## The critical acidification model

Galvele’s critical acidification model first introduced in 1976 allows the estimation of a critical potential, \(E_{Crit}\\) , if the concentration of aggressive species in pits or crevices is known. \(E_{Crit}\) is defined as the potential where a critical current density, \(i_{Crit}\\) , is reached. ^{1} For a given bulk environment, \({i_{Crit}}\) is a function of the pit or crevice depth, \(x\) .

According to Galvele, \(E_{Crit}\) can be estimated by measuring the anodic polarization behavior of the material of interest in pit- or crevice-like solutions prepared with reagent grade chemicals. In his seminal 1976 article, Galvele proposes that \(E_{Crit}\\) can be estimated as shown in Equation \eqref{eq:1}:

\[{E_{Crit}} = E_{OC}^* + \eta + \phi + {E_{Inh}} \label{eq:1} \tag{1}\]

where \(E_{OC}^*\) is the corrosion potential of the metal/alloy in the solution inside the pit or crevice, \(\eta\) is the anodic polarization required to reach \(i_{Crit}\) at the bottom of a pit or crevice, \(\phi\) is the ohmic potential drop along the pit or crevice, and \(E_{Inh}\) is an additional polarization required in presence of buffers or inhibitors.

### Application to engineering alloys

Although Galvele’s model provides a methodology to estimate critical potentials leading to either stable pitting or crevice corrosion, limited work has been done to verify the validity of the model using engineering alloys. In this regard, it was Galvele, Lumsden, and Staehle who first applied the critical acidification model when investigating the effect of molybdenum on the pitting corrosion resistance of stainless steels. ^{2} More recently, Rodriguez, Carranza, Hornus, Rebak, and Rincón-Ortiz applied Galvele’s equation on nickel-based alloys and some stainless steels. ^{3}^{,} ^{4}^{,} ^{5}^{,} ^{6}

The objective of this work, published in CORROSION Journal’s issue dedicated Galvele, was to evaluate the validity of the critical acidification model to predict the localized corrosion behavior of a duplex and a super duplex stainless steel in seawater exposure.

## Abstract

Crevice corrosion affects the integrity of stainless steels used in components exposed to seawater. Traditionally, crevice corrosion testing involves the use of artificial crevice formers to obtain a critical crevice potential, which is a measure of the crevice corrosion resistance of the alloy. The critical acidification model proposed by Prof. J.R Galvele predicts that the critical crevice potential is the minimum potential required to maintain an acidic solution with a critical pH inside either a pit or a crevice. Application of Galvele’s model requires an estimation of both the diffusion length and the i vs. E behavior of the metal in the solution inside the crevice.

In this work, the crevice corrosion resistance of a 22% Cr duplex stainless steel (UNS S31803) and a 25% Cr super duplex stainless steels (UNS S32750) was investigated. The \(i\) vs. \(E\) response of the two stainless steels was determined in acidified solutions of various chloride concentrations, which simulate those found in an active crevice. Critical potentials predicted by the critical acidification model were compared with critical crevice potentials measured in simulated seawater.

Results showed that despite the various assumptions and simplifications made by Galvele, the model correctly predicted the occurrence of crevice corrosion of both UNS S32750 and UNS S31803 close to room temperature in a 3.5 wt.% NaCl environment. Critical potentials obtained by Galvele’s model were similar if assuming that the chloride concentration of the simulated crevice solutions was between 7 M and 12 M acidified to a pH of 0.

## Citation

Kappes, M. A., Rincón Ortíz, M., Iannuzzi, M. & Carranza, R. M. “Use of the Critical Acidification Model to Estimate Critical Localized Corrosion Potentials of Duplex Stainless Steels.” Corrosion 73, 31–40, doi: http://dx.doi.org/10.5006/2142 (2017).

## Acknowledgements

We thank General Electric (Oil & Gas), the Argentinean Atomic Energy Committee (Comisión Nacional de Energía Atómica, in Spanish), and the Instituto Sabato for sponsoring this work.

## References

- Galvele, J. R. Transport Processes and the Mechanism of Pitting of Metals. J. Electrochem. Soc. 123, 464-474, doi:10.1149/1.2132857 (1976). ↩
- Galvele, J. R., Lumsden, J. B. & Staehle, R. W. Effect of Molybdenum on the Pitting Potential of High Purity 18% Cr Ferritic Stainless Steels. J. Electrochem. Soc. 125, 1204, doi: 10.1149/1.2131650 (1978). ↩
- Rincón-Ortíz, M., Rodríguez, M. A., Carranza, R. M. & Rebak, R. B. Determination of the Crevice Corrosion Stabilization and Repassivation Potentials of a Corrosion-Resistant Alloy. Corrosion 66, 105002-105002-105012, doi: 10.5006/1.3500830 (2010). ↩
- Hornus, E. C., Giordano, C. M., Rodriguez, M. A., Carranza, R. M. & Rebak, R. B. Effect of Temperature on the Crevice Corrosion of Nickel Alloys Containing Chromium and Molybdenum. J. Electrochem. Soc. 162, C105-C113, doi: 10.1149/2.0431503jes (2014). ↩
- Martínez, P. A., Hornus, E. C., Rodríguez, M. A., Carranza, R. M. & Rebak, R. B., "Crevice Corrosion Resistance of Super-Austenitic and Super-Duplex Stainless Steels in Chloride Solutions.," CORROSION/15, paper no. 5740 (Dallas, Texas: NACE International, 2015). ↩
- Hornus, E. C., Rodríguez, M. A., Carranza, R. M. & Rebak, R. B. Comparative Study of the Crevice Corrosion Resistance of UNS S30400 and UNS S31600 Stainless Steels in the Context of Galvele’s Model. Corrosion 73, 41-52, doi: 10.5006/2179 (2016). ↩