The Unexpected Role of Carbonate Impurities in Polyphosphate Corrosion Inhibition

Suzanne Morsch,Stuart B Lyon,Matthew D Goodall,Simon R Gibbon,Seyedgholamreza Emad,Lee A Farren

doi:10.1038/s41598-018-35512-2

Suzanne Morsch, Stuart B Lyon + Show 4 more

Open Access

https://doi.org/10.1038/s41598-018-35512-2

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Abstract

Polyphosphate corrosion inhibitors are increasingly marketed as chromate replacements for coil coated steel. The mechanisms underpinning corrosion prevention by these species is, however, not fully understood; corrosion inhibition is ordinarily assessed using electrochemical techniques, followed by ex-situ surface analysis. As a result, the formation of a clear film over cathodic sites is known to contribute to corrosion prevention, but little is known about its formation. Here, we apply advanced microscopy techniques (in-situ fluid cell AFM, SEM-EDX, and AFM-IR nano-chemical analysis) to examine early cathodic film formation by strontium aluminium polyphosphate (SAPP) in detail. For a model cut edge system, it is found that cathodic inhibition dominates during the first 24 hours of immersion, and surprisingly, that strontium carbonate impurities play a significant role. Rapidly precipitated zinc carbonate provides protection almost immediately after immersion, before the film structure evolves to include (poly)phosphate species. This suggests that the purposeful inclusion of carbonates may provide a new, environmentally sound approach to enhancing inhibitor efficacy.

Highlights

Polyphosphates have emerged as commercially viable chromate replacements for some applications, but their performance is not well-understood[5,6]
Electrochemical tests were performed in saturated strontium aluminium polyphosphate (SAPP, Huebach) in electrolyte solution (3.5. % w/w NaCl)
The diffraction pattern was analysed with the X’Pert HighScore Plus Software (Malvern Panalytical) and the crystalline phases of the SAPP pigment were identified with reference to the International Centre for Diffraction Data (ICDD) database

Summary

Corrosion Inhibition

Suzanne Morsch[1], Seyedgholamreza Emad[1], Lee A. We apply advanced microscopy techniques (in-situ fluid cell AFM, SEM-EDX, and AFM-IR nano-chemical analysis) to examine early cathodic film formation by strontium aluminium polyphosphate (SAPP) in detail. Precipitated zinc carbonate provides protection almost immediately after immersion, before the film structure evolves to include (poly)phosphate species. This suggests that the purposeful inclusion of carbonates may provide a new, environmentally sound approach to enhancing inhibitor efficacy. For coil coated steel structures (e.g., architectural cladding), the cut edge remains a uniquely vulnerable point for the initiation of atmospheric corrosion. The early stages of film deposition are monitored using EIS and in-situ AFM, and analysed with EDX and the recently developed AFM-IR technique

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Results and Discussion

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