Marcus P. Corrosion mechanisms in theory and practice

Подождите немного. Документ загружается.

Intergranular corrosion does not necessarily correlate with SCC in aluminum

alloys. The 6000-series (Al-Mg-Si) alloys such as 6061 can suffer intergranular

corrosion but never fail by SCC. Examination of fractured specimens following

intergranular corrosion shows crystallographic tunneling attack on either side of

the grain boundary.

Precipitation strengthening of steels, such as the PH grades of ferritic stainless

steel containing Cu, affects SCC when the operative mechanism is hydrogen

embrittlement [91]. Precipitation, without strengthening, can occur at grain

boundaries and is responsible for the most expensive form of SCC: the cracking of

412 Newman

Figure 10 Direct and indirect evidence for a role of dealloying in SCC: SCC and

dealloying of Cu-Zn and Cu-Al monocrystals in cuprous ammonia solution as a function

of solute content [72].

Figure 11 SCC penetration as a function of potential for carbon steel and low-alloy Cr-Mo

steel tested in hot NaOH solution, showing a cracking regime at higher potential induced

by P segregation to grain boundaries [66]. See also Figure 22.

weldsensitized austenitic stainless steel in oxygenated high-temperature water

(boiling-water reactor pipe cracking) [59]. The sensitization process in AISI type 304

stainless steel consists of the precipitation of chromium carbides at the grain

boundaries in the zone either side of a weld where the temperature has reached

about 600–800°C. This leaves narrow Cr-depleted zones that repassivate more

slowly than the matrix and therefore provide an active crack path. Effective remedies

are the reduction of carbon content to about 0.03% (the 304L grade) or the addition

of alternative carbide formers (Ti, Nb, e.g., the 321 grade). The fraction of sensitized

grain boundaries required to provide a continuous crack path in slow strain rate

tests has been shown to conform to a percolation model with a percolation threshold

of about 23% [92] (Fig. 13). Under static loads the required fraction of sensitized

boundaries may be higher.

Other phase transformations occuring in austenitic and duplex stainless steels

(σ phase, α′ phase) generally produce discontinuous Cr-depleted zones around

these Cr-rich phases, so crack growth is not necessarily enhanced, although pitting

resistance is greatly degraded by σ phase, so SCC is initiated much more easily

[91]. The austenite-ferrite transformation will be considered later.

Martensite is very susceptible to hydrogen embrittlement, and strong low-alloy

steels based on tempered martensite have an inescapable tendency for hydrogen-

induced SCC in saltwater or other neutral environments [93]. Bainite can be the basis

of strong steels (although not as strong as lightly tempered martensites) that are

slightly less susceptible. Depending on the flux of hydrogen into the steel (highest in

acidic H

S, lower in NaCl or fresh water, lower still in moist air), curves of K

ISCC

versus yield strength can be drawn for martensitic steels as shown in Figure 14.

A current challenge is to improve the hydrogen embrittlement resistance of quenched

and tempered steels in the 700–850Mpa range of yield strengths so that they can be

used in wet H

S environments; one strategy is to exploit the greater resistance of

Stress-Corrosion Cracking Mechanisms 413

Figure 12 Schematic drawing of the proposed mechanism of action of group 5B elements

on chloride-SCC of austenitic stainless steels [88].

recrystallized ferrite that forms in some of these steels. Some specialized

high-strength steels can show superior performance, such as cold-drawn pearlite [94]

(prestressing wire or tire cord) and especially work-hardened, high-nitrogen austenite

[58], which is virtually immune to hydrogen embrittlement. Neither of these materials

is suitable as a general-purpose structural steel. Cold-drawn steel wire derives its

414 Newman

Figure 13 Dependence of SCC on the fraction of sensitized grain boundaries for slow

strain rate tests of type 304 stainless steel in sodium thiosulfate solution, showing behavior

in accordance with a 3D percolation model for linkage of sensitized grain boundary facets

with a sharp cracking threshold at 23% [92]. The thresholds at 65% and 89%, representing

growth of the crack as a continuous rumpled sheet, may be relevant in constant-load tests

where ductile ligaments can arrest the cracks.

Figure 14 Dependence of K

ISCC

and K

on yield strength for AISI 4340 high-strength

steel tested in flowing seawater. (Redrawn from Refs. 93 and 152.)

SCC resistance from its highly directional microstructure; the resistance of such

microstructures to transverse, intergranular SCC is well known in many systems

including aluminum alloys and steels (Fig. 15).

Duplex structures are defined as alloy microstructures in which two phases

are present in comparable proportions. Duplex (austenoferritic) stainless steels

have become very popular materials because they combine strength and SCC

resistance and can be welded with good microstructural control [91,95]. Their pitting

and crevice corrosion resistance compare favorably with those of austenitic steels

of similar Mo content.

Stress-Corrosion Cracking Mechanisms 415

Figure 15 Example of transverse SCC resistance in a highly directional microstructure:

5-mm-thick wires of martensitic prestressing steel, notched and tested at a constant load

[higher in (a) than in (b)] in 0.6 M NaCl solution. (Courtesy of A.S. Doughty, unpublished data.)

The SCC resistance of duplex stainless steels is manifested as a high value

of the threshold stress intensity, K

ISCC

, compared with austenitic steels [62] (Fig. 16).

This arises from the different chemistry of the individual phases, which gives

them, individually, different optimal ranges of potential for SCC (higher for γ,

lower for α). Because there can only be one electrode potential at the tip of the

crack, at least one of the phases must be below or above its best cracking potential

(Fig. 17). If the individual phases are synthesized and tested as bulk materials,

they both crack easily with very low K

ISCC

values [62,95], but in modern duplex

steels SCC occurs in the ferrite and is arrested by the austenite (Figs. 18a and 4c).

However, if there is a strong oxidant present, or if oxygen is supplied through a

thin boiling layer (as in evaporating seawater, which leaves behind a layer of

saturated MgCl

solution), then the austenite is no longer protected by the ferrite

(Fig. 18b) and K

ISCC

drops significantly. Such results are consistent with the

reported effect of anodic polarization in deaerated MgCl

solution [91].

Cold work reduces the ductility and fracture toughness but does not necessarily

increase the SCC velocity or reduce K

ISCC

. In austenitic stainless steels that transform

partially to martensite, the region II crack velocity in hot chloride solution is increased

by cold work, and the crack path becomes partly intergranular [57,96,97]. This

may be associated with a change of mechanism. In alloy 600 exposed to reducing

high-temperature water, the velocity of intergranular SCC increases dramatically

with increasing cold work [59,60,98]; according to Scott and Le Calvar [60],

416 Newman

Figure 16

-K curves for a duplex stainless steel and its individually synthesized phases

in hot chloride solution [62].

this may be related to enhanced intergranular oxidation, a familiar effect in

high-temperature gaseous environments.

THE ELECTROCHEMISTRY AND MECHANISMS OF SCC

There are at least five electrochemical conditions that can lead to SCC in electrolytes,

given that the material has a susceptible metallurgy; these are shown schematically

in Figure 19:

A: A state of imperfect passivity near an active-passive transition, e.g., carbon steel

in aqueous hydroxide, nitrate, or carbonate-bicarbonate solutions [26,27].

B: Astate of slow, chloride-induced localized corrosion in stainless steels, aluminum

alloys, or titanium alloys [54,68].

C: A state of surface dealloying with no continuous oxide, e.g., gold alloys in many

aqueous solutions [75,99]. Dealloying may also occur within localized corrosion

sites in passive alloys such as austenitic stainless steels.

D: Formation of unusual surface films that may play a casual role in SCC, e.g.,

nitrides formed on steel in anhydrous ammonia [69,100]. There are a number

of related, room-temperature gaseous systems such as Zr/I

[101] and high-

strength steel/Cl

[102,103]. Where such films cause only intergranular cracking

and this occurs only at elevated temperatures, it is almost certainly due to

penetration of atoms of the gaseous species along grain boundaries [60] and

not to surface mobility as proposed by Bianchi and Galvele [104].

E: An active state leading to hydrogen-induced SCC, usually in high-strength steel

[52,93,105,106] or medium-strength steels in H

S media [53,107].

The most extensive mechanistic investigations have been carried out on passive

systems showing cracking of type A or type B. All SCC mechanisms rely on the

exposure of bare metal to the environment, and if this is too brief (owing to

Stress-Corrosion Cracking Mechanisms 417

Figure 17 Schematic electrochemistry of chloride-SCC of duplex stainless steel.

immediate repassivation), none of the likely cracking agents, such as dissolution

or hydrogen, can operate. This explains the importance of a slow repassivation

rate [26] (Fig. 20). The kinetics of repassivation on bare metal surfaces have been

measured by numerous potentiostatic techniques such as scratching, rapid straining,

and laser illumination [108]. As well as active-passive transitions with potential,

similar transitions occur with pH under free-corrosion conditions and may lead to

critical pH requirements for SCC, notably for α-brass in near-neutral ammoniacal

solutions [109,110].

418 Newman

Figure 18 (a) Typical cracking pattern of chloride-SCC in a duplex stainless steel, showing

cracking of ferrite and partial arrest by austenite (courtesy of W. J. R. Nisbet). (b) Cracking

of a similar steel in evaporating seawater, showing little or no restraint of the crack by the

austenite under this more oxidizing condition (courtesy of H. Ezuber).

This classification of SCC is useful in practice, as it can lead to remedial

measures or material developments without necessitating a complete understanding

of the atomistic mechanism. For example, in type B SCC it is sufficient to prevent the

Stress-Corrosion Cracking Mechanisms 419

Figure 19 Correlation between SCC and various types of electrochemical behavior. The

letters A, B, C and E refer to the types of SCC defined in the text. The arrows indicate

possible potential ranges of SCC.

Figure 20 Correlation between inverse passivation rate (difference between current

densities in fast and slow polarization scans) and slow strain rate SCC behavior for carbon

steel in three SCC environments [26].

localized corrosion process or to ensure that it occurs too rapidly to allow crack

nucleation [68,111,112]. This topic will be discussed in detail later.

Type A SCC normally occurs by intergranular slip-dissolution and is related to

grain boundary segregation or precipitation. Most of our present understanding

has been developed by studying the passivation characteristics of bulk material

[26,27], but alloys or compounds simulating the grain boundary composition can be

useful for assessing the effect of segregation or precipitation on the repassivation

kinetics [25,84,85] (Fig. 21). Rapid solidification can be used to incorporate large

concentrations of segregants such as phosphorus [85] (Fig. 22). In a number of

systems it has been shown that the region II SCC velocity (

) correlates with the

maximum (reasonably sustained) anodic current density on a bare surface (i

max

), via

420 Newman

Figure 21 Delayed repassivation of material simulating the chromium-depleted grain

boundary zone in sensitized stainless steel, measured in sodium thiosulfate solution using

a scratch method [32].

where A is the atomic weight (relative atomic mass) of the metal and s is its

density—that is, crack growth occurs by strain-enhanced intergranular corrosion

[26]. The role of stress is to produce plastic strain in the metal, which fractures a

brittle oxide layer.

SCC data are presented as velocity versus static stress parameter such as K

, yet

it is the dynamic plasticity at the crack tip that actually features the newly formed

oxide film [67,113]. This problem was considered by Vermilyea [24], who showed

that under a static stress the corrosion process could advance the crack tip into a

region that had reached an equilibrium distribution of plastic strain (ε

), and produce

a strain transient (Δε

) that could fracture a newly formed oxide (Fig. 23). Later

Sieradzki [44] showed that the depth of transient corrosion required in Vermilyea’s

original model was unreasonably large (~ μm), confirming that static slip-dissolution

models can apply only to intergranular SCC, where there is a directional active path

with a very low repassivation rate [33]. It is not clear how the shielding of the crack

tip stress intensity by intact metal ligaments [39] enters into this argument—it

certainly makes real cracks much sharper than one would expect from fracture

mechanics.

Stress-Corrosion Cracking Mechanisms 421

Figure 22 Anodic current decays for Fe, Fe-3Ni-2Mo, and an Fe-10P amorphous alloy

(simulating a grain boundary) in 9 M NaOH solution at 98°C and –400 mV (Hg-HgO) [85].

The original i(t) decays have been converted into q(t) (charge density versus time) and then

into q(t)/t for comparison with a slip-dissolution model of SCC where the crack velocity is

proportional to q(τ)/τ with τ being the interval between film rupture events at the crack tip.

The indicated crack velocities measured for Fe-Ni-Mo and Fe-Ni-P alloys are consistent with

a single value of τ(6 ± 3 s), giving some validity to the use of a simulated grain boundary alloy.

Figure 23 Plastic strain distribution ahead of a stress-corrosion crack tip and the effect

of corrosion, according to Vermilyea [24].