The Unexpected Role of Carbonate Impurities in Polyphosphate Corrosion Inhibition

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.

Spectroscopic insights into adhesion failure at the buried epoxy‐metal interphase using AFM‐IR

“…Absorbance of infrared radiation induces abrupt thermal expansion of the sample, and this is detected by deflection of an AFM probe in contact with the surface, Figure 1. Further details of the AFM‐IR technique may be found in our previous publications where we have shown that AFM‐IR spectra obtained using this method closely match to those obtained by conventional FTIR techniques 14–17 . For the present study, AFM‐IR height images were collected in contact mode at a scan rate of 1 Hz using a gold‐coated silicon nitride probe (0.07–0.4 N/m spring constant, 13 ± 4 kHz resonant frequency, Bruker).…”

Infrared spectroscopic monitoring of solid-state processes

“…For example, steel corrosion inhibition by polyphosphonates, which is marketed, although not fully understood, was investigated by combination of RAIRS-AFM and bulk ATR IR spectroscopy. 329 It has been shown that cathodic inhibition dominates at the galvanised steel cut edge in the presence of a dissolved strontium aluminium polyphosphate inhibitor, which is caused by the rapid precipitation of the thick film to cathodic regions. EDX elemental analysis and the AFM-IR technique shown that the rapid formation of a cathodic film is in fact due to the presence of highly soluble strontium carbonate impurities in the commercially available pigment, which leads to the capture of zinc ions in the form of a zinc carbonate layer (Fig.…”

Role of Smart-Release Pigments in Preventing Corrosion Driven Cathodic Disbondment of Organically Coated Hot Dip Galvanised Steel

“…This and the previous finding is consistent with the literature; It has previously been shown that carbonate (CO 3 2− ) ions can provide modest corrosion inhibition of HDG substrates by the formation sparing soluble zinc carbonate layer (K sp = 1.5 × 10 −10 ). 72 The inability of HT CO 3 2…”