In situ monitoring of corrosion mechanisms and phosphate inhibitor surface deposition during corrosion of zinc–magnesium–aluminium (ZMA) alloys using novel time-lapse microscopy

Abstract

In situ time-lapse optical microscopy was used to examine the microstructural corrosion mechanisms in three zinc-magnesium-aluminium (ZMA) alloy coated steels immersed in 1% NaCl pH 7. Preferential corrosion of MgZn(2) lamellae within the eutectic phases was observed in all the ZMA alloys followed by subsequent dissolution of Zn rich phases. The total extent and rate of corrosion, measured using time-lapse image analysis and scanning vibrating electrode technique (SVET) estimated mass loss, decreased as Mg and Al alloying additions were increased up to a level of 3 wt% Mg and 3.7 wt% Al. This was probably due to the increased presence of MgO and Al(2)O(3) at the alloy surface retarding the kinetics of cathodic oxygen reduction. The addition of 1 × 10(-2) mol dm(-3) Na(3)PO(4) to 1% NaCl pH 7 had a dramatic influence on the corrosion mechanism for a ZMA with passivation of anodic sites through phosphate precipitation observed using time-lapse image analysis. Intriguing rapid precipitation of filamentous phosphate was also observed and it is postulated that these filaments nucleate and grow due to super saturation effects. Polarisation experiments showed that the addition of 1 × 10(-2) mol dm(-3) Na(3)PO(4) to the 1% NaCl electrolyte promoted an anodic shift of 50 mV in open circuit potential for the ZMA alloy with a reduction in anodic current of 2.5 orders of magnitude suggesting that it was acting primarily as an anodic inhibitor supporting the inferences from the time-lapse investigations. These phosphate additions resulted in a 98% reduction in estimated mass loss as measured by SVET demonstrating the effectiveness of phosphate inhibitors for this alloy system.

The galvanic corrosion of welded ultra-high strength steels used for automotive applications

“…In situ time lapse optical microscopy, which allowed the imaging of immersed corroding samples at a microstructural level, was employed following a methodology developed previously [29][30] . MT8000 microscope was then manoeuvred so as to image the exposed sample area.…”

Corrosion protection by zinc‐magnesium coatings on steel studied by electrochemical methods

“…Moreover, the formation of simonkolleite is possible also in the presence of chlorides under atmospheric conditions. The protection mechanism of Mg‐containing Zn coatings has been also investigated under immersion conditions . It has been shown that magnesium delays the formation of zincite for Zn‐Mg‐Al coatings immersed in different electrolytes with pH 9 .…”

“…The protection mechanism of Mg-containing Zn coatings has been also investigated under immersion conditions. [15,18,20,21,[23][24][25][26] It has been shown that magnesium delays the formation of zincite for Zn-Mg-Al coatings immersed in different electrolytes with pH 9. [19][20] This effect was attributed to the buffering effect of magnesium and to the rapid precipitation of magnesium hydroxide on the metallic coating.…”

Mapping of Corrosion Sites in Advanced Gas-cooled Reactor Fuel Cladding in Long Term Pond Storage

“…Time-lapse Microscopy (TLM) was conducted using a Meiji MT8000 metallurgical microscope, fitted with an Infinity 2-5C camera. The technique has been well described in previous studies [149] [88] and in the experimental chapter. Once an NbC inclusion of suitable size was located (a set of three inclusions in a ≈20 µm 2 area was selected) , the specimen was connected to a Palmsens 3 potentiostat.…”

“…Breakdown potential at pit and crevice sites can be hundreds of millivolts lower than that of the metal surface; as a consequence of IR drop (which is related to pit and crevice geometry) [88]. These environments are more harsh than that of the bulk electrolyte, which help the continued aggressive growth of the excavation [13] [71].…”