Vapor Corrosion Program Plan Page: 6 of 20
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WSRC-TR-2002-00065
2 TECHNICAL APPROACH
The approach to understand the vapor and beach-line corrosion will consist of four distinct
phases:
1. Determining the relevant environments in the vapor space of the tanks.
2. Determining the material behavior in these environments.
3. Coupon testing in vapor space of tank.
4. Development of mitigation techniques of vapor corrosion, if found to occur.
The scope of this plan is limited to Phases 1 and 2. The results of the experimentation will be
used to develop Phases 3 and 4.
The focus of the experimental investigation to date has been to attempt to reproduce crack
growth in specimens of waste tank steel in the vapor above a sodium nitrate solution known to
induce cracking. Tests on fatigue-pre-cracked ASTM A285 Grade C wedge-opening-loaded
(WOL) specimens revealed crack growth in the sodium nitrate solution at 100 C, but no growth
in the vapor (at -85 C) above the solution. No growth was seen even in a specimen that showed
growth in the liquid phase before its removal to the vapor phase. Several factors may have
contributed to SCC inhibition. The experimental technique allowed the specimens to dry each
time they were measured for crack growth. This drying may have allowed a protective oxide
film to inhibit SCC. Another factor that may have contributed to the inhibition or prevention of
SCC was the high general corrosiveness of the vapor environment. Specimens in the vapor
phase readily condensed the 100% relative humidity (RH) vapor in the sealed test vessels, and
considerable general corrosion resulted from the condensation.
Stress corrosion cracking is unlikely to occur under active corrosive conditions. SCC requires
the steel to be in an electrochemical range termed the active-passive transition. SCC exists under
conditions that are generally protective of the greater part of the metal surface, with metal
oxidation occurring only at the crack tip, but not along the passive walls of the crack. In a
generally corrosive electrochemical environment, the aqueous metal ion is thermodynamically
stable until it reacts with water, and no protective oxide layer forms and thus no part of the metal
is passive. In a locally corrosive environment an oxide layer (of varying degrees of
protectiveness) is thermodynamically favored, but is unstable due to the action of small
concentrations of aggressive species, such as nitrate (on carbon steel) or chloride (on stainless
steel).
3 ENVIRONMENTAL CONDITIONS
3.1 Vapor Corrosion Mechanism
It is hypothesized that a crack may propagate in the vapor space environment of a waste tank in
the special conditions created by relative humidity and the deliquescence behavior of aggressive
species. Specifically, sodium nitrate salt is assumed to be present on the wall and cooling coils
of waste tanks. Several mechanisms have been proposed for the deposition of aggressive species
on the tank wall within the vapor space. In dry environments the salt may exist as a solid residue
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Subramanian, K.H. Vapor Corrosion Program Plan, report, May 22, 2002; South Carolina. (https://digital.library.unt.edu/ark:/67531/metadc738714/m1/6/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.