Kutz M. Handbook of materials selection

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68 STAINLESS STEELS

1 EFFECT OF ALLOYING ELEMENTS

The corrosion behavior of the alloying elements in pure form inﬂuences the

corrosion properties of the alloys.

Chromium is the ﬁrst example, with outstanding corrosion resistance in the

passive state. In solutions of neutral pH, dissolved oxygen from the air is suf-

ﬁcient to maintain passivity. But in low-pH solutions, stronger oxidizing agents

must be present, and halogen or sulfuric acids absent, in order to stabilize the

passive condition. Chromium metal is not resistant to corrosion by reducing

acids.

Some examples, from Uhlig,

of corrosion resistance of electrodeposited chro-

mium:

Acid or Salt

Concentration

(%)

Temperature

⬚C ⬚F

Corrosion Rate

(mm/ yr) (mils/ yr)

Acetic 10 58 136 0.38 15

Ferric chloride 10 58 136 0.41

Formic 10 58 136 30 1200

Hydrobromic 10 58 136 4.7 186

Hydroﬂuoric 10 12 54 2.5 1000

Phosphoric 10 58 136 0.86 34

Sulfuric 10 12 54 0.28 11

Sulfuric 10 58 136 250 10,000

Sulfuric 100 12 34 0.76 30

Sulfuric 100 58 136 1.8 69

Pitting occurred, this number does not reﬂect uniform corrosion.

Three points can be made from this data. First, chromium as an alloying

element is not particularly effective in promoting resistance to reducing or hal-

ogen acids. Second, in solutions of some halogen salts the passive layer was

maintained by oxygen dissolved in the solution. Third, sulfuric acid behaves as

a reducing acid in lower concentrations but as an oxidizing acid in concentrated

form. When selecting alloys to resist sulfuric acid, one must bear this in mind.

Stainless steels containing only chromium and iron, speciﬁcally the ferritic

and martensitic stainlesses, likewise have poor resistance to sulfuric acid solu-

tions but may resist nitric acid. These chromium–iron alloys are not resistant to

corrosion by halogen acids or by chloride salts. Those ferritic alloys that do have

good to excellent chloride pitting resistance, such as E-BRITE

威 and AL29-4C威,

gain that resistance by the addition of 1 and 4% Mo, respectively.

Austenitic nickel alloys with resistance to concentrated (oxidizing) sulfuric

acid require high chromium, such as the Krupp VDM alloy 33, or silicon as in

Haynes

威 Mickel alloy D-205, and the stainless grades A610, A611, and

Sandvik

威 SX.

There are a few highly corrosion resistant nickel alloys with little or no chro-

mium, including the various Ni–Mo ‘‘B’’ grades and the 67Ni–31Cu alloy 400.

Excellent in reducing environments, they have almost no tolerance to oxidizing

compounds in the environment. A newly developed nickel–molybdenum alloy,

1 EFFECT OF ALLOYING ELEMENTS 69

B-10, includes 8% Cr in its composition for limited resistance to low levels of

oxidizers.

Molybdenum, in contrast to chromium, has very low resistance to oxidizing

solutions but does resist reducing and halogen acids. Oxidizing acids such as

nitric, aqua regia, and concentrated sulfuric acids readily dissolve molybdenum

metal. Hydroﬂuoric acid does not affect Mo, and hot (110

⬚C) hydrochloric acid

attacks molybdenum metal only slowly.

In both stainless steels and nickel-base

alloys, molybdenum as an alloying element is required for resistance to halogen

acids and to pitting by acid or oxidizing chlorides. The amount used ranges from

2% in 316 L stainless up to 24–30% in alloys B through B-10. As an alloying

addition, molybdenum improves the stability of the passive layer in the presence

of halogens.

Tungsten at the 2–4% level is used, along with molybdenum, to improve

chloride pitting corrosion resistance.

Nickel metal in general is attacked by oxidizing solutions, while reducing

solutions are less aggressive. Some examples

follow:

Acid Notes

Concentration

(%)

Temperature

⬚C ⬚F

Corrosion Rate

(mm/ yr) (mils/ yr)

Hydrochloric Air saturated 10 30 86 2 80

Hydrochloric N

saturated 10 30 86 0.25 10

Sulfuric Aeration by convection 10 77 170 0.31 12.1

Sulfuric Air saturated 10 82 180 4 160

Nickel metal is strongly attacked by phosphoric acid solutions containing ferric

(oxidizing) salts, whereas it resists phosphoric acid solutions that are free of

oxidizing compounds. Nickel metal resists neutral chloride solutions, such as

sodium chloride, but is attacked by acid or oxidizing chloride salts. Alloys for

use in reducing acids invariably have a considerable nickel content, ranging from

as little as 8% in 304L to as much as 71% in alloy B-2.

Copper is generally resistant to reducing acid solutions containing only low

levels of oxygen but is readily attacked by oxidizing acids. These include nitric,

sulfurous, and concentrated sulfuric acids, as well as solutions containing oxi-

dizing salts, such as ferric chloride. Copper in solution tends to reduce the

corrosion rate of stainless alloys in (reducing) sulfuric acid. Those alloys in-

tended for use in environments containing much sulfuric acid invariably have

some copper as an alloy addition. These include 904L, 20Cb-3, 825, Nicrofer

威

3127 hMo (alloy 31), and the Hastelloy威 alloys G, G-3, and G-30.

Additions of some 4 or 5% silicon increase corrosion resistance to oxidizing

environments. The silicon is primarily used in alloys meant to withstand con-

centrated, hence oxidizing, sulfuric acid. Such alloys in current production in-

clude A610, A611, Sandvik SX, and Haynes D-205. The use of silicon as an

alloying element for corrosion resistance dates back to before Word War I. Al-

though not a stainless, one of the oldest and most generally corrosion-resistant

alloys ever developed is the 14.5% silicon cast iron, Duriron. A silicon oxide

ﬁlm is believed responsible for this grade’s useful resistance to environments

70 STAINLESS STEELS

ranging from oxidizing to reducing. This includes seawater, organic and many

inorganic acids, though not halogen acids. Lack of strength and ductility limit

the cast iron’s range of use.

When one speaks about the effect of this or that pure chemical on an alloy,

one must emphasize that real industrial environments are complex mixtures of

chemicals. These mixtures may behave in surprising ways; quite unlike what

one might expect from the behavior of alloys in pure, laboratory-controlled en-

vironments. Corrosion rates depend not only upon the concentrations of various

chemicals but also on the temperature. The temperature of liquid inside a vessel

is one point that can be measured, but the temperature at the surface of sub-

merged heating coils in that vessel is another, and higher value. Likewise the

concentration of, say an acid, in the vessel is not the same as the concentration

at the point where that acid is introduced to the mixture.

The most commonly used corrosion-resistant alloys are the stainless steels

304 (18% Cr 8% Ni, commonly known as 18–8 stainless) and 316 (17% chro-

mium 11% nickel 2% molybdenum). The more corrosion-resistant nickel alloys,

such as C-276, have much higher levels of nickel, 57%, and molybdenum,

15.5%. Commercially pure nickel, and nickel–copper alloys are used for special

environments.

Oxidizing and reducing environments are deﬁned chemically with respect to

whether hydrogen is oxidized or reduced under the environment in question. In

an oxidizing environment, hydrogen will only be present chemically combined

with some other element, for example, with oxygen to form H

O. In a reducing

environment, that H

⫹

will be reduced to hydrogen gas, H

Common oxidizing chemicals are nitric acid, HNO

, and certain salts such as

ferric chloride, FeCl

, and cupric chloride, CuCl

. The ferric and cupric ions are

at a relatively high valences,

⫹3 and ⫹2, respectively, and readily accept elec-

trons, or oxidize, other materials, to get their own valences reduced to a more

stable level. Sulfuric acid, H

, is normally a reducing acid. At high concen-

trations, above about 95%, sulfuric acid changes its character and becomes an

oxidizing acid. Of course, dissolved oxygen contributes to the oxidizing char-

acter of an environment. To some extent so does dissolved elemental sulfur.

To resist oxidizing conditions an alloy must contain some amount of chro-

mium. For oxidizing acid service simple materials such as 304 (18% Cr 8% Ni)

or 310 (25% Cr 20% Ni) are often used. An unusually high level of chromium,

33%, is present in a newly developed alloy, UNS R20033, meant to resist very

oxidizing acids. In any of these alloys the nickel content is necessary to make

a stable austenitic alloy, but it does not contribute speciﬁcally to oxidizing acid

resistance. Small additions of molybdenum or copper may be tolerated in these

alloys to enhance resistance to chlorides or sulfuric acid. But neither Mo nor

Cu themselves are helpful in resisting strongly oxidizing chemicals.

A common, and severe, test for resistance to oxidizing acids is boiling 65%

nitric acid. The test is run for ﬁve periods of 48 h each, specimens being weighed

after each test period, and the results averaged. This test is a good measure of

resistance to intergranular corrosion in a sensitized alloy, as well as to general

corrosion in nitric acid. Test results

show 2205 0.13–0.20 mm/yr, which is

good, 304 0.23 mm/yr, and RA333

威, which has been stabilize annealed 1700⬚F,

at 0.29 mm/yr. In the case of RA333 it is the high chromium that helps, in spite

2 SOME FORMS OF CORROSION 71

of 3% Mo. Other molybdenum bearing grades do not fare so well, 316L (2%

Mo) at 0.87 mm/yr after only 24 h, AL-6XN

威 (6.3% Mo) at 0.74 mm/yr, 625

(9% Mo) at 0.76 mm/yr, and C-276 (15.5% Mo) at 0.74 mm/yr. These results

do not mean that one cannot successfully use a higher Mo alloy in the presence

of any nitric acid at all. They do indicate that high-molybdenum alloys may not

behave at all well in hot, concentrated oxidizing industrial environments.

One cannot readily ﬁnd boiling 65% nitric acid (ASTM A262C) data for the

66% Ni–31% Ni alloy 400 (Monel

威 400) or for the assorted B alloys—B, B-2,

B-3, or B-4. Their corrosion rates in nitric acid are simply too high for the test

to have any practical value. Alloys 400, B, and B-2 have no deliberate chromium

addition, B-3 and B-4 only about 1.3% Cr. These grades have excellent resis-

tance to various reducing environments, but because there is essentially no chro-

mium present, they will literally dissolve in nitric acid. Likewise, they are

attacked by ferric, cupric, and chlorate ions, and even dissolved oxygen in HCl.

The common ‘‘reducing’’ acids are sulfuric under about 95%, phosphoric

), and hydrochloric. Of these by far the most corrosive is HCl, phosphoric

being the less troublesome. Because reducing industrial environments often do

contain some oxidizing salts or oxygen from the air, most alloys used to with-

stand reducing chemical environments will contain chromium, at least 15%. The

alloy additions used to resist the reducing components of the environment are

nickel (Ni), molybdenum (Mo), and copper (Cu).

In sulfuric acid some amount of copper is usually used, such as in 20Cb-3

stainless, 904L, or 825. Even copper salts in the acid will reduce corrosion attack

of stainless. 20Cb-3 uses carefully balanced proportions of Cu and Mo to resist

sulfuric acid corrosion.

2 SOME FORMS OF CORROSION

2.1 General Corrosion

This is the most common form of corrosion, accounting for the greatest tonnage

loss of metal. It is characterized by relatively uniform attack of the entire area

exposed to the corrosive environment. The passive ﬁlm slowly dissolves but

continually reforms. Since the attack is linear with time, the life of equipment

subject to general corrosion is reasonably predictable. If the passive ﬁlm is

locally disrupted, as by chlorides, corrosion modes such as pitting, crevice, and

stress corrosion may occur. These are more difﬁcult to predict and tend to cause

premature equipment failures. Erosion may also remove the passive ﬁlm and

contribute to much higher than expected general corrosion rates.

Stainless steel passivates simply by being exposed to air. A metallographic

specimen of AL-6XN, for example, must be etched immediately after polishing.

Otherwise it will passivate in air so that a uniform etch cannot be achieved.

Passivation in acid is not required. But, during normal fabrication practice,

enough iron is picked up to cause surface rusting in damp weather. A treatment

in nitric–hydroﬂuoric acid may be used to remove this surface iron contamina-

tion.

Uniform corrosion rates may be stated as an average metal thickness loss with

time, mils per year, or millimeter per year. A convenient rating for metals subject

to uniform attack based on corrosion rates follows:

72 STAINLESS STEELS

Excellent—rate less than 5 mils/yr (0.13 mm/yr). Metals suitable for making

critical parts.

Satisfactory—rate 5 to 50 mils/yr (0.13–1.3 mm/yr). Metals generally suit-

able for noncritical parts where a higher rate of attack can be tolerated

Unsatisfactory—rates over 50 mils/yr (1.3 mm/yr). Metals usually not ac-

ceptable in the environment.

An approximate ranking of a few common alloys by increasing resistance

to general corrosion would be 304L, 316L, 20Cb-3/825, AL-6XN, 625, and

C-276. Alloy selection does depend upon the exact corrosive environment in

question. Some speciﬁc examples include hot concentrated caustic, where com-

mercially pure nickel or the 76% nickel alloy 600 are used. For sulfuric acid

alloys 20Cb-3 or 825 are usually chosen—however, if chlorides are present in

the acid, one of the 6% molybdenum grades such as AL-6XN would be pre-

ferred. AL-6XN is used for organic acids, such as napthenic acid in reﬁnery

service. For nitric acid service chromium is beneﬁcial, molybdenum not. Alloys

commonly selected include 304L or a low carbon version of 310. RA333 is used

when the same piece of equipment must see very high temperatures, in the red

heat range, in one zone and aqueous corrosion in another.

2.2 Stress–Corrosion Cracking

For just about every alloy there is some chemical environment that, combined

with stress, will cause cracking. For brass that environment is ammonia or other

nitrogen compounds. The source of stress is usually residual forming and weld-

ing stresses, which may reach the yield point of the material. Operating stress

is rarely the issue.

For austenitic stainless steels chlorides are the major cause of stress–corrosion

cracking (SCC). An example is hot potable water under heat-transfer conditions,

which permit chlorides to concentrate locally. Susceptible alloys include 304L,

316L, 321, and 347. Some 95% of 316L chemical plant equipment failures may

be attributed to chloride stress–corrosion cracking. The chlorides concentrate

from trace amounts present in steam for heating, or the cooling water in heat

exchangers, as well as from the product.

Chloride SCC occurs most quickly in stainless steels with about 8–10%

nickel, alloys with much lower, or much higher, nickel content being less sus-

ceptible. As thermal stress relief is rarely practical with stainless fabrications,

the metallurgical solution is a change in alloy. Nickel-free ferritic steels, such

as E-BRITE are highly resistant to chloride SCC but impractical to fabricate

into a vessel.

The traditional solution in the United States has been to go to a higher nickel

alloy. Alloys with about 30% or more nickel are generally considered to be good

engineering solutions to most chloride SCC problems, although they will crack

under very severe conditions. 20Cb-3 at about 34% and 825 at 40% nickel have

long been chosen for this service. Likewise the ﬁne-grained Incoloy

威 800, UNS

2 SOME FORMS OF CORROSION 73

N08800, 31%Ni, had been used in years past. However, this low-carbon, ﬁne-

grained version of 800 is now rarely available. Regardless of what it is called,

‘‘800’’ today usually is 800HT, UNS N08811, a higher carbon, coarse-grained

version. This grade is designed to maximize creep rupture strength for high-

temperature applications.

Since the mid-1980s the 6% molybdenum superaustenitics have become avail-

able. Grades such as 254 SMO

威 with only 18% nickel, or AL-6XN at 24%

nickel, have been used effectively to resist chloride SCC. The material cost of

the ‘‘6-moly’’ grades is approximately three times that of 316L stainless. Al-

though lower in nickel, molybdenum contents above 2% tend to decrease sus-

ceptibility of austenitic stainless to chloride SCC.

For greater resistance to both corrosion and chloride SCC the most used grade

has been C-276, at 57% Ni 15.5% Mo. There are now a number of alloys in

this class, including ALLCORR

, C-22, Inconel威 686, C-2000, 59, and a new

Japanese grade, MAT 21. These very high nickel alloys can easily reach ﬁve

times or more the cost of 316L stainless. They are metallurgically excellent

solutions to chloride SCC, particularly in severe environments. However, they

are expensive choices for service conditions under which 316L lasted a few

years before cracking. There is a less expensive choice, one which has long been

used in Europe. That is a duplex stainless steel, which is about half austenite

and half ferrite. Duplex stainless steel is a practical solution to most 304L or

316L SCC failures. The most commonly available duplex in North America is

2205, at a cost roughly 20% above that of 316L.

Other forms of stress–corrosion cracking in stainless steels include caustic

cracking and polythionic acid stress–corrosion cracking. Caustic may crack car-

bon steel as well as stainless. High nickel alloys, such as alloy 600 or, better,

commercially pure nickel (UNS N02201) are used.

Polythionic acid stress–corrosion cracking (PASSC) is caused by sulfur com-

pounds in the environment and most often encountered in reﬁneries. Any stain-

less or nickel alloy that has been sensitized can be subject to PASCC. High

nickel does not help, even 600 alloy will crack when sensitized.

To resist this

form of SCC the alloy must contain a strong carbide-forming element, or ‘‘sta-

bilizing’’ element, such as columbium or titanium. Examples include 321, 347,

20Cb-3, 825, and 625. In addition the alloy must be given a stabilizing anneal

so that the carbon is effectively combined with the Cb or Ti. RA333, because

of its tungsten and molybdenum content, resists PASCC when stabilize annealed

about an hour at 1700

⬚F (927 ⬚C).

Normally, 304H or 316H would be quite sensitive to polythionic acid stress

cracking, as these higher carbon, solution-annealed alloys readily sensitize. The

matter has been addressed at one reﬁnery by fabricating the equipment from one

of these H-grade stainless steels, then stabilize annealing the completed fabri-

cation. A temperature of about 1650

⬚F (900⬚C) for a minimum of 1 h is used.

This does precipitate carbides at the grain boundaries, but temperature is high

enough to permit chromium to diffuse back into the Cr-depleted grain boundary

zone. In addition, this treatment relieves over half of the residual fabricating

stress, thus reducing susceptibility to chloride stress–corrosion cracking as well.

74 STAINLESS STEELS

2.3 Pitting Corrosion

Pitting is an extremely localized form of corrosion that results in holes in the

metal. Although total metal loss may be small, the equipment may be rendered

useless because of perforation. Pitting usually requires a long initiation period

before attack is visible. Once a pit has begun, the attack continues at an accel-

erating rate. Pits tend to grow in a manner that undermines or undercuts the

surface. Typically a very small hole is seen on the surface. Poking at this hole

with a sharp instrument may reveal a rather cavernous hole under what had

looked like solid metal. In effect, a pit may be considered a self-formed crevice.

Pitting attack increases with temperature.

Chloride solutions are the most common cause of pitting attack on stainless

steels and nickel alloys. The alloying additions of molybdenum, nitrogen, and,

to some extent, chromium, all contribute to pitting resistance. A laboratory mea-

sure of resistance to pitting corrosion is the critical pitting temperature, or CPT,

which is the highest temperature at which an alloy resists pitting in a given

environment. Alloy ranking with respect to chloride pitting resistance would be

304L (0% Mo, poor), followed by 316L (2% Mo), then four austenitics each

with about 3% molybdenum, 20Cb-3/825/317L/RA333, the duplex 2205 (3%

Mo 0.16% N), AL-6XN (6.3% Mo 0.22% N), 625 (9% Mo), and C-276 (15.5%

Mo). Alloys AL-6XN and higher have chloride sufﬁcient resistance to be used

in hot seawater service. The lower molybdenum grades, including 2205 with 3%

Mo, are unsuitable for use in seawater.

2.4 Crevice Corrosion

Crevice corrosion, more so than pitting, is the limiting condition that often pre-

vents the use of conventional austenitic stainless in chloride environments. The

attack usually occurs in small volumes of stagnant solution under gasket sur-

faces, lap joints, marine fouling, solid deposits, and in the crevices under bolt

heads and the mating surfaces of male and female threads. The mechanism

involves oxygen depletion in the crevice, followed by chloride ion concentration

and increase in acidity (decrease in pH) within the crevice. In a neutral, pH 7,

chloride solution service, the liquid within a crevice may contain 3–10 times as

much chloride as the bulk solution, and have a pH of only 2–3. Susceptibility

to crevice corrosion increases rapidly with temperature. Molybdenum and nitro-

gen additions to nickel–chromium–iron alloys improve their resistance to crevice

corrosion. Together with the use of appropriate materials, design practice to

minimize crevices and maintenance procedures to keep surfaces clean are re-

quired to combat the problem.

The usual laboratory measure of resistance to crevice corrosion is the critical

crevice corrosion temperature, or CCCT, which is the highest temperature at

which an alloy resists crevice corrosion in a given environment. For a given

environment the CCCT is usually signiﬁcantly lower than the CPT. Crevice

corrosion resistance as measured by the ferric chloride test relates, to a degree,

to performance in seawater. Here are the results for a number of alloys

—

temperature for initiation of crevice corrosion in ferric chloride (FeCl

䡠6H

O),

10% FeCl

䡠6H

O, per ASTM G48 Practice B, (PRE) N ⫽ Cr ⫹ 3.3% Mo ⫹

30% N:

2 SOME FORMS OF CORROSION 75

Alloy

(%)

Temperature

⬚C ⬚F

Pitting Resistance

Equivalent (PRE),

N Ref.

316L 2.1 ⫺327 23 5

825 2.7

⫺327 30 5

317L 3.2 2 35 29 5

2205 3.1 20 68 38 5

317LMN 4.4 20 68 34 5

28 3.5 24 75 38 6

904L 4.4 24 75 35 5

904L 4.4 25 77 35 7

G 6.5 30 86 43 5

28 3.5 35 95 39 7

2507 4.0 35 95 47 7

1925hMo

威 6.2 40 104 47 7

33 1.4 40 104 50 7

AL-6XN 6.2 43 110 48 5

625 9.0 45 113 51 5

625 9.0 55 131 51 6

31 6.5 55 131 54 6

G-30

威 5.5 50 122 48 7

C-276 15.4 55 130 66 5

2.5 Intergranular Corrosion

Intergranular corrosion consists of localized attack along the grain boundaries

of the metal. Sensitization to this attack in stainless or nickel alloys is caused

by precipitation of chromium-rich carbides in the grain boundaries, at a tem-

perature low enough that a chromium-depleted zone forms. This precipitation

most commonly occurs from the heat of welding. It may also result from a slow

cool after annealing, or from prolonged exposure to intermediate temperatures,

roughly 850–1470

⬚F (450–800⬚C), in service. For exposures at very long times,

or around the high end of this range, diffusion of chromium back into the de-

pleted zone will restore the corrosion resistance.

A most effective means of combating intergranular corrosion is to restrict the

carbon content of the alloy. In the stainless ‘‘L’’ grades 0.03% maximum is

considered sufﬁcient. High chromium and molybdenum additions, as in AL-

6XN, also reduce the chance of intergranular attack. Another approach is to add

columbium or titanium to tie up the carbon, the same as is done to resist poly-

thionic acid stress–corrosion cracking. 20Cb-3 stainless takes both approaches,

being melted to low carbon, as well as having a columbium addition.

2.6 Galvanic Corrosion

An electrical potential, or voltage, difference will exist between two different

metals that are in electrical contact and immersed in a corrosive solution. This

potential difference causes current to ﬂow and the less noble, or more anodic,

metal suffers increased corrosion rate. The severity of attack depends upon the

relative voltage difference between the metals, the relative exposed areas of each,

and the particular corrosive environment.

The most common example is the old-fashioned ﬂashlight battery, or dry cell.

It has a shell of zinc metal (less noble, or anodic), ﬁlled with a moist, corrosive

76 STAINLESS STEELS

chloride paste that conducts electricity. The center post is made of graphite,

which is quite noble (cathodic, does not tend to corrode). The potential (voltage)

difference between zinc and graphite happens to be about 1.5 V. When an electric

connection is made in a ﬂashlight, the zinc corrodes, giving up electrons, which

ﬂow through the lightbulb toward the graphite cathode, the positive pole. In this

case, because generating electricity is the point, no one minds that the zinc

corrodes and gets used up.

The ratio of cathodic (noble) to anodic areas is an important factor in galvanic

corrosion. An undesirable situation is a large cathode connected to a small anode,

or less noble metal. This can develop high current density, hence severe corro-

sion, at the anode. In that common zinc dry cell the zinc anode has a much

larger area than the graphite cathode, so it has some useful working life before

corroding through the zinc case.

For example, a large area of stainless in contact with a small surface area of

carbon steel is undesirable. The potential difference will tend to corrode the

carbon steel, and the very large area of stainless will make that corrosion occur

quickly. The reverse condition is preferred. That is, a small area of stainless (or

more noble metal) may be coupled with a much larger area of carbon steel

(anodic) without signiﬁcant problems. ‘‘Signiﬁcant’’ depends upon the applica-

tion. In the past, when ferritic stainless trim was used on carbon steel automobile

bodies, the steel would tend to corrode most severely underneath the trim. In

part that was because the crevice trapped salt, but it was accentuated by the

galvanic difference between ferritic stainless and carbon steel.

There is some potential difference among the various stainless and nickel

alloys. In practice, galvanic corrosion is rarely a problem among these various

alloys. There is, however, a signiﬁcant potential difference between copper alloys

and stainless. So long as the stainless is passive (not actively corroding) it is

enough more noble than copper to corrode the copper alloy. An example is when

a heat exchanger with a Muntz metal (60% Cu, 40% Zn) tubesheet is retubed

with AL-6XN alloy instead of the original copper alloy tubes. The potential

difference is enough to corrode the copper alloy tubesheet. One ought either to

replace the tubesheet as well, with stainless, or retube using a copper base alloy.

Graphite is at the noble end of the galvanic series. If graphite is in contact

with stainless or nickel alloys in a corrosive environment, those alloys may

corrode preferentially.

Galvanic effects have a positive side and may be used to protect equipment

from corrosion, a common example being a zinc coating on steel. The zinc

corrodes preferentially, and in doing so protects the steel from corrosion (rust-

ing). Zinc or magnesium anodes are often connected to equipment from chemical

process to steel ship hulls to protect them from corrosion.

3 AOD, DUAL CERTIFICATION, AND CHEMISTRY CONTROL

Most stainless steels, and a few higher nickel alloys, are available with different

levels of carbon. For resistance to intergranular corrosion, a low carbon is pre-

ferred, usually 0.03% carbon maximum in stainless. Such a stainless is referred

to as an ‘‘L’’ grade, e.g., 304L and 316L. With respect to aqueous corrosion

resistance, the lower the carbon, the better. For high-temperature service the

opposite is true, and some minimum amount of carbon is required for both

tensile and creep rupture strength.

3 AOD, DUAL CERTIFICATION, AND CHEMISTRY CONTROL 77

The argon–oxygen decarburization (AOD) process for reﬁning stainless steel

was introduced in the 1970s. This made profound changes in how existing grades

were produced, as well as permitting totally new grades to be developed. Three

of these changes are worth discussing—carbon, sulfur, and precise control of

chemistry.

Prior to the AOD, carbon could not be removed in the reﬁning process without

also removing chromium. Low-carbon grades could only be produced by starting

with low-carbon raw materials, speciﬁcally low-carbon ferrochrome. The ex-

pense of low-carbon ferrochrome meant that the L grades were inherently more

expensive. The AOD now permits reﬁning carbon to very low levels, even with

starting stock of higher carbon.

Industrywide speciﬁcations such as the American Society for Testing and

Materials (ASTM) were written prior to the introduction of this new melting

process. For example, ASTM A 240 for 304 stainless, UNS S30400, calls out

0.08% carbon maximum, no minimum, 30,000 psi minimum yield strength. Low

carbon 18–8, 304L, S30403 is limited to 0.03% carbon, with a consequent lower

limit for yield strength, 25,000 psi minimum. In addition, there is a 304H, meant

for high-temperature use, with carbon speciﬁed as a range, 0.04–0.10%, and

annealing and grain size requirements. This constitutes three separate grades. It

is more economical if the mills can melt steel to only two, not three, different

levels of carbon, and dual certify. Consider 304, UNS S30400. As the carbon is

speciﬁed only as a maximum, it might be possible to melt 304 to 0.03 max

carbon. Lower carbon would also result in lower than the 30,000 psi yield

strength required. However, using the AOD it is now possible to add a very

small, precisely controlled amount of nitrogen. This does not harm intergranular

corrosion resistance but it does tend to increase room temperature tensile prop-

erties. With care in annealing practice, it is possible to produce 304 with low

enough carbon to meet the 304L speciﬁcation, yet with high enough yield

strength to meet 304 requirements. As this metal meets all speciﬁed requirements

of both 304L and 304, the mill test report will show both S30403 and S30400,

i.e., dual certiﬁed.

S30403/S30400 is appropriate for corrosion service but not for high-

temperature mechanical properties. For useful creep rupture strength some min-

imum amount of carbon is required, typically 0.04%. The situation was

addressed a few years ago by adding a number of H grades to ASTM A240,

with controlled carbon for high-temperature strength. The stainless 304H,

S30409, has carbon speciﬁed 0.04–0.10% for high-temperature strength. In ad-

dition, there are grain size and minimum anneal temperature requirements. The

304, S30400, has no requirement for minimum carbon, control of grain size, or

annealing temperature. Therefore any 304H containing no more than 0.08%

carbon will meet 304 requirements, and may be dual-certiﬁed with 304. One

should note that dual certiﬁed 304L/304 is suited only for aqueous corrosion

service but would have rather low strength at high temperature. Likewise dual-

certiﬁed 304/304H is meant for high-temperature service but may be unsatis-

factory for welded construction in a wet corrosive environment. In practice, there

is rather little actual S30400 produced as sheet or plate at this time. Most is

dual certiﬁed, one way or another.

Like carbon, sulfur can now readily be reﬁned to very low levels, typically

less than 0.005%. Compare this with typical ASTM A 240 levels of 0.030% S