Real‐time corrosion monitoring of aluminium alloys under chloride‐contaminated atmospheric conditions

Abstract

In this study, the use of electrical resistance (ER) sensors to monitor the corrosion of Al94Cu6 alloy is assessed and compared with 2024‐T3 coupons. Under uniform corrosion, a good correlation was found between the ER sensors and mass loss on coupons. Three different chloride depositions are studied: (i) pre‐contamination with dry/wet cycles, (ii) Volvo standard accelerated corrosion test and (iii) neutral salt spray test. The obtained results show good reproducibility of the ER sensors under all tested conditions. This suggests that ER sensors more levelled the effect of localised corrosion through a large surface evaluation compared with cross‐sections. The corrosion thickness obtained with the ER sensors does not correspond to the mean depth obtained by cross‐sections. This can be explained by the distribution and size of the localised corrosion events according to a finite element model proposed. The ER method allows obtaining useful real‐time corrosion data for the understanding of the corrosion mechanisms and the development of accelerated tests. The chloride concentration, the frequency of salt application and wet/dry cycles have a strong influence on the corrosion rate of aluminium alloys.

Corrosion Monitoring in Atmospheric Conditions: A Review

“…Although the CD calculated from the ER probes was not in agreement with the mean CD obtained by post mortem cross-sectioning, the good reproducibility of the former parameter was in contrast with the large deviation of the latter one. The authors proposed that the ER sensors could level the localised corrosion effect through a large surface evaluation compared to cross-sections, and verified this hypothesis using finite element modelling [29]. Li et al presented a steel thin film ER (TFER) sensor of two specific patterns, one with a single wide metal track and another with ten narrow tracks, as shown in Figure 4.…”

“…Along with the data processing uncertainties, a drawback of the ACM is its reliance on the electrolyte layer's presence. When the electrolyte is too resistive or discontinuous, the measurement is interrupted, while the corrosion process subsists [29,30]. Similarly, if the insulating material between the electrodes is hydrophobic to any degree, the electrolyte may not spread across [13].…”

“…According to their results, the electrolyte layer was discontinuous up to 30% RH, and the EIS spectra reflected the character of the electrode; in the range of 40-60% RH, the spectra corresponded to the surface electrolyte conductivity while no corrosion process was taking place; and finally, at the RH above 70%, NaCl dissolved on the surface, causing the corrosion rate to increase, and the corrosion rate was evaluated as R ct −1 [71]. It should be noticed, however, that the corrosion rate might remain non-null, even under lower RH, due to the formation of soluble salts with a low deliquescence point [29] which is not detectable by EIS.…”

“…This is avoided for metals prone only to uniform corrosion in any given environment, and ER probes were thus traditionally used for uniform corrosion detection. The appearance of non-uniform corrosion that can be recognised by a sudden sharp thickness loss was claimed to lead to underestimation or overestimation of the real corrosion rate [12,29,100]. However, in last few years, the ability of ER probes to detect maximum CD attracted more attention.…”

“…The authors emphasised that the maximum corrosion depth is important from a practical point of view and is often used as the main parameter defining corrosion degradation of automotive materials [101]. Diler et al applied the ER method for the assessment of aluminium corrosion during a chloride ACT [29]. Localised corrosion, i.e., pitting, intergranular corrosion and exfoliation, typically occurs on aluminium alloys in the presence of chlorides.…”