Marcus P. Corrosion mechanisms in theory and practice

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on nickel alloys; thus, an “equivalent EPR” is used to represent equivalent

chromium depletion profiles in nickel alloys.

For alloy 182 weld metal, an equivalent EPR in the range 10 to 20 C/cm

is used. For fully solutionized (or fully healed, i.e., no Cr depletion) alloy

600, an equivalent EPR of 0 to 5 C/cm

is appropriate, and for sensitized

alloy 600 a value of 10 to 20 C/cm

is used. Alloy 82 is higher in chromium

than alloys 600 or 182 and has less tendency to sensitize; thus an equivalent

EPR of 0 to 5 C/cm

is justified for the as-welded condition, and 5 to 10 C/cm

is appropriate for the postweld heat-treated condition. Clearly, more work is

needed to evolve a test to quantify the grain boundary compositional effects

in nickel alloys.

• Both alloy systems involve ductile face-centered cubic (fcc) structures with

similar mechanical properties. Thus, it is reasonable, in preliminary model

derivations, to use the same crack tip strain rate algorithms for ductile

nickel-base alloys as for stainless steels.

• The solubilities of the (NiCr) oxides and (FeCr) oxides are similar in BWR

water at 288°C, and thus from an electrochemical viewpoint the

rate-determining steps in the bare surface oxidation kinetics are likely to

be comparable for the nickel-base and austenitic stainless steel systems. Thus,

it is to be expected that the i

, t

and n values in Eq. (5) will be similar in the

634 Ford and Andresen

Figure 26 Crack growth rate vs. stress intensity in 288°C water for alloy 600 and alloy

182 weld metal. Data are from many sources, many of which provide only nominal water

and material chemistry conditions. This inadequate definition results in orders of magnitude

spread in crack growth rate that can mistakenly be interpreted, e.g., for a very high

dependence on stress intensity.

two systems and, hence, the general propagation Eq. (11) can be used for

nickel-base alloys.

• Studies show that the cracking responses of sensitized alloy 600/182 and

sensitized stainless steel are very similar as a function of temperature [124]

and water chemistry [125].

Despite these assumptions, the prediction capabilities are in remarkably

good agreement with the observations of stress corrosion and corrosion fatigue in

the nickel-base alloy/BWR systems. For instance, the observed crack propagation

rate–stress intensity data for alloys 182 and 600 are in reasonable agreement with

the theoretical predictions for a variety of conductivities in 288°C water containing

200 ppb oxygen (Fig. 27). Similarly, the dependence of the crack propagation rate

on the BWR water conductivity is accurately predicted for 1TCT specimens of

alloy 182 exposed to BWR recirculation water (Fig. 28). Finally, the incidence of

cracking of alloy 600 shroud head bolts as a function of average plant conductivity

is also predicted with reasonably accuracy (Fig. 29).

Corrosion in Nuclear Systems 635

Figure 27 Plot of crack propagation rate vs. stress intensity showing a comparison of

the predicted and observed crack growth rates for data on alloy 600 and alloy 182 weld

metal tested at or near constant load in 288°C water containing 200 ppb oxygen.

636 Ford and Andresen

Figure 28 Comparison of the predicted and observed crack growth rates vs. solution

conductivity for in-plant GE “CAVS” data on alloy 182 weld metal tested at constant load

in BWR water of varying chemistry (corrosion potential).

Figure 29 Predicted and observed effects of average plant water purity on crack incidence

in field correlations of creviced alloy 600 shroud head bolts.

Pressurized-Water Reactor Systems

A variety of factors distinguish most PWR systems from BWRs. On the primary

side of commercial PWRs, additions of about 1000 ppm H

, several ppm LiOH,

and more than 1 atmosphere of hydrogen are commonly made. The H

and

LiOH levels are shifted throughout the fuel cycle and provide an elevated, buffered

pH. The higher pH coupled with the hydrogen addition (which suppresses radiolysis)

yields low corrosion potentials (e.g., –700 to –950 mV

she

) and dramatically reduces

or eliminates the potential gradient in the crack which drives the anion concentration

and pH-shifting mechanism in BWR environments. On the secondary side,

allvolatile treatments (AVTs) are generally used, which also tend to produce low

corrosion potentials. However, there is a possibility for elevated corrosion potentials

from oxygen in-leakage and the presence of reducible copper ions. In addition, the

presence of sludge piles and thermal gradients in the steam generator provides an

opportunity for significant shifts in local chemistry on the secondary side.

Regardless of these chemistry differences, however, PWRs and BWRs do not

represent fundamentally different systems in terms of the concepts or modeling of

crack propagation. For example, the crack enclave in either case is fully deaerated.

Also, the presence of crack enclave potential gradients does not play a necessary or

fundamental role in environmentally assisted cracking, in that high crack propagation

rates have been observed in fully deaerated water if it is sufficiently acidic or basic

[70]. Metallurgical effects also follow a continuum, in that the minimal importance of

grain boundary chromium depletion observed in PWR environments also applies to

BWRs where the potential is lowered (e.g., due to hydrogen water chemistry) or the

crack tip pH is maintained near neutral. In PWRs other depassivation mechanisms

can come into play; e.g., at higher pH values the presence of sulfide (or other forms of

sulfur that can reduce to sulfide) above ≈ 100 ppm inhibits repassivation.

Unfortunately, few crack propagation rate data have been generated in PWR

systems; much more focus has been placed on various types of U-bend specimens.

Preliminary efforts [126] indicate that the crack growth data in PWR environments

are accurately predicted using the same modeling methodology. Given the similarity

in alloys, temperature, (crack tip) environments, etc., this is not surprising, unless

one insists that cracks propagate by fundamentally different mechanisms (e.g., slip

dissolution in BWR and hydrogen embrittlement in PWR systems).

CONCLUSIONS

A summary was presented of the unique characteristics of environmentally assisted

cracking of structural materials in high-temperature, dilute aqueous environment

systems associated with LWR operation. Candidate mechanisms of environmental

crack advance were discussed, and quantification and use of the slip dissolution

model were presented. The predictive model was shown to be quantitatively accurate

for the stainless steel–BWR water system. Extensions to address the special

characteristics of irradiation, nickel-base alloys, low-alloy and carbon steels, and PWR

environments were also presented. The advantage over existing, mechanics-based

codes of quantitative, fundamental models of environmentally assisted crack

propagation in addressing the design and life evaluation issues was also highlighted.

Corrosion in Nuclear Systems 637

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642 Ford and Andresen

Corrosion of Microelectronic and

Magnetic Data-Storage Devices

Gerald S. Frankel

The Ohio State University, Columbus, Ohio

Jeffrey W. Braithwaite

Sandia National Laboratories, Albuquerque, New Mexico

INTRODUCTION

Technological advances in microelectronic and magnetic-storage devices continue at

an incredible rate. For example, over the past 30 years, microprocessor power has

doubled every 1 ½ to 2 years. Associated with this performance improvement is the

miniaturization of integrated circuit (IC) packaging. The width and separation of

conductor lines have been cut in half approximately every 5 years and are now less

than 0.5 μm on some devices. Relative to data-storage media, the aerial bit density of

thin magnetic films has doubled every 2 years and the separation distance (fly height)

between heads and disks has been reduced to ~250 Å. These accomplishments have

been coupled with increasingly long service life and more stringent reliability

requirements.

Intuitively, because of their general use in relatively benign external

environments, the vulnerability of electronic components to corrosion-induced

failure could be considered to be less of an issue than that associated with other

equipment and structures that are commonly exposed to severe environments

(e.g., automobiles, bridges, airplanes, chemical processing, and oil production).

However, there are several reasons why corrosion in electronics remains a

major reliability concern. First and foremost, corrosion itself can directly cause

failure of the device. This first-order result is in contrast to many other corrosion

effects in which a material property is degraded and another subsequent process

actually causes the failure (e.g., metal thinning that leads to mechanical

overload or pitting that breaches housing hermeticity, thus permitting

contamination of internal parts). Second, the two main factors needed for

significant corrosion vulnerability can certainly exist: susceptible metallic materials

and aggressive environments. Metals are selected primarily for their electrical

or magnetic properties. Standard approaches to improving corrosion resistance,

such as alloying with chromium, are often not possible because of delete-

rious effects on either these properties or indirectly on the properties of other

643