the cathode usually on the surrounding surface

(Fig. 12.38). Pitting usually results in worse damage

than uniform corrosion because it can lead to perfora-

tion in a very short period of time.

When evaluating attack caused by pitting the fol-

lowing should be taken into account

•

the number of pits per unit area,

•

the diameter of the pits, and

•

the depth of the pits.

The number of pits per unit area and the pit diameter are

easily determined by comparison with a pictorial stan-

dard. In the case of pit depth it is usually the maximum

depth that is determined. In some cases this means the

depth of the deepest pit observed on the sample exam-

ined, but in others the mean value of, for example, the

ﬁve deepest pits. The depth of the pits can be measured

with a microscope by focusing ﬁrst on the bottom of the

pit and then on the surface of the uncorroded metal. In

this way the distance between the two focusing levels

is obtained. The depth of pitting can also be determined

Part D 12.5

702 Part D Materials Performance Testing

500 μm

Fig. 12.37 Pitting in stainless steel

Pit: Anode

• high Cl

–

concentration

• low pH

Oxidation

Free surface: Cathode

• high pH

• high O

concentration

Reduction

–

Fig. 12.38 Principle of pitting corrosion

with a micrometer or by cutting a cross section through

the pit followed by direct measurement – possibly with

the aid of a microscope.

The ratio of the maximum pit depth (P

max

)andthe

average penetration (P

aver

) is called the pitting factor

(F)(Fig.12.39)

F =

max

aver

. (12.75)

Pitting can occur in most metals. Amongst passivated

metals this type of corrosion is initiated only above

a certain electrode potential, the pitting potential.

Especially stainless steels where the protective ef-

fect results from passivation of the steel are prone to

pitting under certain environmental conditions. Stain-

less steels are iron-based alloys in which chromium is

the main alloying additive at a concentration of at least

Initial surface

aver

max

Fig. 12.39 Pitting factor F = P

max

aver

(after [12.10])

12%. Because of the chromium content, stainless steels

are easily passivated and hence have good corrosion re-

sistance in many of the more common environments. In

unfavorable conditions, however, even stainless steels

can be subject to, for example, uniform corrosion, pit-

ting, crevice corrosion, intergranular corrosion or stress

corrosion cracking.

One can distinguish between different types of

stainless steels depending on their structure, e.g. ferritic,

austenitic and ferritic–austenitic steels. Differences in

structure convey differences in corrosion characteristics

and even differences in weldability, hardening capacity

and magnetic properties. Ferritic and ferritic–austenitic

steels are magnetic in contrast to pure austenitic steels.

TheActiveandPassiveStates

The conditions for passivation are shown in the an-

odic polarization curves of the steels. If, in the case

of a stainless steel in sulphuric acid solution, the elec-

trode potential is increased then the current density rises

to a maximum, with dissolution of the metal taking

place in the active state; the current density is an ex-

pression of the dissolution rate. At a certain potential,

the passivation potential, the corrosion current density

is drastically reduced and the metal surface becomes

passivated. Passivation is associated with the forma-

tion of a thin, protective coating which largely consists

of a mixed iron–chromium oxide and hydroxide. If the

potential is further increased to very high values the cur-

rent density will increase again as a result of, so-called,

transpassive corrosion.

Mechanism of Pitting and Crevice Corrosion

When using stainless steel in an environment with

a high chloride content, such as sea water or the bleach-

ing liquors used in the pulp industry, localized corrosion

will often occur in the form of pitting – which can some-

Part D 12.5

Corrosion 12.5 Corrosion Without Mechanical Loading 703

times result in perforation of pipe walls – or in the form

of crevice corrosion, e.g. in ﬂanged joints. These two

corrosion types are related.

Stainless steels are sensitive to localized corrosion,

especially in the presence of halogen ions. Amongst

these the chloride ion is the most corrosive and also of

greatest practical importance.

Addition of sulphide considerably increases the cor-

rosiveness of the environment and may take place, e.g.

due to the dissolution of sulphide inclusions in the steel

surface.

Pitting in stainless steel also affects the shape of the

anodic polarization curve. Thus, if the potential is in-

creased above a certain critical value, referred to as the

breakdown potential, the current density will begin to

increase and the curve often shows thereafter a series

of current peaks. Since this rise marks the beginning of

pitting the breakdown potential is in this case called the

pitting potential. If the potential is then decreased, pas-

sivation is achieved again, but only when a protection

potential, which is a little below the pitting potential,

is reached. A similar development occurs with corro-

sion in crevices or under surface deposits. The existence

and value of the pitting potential can be demonstrated

by using an auxiliary electrode and an applied volt-

age. In practice, the presence of an oxidizing agent,

e.g., oxygen, chlorine or peroxide, in the solution is of-

ten sufﬁcient to raise the potential to the pitting value

with consequent attack. The breakdown potential is not

a well-deﬁned constant but depends to a large extent on

conditions such as chloride concentration, temperature

and the method of measurement.

Localized corrosion is observed only after a cer-

tain incubation time during which the initiation of the

attack takes place. This is followed by the propaga-

tion stage and the growth of the pit. Both initiation and

propagation take place by a mechanism which involves

electrochemical corrosion cells.

Many mechanisms have been proposed for the initi-

ation stage. In some the initiation can consist of a new,

unpassivated metal surface being created as a result of

the dissolution of sulphide or sulphide/oxide inclusions

in the surface. This could then lead to an acidic solu-

tion containing a high sulphide content developing in

the incipient cavities; these conditions would not al-

low passivation of the stainless-steel surface to occur

at these sites. In other cases, initiation can be associated

with depletion of oxygen in a crevice or under a de-

posit. This would give rise to an oxygen concentration

cell with the anode in the crevice or under the deposit,

and the cathode outside these regions. Hydrolysis of an-

odically dissolved metal ions in the various situations

gives rise to acidic, unpassivating conditions at anodes.

When localized attack has been initiated and the

growth has reached steady-state conditions then the pro-

cess can be said to have reached the propagation stage.

Certain characteristic conditions now prevail in the pit,

as detailed below.

The pH value is lower than in the bulk of the so-

lution because anodically dissolved metal ions, such as

and Cr

have been hydrolyzed with the formation

of oxides, hydroxides or hydroxide salts, thus releasing

hydrogen ions. The actual pH value will depend on the

composition of the steel and will be lower the more cor-

rosion resistant the steel. Often a pH value of 0–1 can

arise.

The solution in the pit has a higher chloride con-

centration than the bulk of the solution. This is because

chloride ions migrate against the electric current in

through the mouth of the pit. In practice a chloride

concentration as high as 5 M can occur in the pit.

The resistance of stainless steels towards localized

corrosion can be evaluated by

•

determination of the breakdown potential, i. e. the

pitting potential or crevice corrosion potential; this

can be done by recording the anodic polarization

curve in a solution of chloride (American Society

for Testing and Materials (ASTM) G 61),

•

determination of the critical pitting temperature

(cpt) and the critical crevice corrosion tempera-

ture (cct); the cpt or cct, is the lowest temperature

at which attack takes place, while maintaining the

stainless steel at a constant potential.

•

total immersion testing in a suitable corrosive agent,

e.g. ferric chloride solution (ASTM G 48), followed

by measurement of the maximum pit depth; in this

type of test an elastic string or plastic disc is pressed

against the test piece and the depth of attack is

measured at the points of contact after the exposure

period.

Crevice Corrosion

Corrosion which is associated with a crevice and which

takes place in or immediately around the crevice, is

called crevice corrosion. In some cases crevice corro-

sion can simply be caused by corrosive liquid being held

in the crevice, while surrounding surfaces dry out. If the

crevice and the surrounding metal surfaces are in a solu-

tion, the liquid in the crevice can be almost stagnant. As

a result of corrosion in the crevice the conditions there

can be changed; for example, the pH value can decrease

Part D 12.5

704 Part D Materials Performance Testing

Crevice: Anode

Interstice

• low pH

• low O

concentration

• high concentration of Cl

–

, Me

, H

Oxidation

Free surface: Cathode

• high pH

• high O

concentration

Reduction

–

Fig. 12.40 Principle of crevice corrosion

and the concentration of chloride increase, and as a con-

sequence the corrosiveness can be higher in the crevice

than outside (Fig. 12.40).

When stationary conditions have been established,

anodic attack of the metal usually takes place near the

mouth of the crevice, while cathodic reduction of oxy-

gen from the surroundings takes place on the metal

surfaces outside

Anode reaction: Me →Me

+n e

−

, (12.76)

Cathode reaction:

O+2e

−

−2OH

−

(12.77)

Crevice corrosion can take place in most metals. The

risk of crevice corrosion should be especially heeded

with passive metals, e.g. stainless steel.

Crevice corrosion not only takes place in the

crevices between surfaces of the same metal, but also

when the metal is touching a nonmetallic material.

A combination of crevice corrosion and bimetallic cor-

rosion can take place when two different metals form

a crevice. The presence of Cl

−

,Br

−

or I

−

ions generally

accelerates this type of corrosion. As in the case of pit-

ting, crevice corrosion is initiated only above a certain

electrode potential.

Deposit Corrosion

Corrosion which is associated with a deposit of corro-

sion products or other substances and which takes place

under or immediately around the deposit is called de-

posit corrosion.

This type of corrosion is caused by moisture being

held in and under the deposit. Because the movement of

water is poor, corrosive conditions can be created under

the deposit in a similar way to that described in crevice

corrosion. The result is that a corrosion cell is formed

with the anode under the deposit and the cathode at, or

just outside, the edge.

Deposit corrosion is found, for example

•

under the road-mud in the wheel arch of a car,

•

under leaves which have collected in guttering, and

•

under fouling on ships’ hulls and in sea-water-

cooled condensers.

Selective Corrosion

Selective corrosion can be found in alloys and results

from the fact that the components of the alloy corrode

at different rates.

The most well-known example of selective corro-

sion is the dezinciﬁcation of brass. During dezinciﬁca-

tion the zinc is dissolved selectively, while the copper

is left as a porous mass having poor structural strength.

Similar corrosion processes are the dealuminization of

aluminum bronze and the selective dissolution of tin in

phosphor bronze.

Graphitic corrosion in grey cast iron provides an-

other example of selective corrosion where the metallic

constituents of the iron are removed. The remain-

ing graphite allows the object concerned to maintain

its shape but its strength and weight are severely

reduced.

Intergranular Corrosion

Intergranular corrosion means corrosion in or adjacent

to the grain boundaries of the metal. Metals are usu-

ally built of crystal grains. When a metal solidiﬁes or is

heat treated, several processes take place which cause

the grain-boundary region to take on other corrosion

characteristics than the main mass of the grain.

Intergranular corrosion can occur in most metals

under unfavorable conditions. Most well known is in-

tergranular corrosion in stainless steel (Fig. 12.41)as

a consequence of chromium carbide formation when the

carbon concentration is too high and unfavorable heat

treatment has occurred, e.g. in the heat-affected zone

along a weld.

Intergranular corrosion can occur in certain types

of stainless steel which have a high carbon content

(0.05–0.15% C). This can take place if the stainless steel

is heat treated so that chromium carbide precipitates in

the grain boundaries and the material is subsequently

Part D 12.5

Corrosion 12.6 Corrosion with Mechanical Loading 705

exposed to an acidic solution or sea water. The pre-

cipitation of chromium carbide takes place only under

certain conditions, for austenitic steel this is in the

temperature range 550–850

◦

C. The steel is then said

to be sensitized. The carbide precipitation results in

a narrow zone near the grain boundaries becoming so

depleted in chromium that the steel loses its stainless

character.

Sensitization is not restricted to manufacturing heat-

treatment processes. It can also occur as a result of

welding, since a region of the metal near the weld will

pass through the above temperature range. On expo-

sure to a corrosive environment the chromium-depleted

zones, together with the remaining parts of the grains,

form corrosion cells. In these the chromium-depleted

metal acts as the anode and is attacked, resulting in

intergranular corrosion.

Intergranular corrosion does not usually inﬂuence

the shape of the object but the strength characteristics

can be catastrophically reduced. In a chloride environ-

ment, e.g. sea water, the attack can be visible as pitting.

Intergranular corrosion also results in the loss of metal-

lic sound when the affected component is struck.

Countermeasures are designed so as to counteract

the precipitation of chromium carbide, which is the

cause of the sensitization. The following possibilities

exist.

•

The choice of stainless steel with a low carbon con-

tent (< 0.05% or preferably < 0.03%),

•

The choice of steel which has been stabilized by the

addition of titanium or niobium, which bind the car-

bon as carbides so that the formation of chromium

carbide is prevented,

•

The shortest possible time at 550–850

◦

Cand

•

Solution heat treatment; the material is heated to

about 1050

◦

C to dissolve precipitates of chromium

carbide that have formed, followed by subsequent

rapid cooling so that new chromium carbide precip-

itates have no time to form.

The resistance of stainless steel to intergranular corro-

sion can be evaluated through

Fig. 12.41 Intercrystalline corrosion

•

Strauss test (ASTM A 262 practice E); the test piece

is exposed to a boiling copper sulphate solution

containing sulphuric acid and metallic copper; af-

ter exposure the test piece is deformed by bending,

and the surface is then examined for cracks,

•

Streicher test (ASTM A 262 practice B); the

test piece is exposed to a boiling ferric sulphate-

sulphuric acid solution for 120 h; after the exposure

the weight loss of the test piece is determined, and

•

Huey test (ASTM A 262 practice C); the test piece is

exposed to a boiling 65% nitric acid solution; eval-

uation consists of determining weight loss; Huey

testing is particularly signiﬁcant for material that is

to be used in strongly oxidizing media.

Layer Corrosion

With layer corrosion the attack is localized to internal

layers in wrought metal. As a rule the layer is parallel

to the direction of processing and usually to the surface.

The attack can result in the unattacked layers becom-

ing detached and looking like the pages in a book. But

the result can also be the formation of blisters swelling

the metal surface, because of the voluminous corrosion

products. Layer corrosion is rather unusual. It is best

known amongst certain aluminum alloys.

12.6 Corrosion with Mechanical Loading

When investigating corrosion systems it is advisable to

distinguish between corrosion systems where no exter-

nal mechanical load is applied or residual stresses are

present and systems which include inﬂuences of me-

chanical stresses. The distinction is useful because the

forms of corrosion involved are different from their ap-

pearance as well as their mechanistic background. The

same considerations apply for the test methods. Testing

Part D 12.6

706 Part D Materials Performance Testing

of corrosion systems where mechanical stress is part of

the environmental conditions the metal is subjected to

means to study the inﬂuence of additional parameters,

e.g. mechanical stress or strain, which may be supplied

to the material in a static or dynamic regime. In sum-

mary this requires the study of the interaction of all

parameters involved and very often the mechanical pa-

rameters are of higher importance with respect to the

corrosion behavior than the electrolytic conditions.

There is also completely different equipment and

evaluation of test data. The introduction of mechani-

cal stresses into the test system needs an apparatus to

supply stresses or strains to the system in a static or dy-

namic manner and the type of specimen may be smooth,

notched or precracked, depending on the speciﬁc pa-

rameter to be studied (crack initiation or propagation).

12.6.1 Stress Corrosion

General

Stress corrosion cracking means crack initiation and

propagation in the presence of certain corrosive me-

dia under static tensile stresses. In some cases residual

stresses within the material are sufﬁcient to initiate this

form of corrosion. Contrary to most other manifesta-

tions of corrosion, the failure, due to a sudden brittle

fracture, often occurs without formation of detectable

corrosion products and is not necessarily characterized

by any previously occurring visible damage.

For assessing the corrosion, parameters of the

medium as well as of the material are important. In

practice, where complex corrosion conditions often ex-

ist, adequate knowledge of the corrosion process is

therefore required in the evaluation of failures. Stress

corrosion cracking can be subdivided into an initiation

and a propagation process. In theory, such a differentia-

tion is possible, but experimentally it is very difﬁcult to

detect as the two processes overlap during the stepwise

progress of the cracking [12.68]. Furthermore, metal

surfaces often have incipient cracks and crevices al-

ready present in the as-delivered state which could act

as crack nuclei, which means that crack initiation in

such cases is not necessary at all.

Considerable differences can be found when com-

paring experimentally determined crack-growth rates of

various metal/environment systems. This immediately

indicates that stress corrosion cracking cannot be ex-

plained satisfactorily by only one theoretical model.

The spectra of the proposed mechanisms range from

models where crack growth is regarded as a very fast

local metal dissolution at the crack tip to models that

assume that the adsorption of certain species of the

medium at the metal surface weakens the binding forces

between the metal atoms and favors brittle fracture

along the cleavage planes. A comparison of both mech-

anisms shows that in the ﬁrst case hydrogen-induced

and in the second case brittle fracture is the predomi-

nant factor. Between both cases various transitions are

possible which means that the degree of the corrosion

effect is also different [12.68].

When discussing stress corrosion cracking a dif-

ferentiation has to be made between an anodic and

a cathodic mechanism. Stress corrosion cracking is usu-

ally described as anodic when the propagation rate is

equivalent to the anodic metal dissolution rate at the

crack tip. Cathodic stress corrosion cracking (HISCC)

results from the embrittlement of the material due to

hydrogen, which is generated by cathodic hydrogen

evolution at the metal surface. Although the external ap-

pearance of the damage in both cases shows similarities,

the mechanisms responsible for the failure are different.

Anodic Stress Corrosion Cracking

Anodic stress corrosion cracking requires several pre-

conditions: (i) the electrolyte must have a speciﬁc

effect on the material, (ii) the material itself has to be

susceptible to stress corrosion cracking, and (iii) the

tensile stresses have to be sufﬁciently high. The stress

corrosion cracking susceptibility of a material is not

a material property comparable to the tensile strength

but a manifestation which the material shows under

speciﬁc conditions. It is based on a three-component

system consisting of the material (strength, microstruc-

ture, surface condition), mechanical stresses (external

stresses, residual stresses) and a speciﬁc electrolyte.

Furthermore the existence of protecting layers on the

metal surface is a necessary but not sufﬁcient prereq-

uisite for initiation of stress corrosion cracking. Local

damage of this surface layer – which could result from

chemical as well as mechanical effects – provides start-

ing points for crack formation. The mechanisms of

crack propagation can be subdivided into two groups.

The ﬁrst group considers stress corrosion cracking as

selective electrochemical metal dissolution at the crack

tip while the other group supposes that the reduction

of binding forces of the atoms at the crack tip in

the presence of speciﬁc ions or molecules is responsi-

ble [12.37].

Test Methods for Stress Corrosion Cracking

Corrosion systems with simultaneous corrosion load

and mechanical stresses are very complex as the condi-

Part D 12.6

Corrosion 12.6 Corrosion with Mechanical Loading 707

tions change with time and the changes resulting from

the parameter interactions are often difﬁcult to inter-

pret. It is therefore necessary to set up stress corrosion

cracking tests in such a way that the main parame-

ters are clearly deﬁned and measured during the test.

The existing literature provides standards [12.69, 70]

and summaries [12.71] in which all relevant aspects are

presented and discussed. In the next section the main

parameters will be discussed on the basis of these refer-

ences as these provide a basis for a practice-related test

method for assessing the susceptibility of prestressing

steels. Nearly all mechanical types of stress have been

used for investigating prestressed steels.

Aim of Stress Corrosion Test

From the deﬁnition of stress corrosion it is clear that

stress corrosion cracking is a special case of stress cor-

rosion and that under certain circumstances corrosion

will not lead to the formation of cracks.

Although there is an agreement that crack formation

is the normal test result, other manifestations have also

been found like intergranular corrosion or elongated ﬁne

cracks which are intensiﬁed in the presence of stresses.

There is a large variety of methods to assess the stress

corrosion cracking properties of metals. Each of them

has its speciﬁc advantages and disadvantages.

It is important to recall that the term test in connec-

tion with the resistance or susceptibility to stress cor-

rosion cracking has a special meaning. Whether stress

corrosion cracking occurs in a certain case or not de-

pends on the environmental and mechanical conditions

as well as the material properties of the material. The

word susceptibility does not describe a material property

or quality which can be related to an overall valid scale

as the ranking of a series of alloys can be very different

depending on the exposure conditions.

In an ideal case for a given application the likeli-

hood of occurrence of stress corrosion cracking can only

be determined by carrying out simulation tests under all

possible exposure conditions. In practice this is difﬁcult

and sometimes even impossible. Therefore, on the basis

of practical experience, a certain number of standard test

methods have been developed which give useful guid-

ance for the likely practical behavior in speciﬁc cases

of application. These laboratory standard test methods

are only suitable for such practical conditions if they are

based on experience or where relevant relations exist,

even if they are only empirical.

The fact that a given material may or may not pass

a test which was previously useful for another mater-

ial may not be signiﬁcant and, similarly, a test which

correctly distinguishes between two materials for one

application does not necessarily provide reliable evi-

dence when exposure conditions are different. Therefore

the use of a standard test for conditions outside the scope

of the test requires validation.

Selection of the Test Method

Prior to starting a programme for stress corrosion crack-

ing testing it is necessary to decide which kind of test is

suitable. Such a decision mainly depends on the purpose

of the test and the type of information to be obtained.

While some test methods try to simulate practical con-

ditions as far as possible, this being of great value to

plant engineers, other tests are directed towards eval-

uating certain mechanistic effects of failure. In the ﬁrst

case limitations with respect to e.g. material, space, time

etc. can lead to relatively simple test methods while un-

der different conditions reﬁned methods are necessary.

For example, investigations on the crack growth rate can

require the use of cracked specimens, but these are un-

suitable for evaluating the effects of surface treatments.

Although improved techniques are available the use of

a simple test can be valuable for conditions where the

more complicated techniques cannot be used.

In selecting a test method which only gives a fail-

ure/nonfailure result it is important to ensure that it is

not so severe that it leads to rejection of a material which

has already proven its suitability in a special practical

application. On the other hand it must not be so mild

that it encourages the use of a material under conditions

which could lead to rapid failure.

In general the aim of a stress corrosion cracking

test is to obtain quicker information on the prediction

of practical behavior than is possible from practical ex-

perience. There are various possibilities to achieve this,

such as using higher stresses, continuous slow straining,

cracked specimens, higher concentration of certain sub-

stances in the test environment and higher temperatures,

i. e. compared to service conditions, and also electro-

chemical stimulation. However, it is very important that

the methods used and parameters selected do not change

the details of the failure mechanism.

Loading Systems

General. The methods for loading the specimens

(smooth, notched or cracked) are usually classiﬁed as

follows

•

constant strain,

•

constant load,

•

slow strain rate.

Part D 12.6