The combination of minimally invasive electrochemical investigations and FTIR‐spectroscopy to analyze atmospheric corrosion product layers on zinc

Abstract

The present work describes the combination of electrochemical investigations by using a gel-type electrolyte with Fourier-transformed infrared spectroscopy to investigate partially extremely thin corrosion product films on titanium-zinc. The gel pad method enables the determination of corrosion relevant parameters such as the potential and the linear polarization resistance without altering the corrosion product layers, which are extremely prone to re-dissolution when freshly formed. Complementary infrared spectroscopy enables the determination of main compounds of even very thin surface layers of few tenth of nanometers with a certain lateral resolution. It was found that zinc forms mostly zinc carboxyhydroxides such as hydrozincite, under various exposure conditions. The protective properties of these hydrozincite layers depend on the structure of the corrosion product film rather than on its thickness. In mid-term exposure tests, shallow corrosion pits were found even in the absence of corrosive agents such as chloride. K E Y W O R D Scorrosion investigations, gel-type electrolytes, infrared spectroscopy, titanium-zinc | INTRODUCTIONZinc plays a prominent role in corrosion prevention. Almost half of the 13 million tons of the annual production is used as anti-corrosive layer for steel components [1] particularly under atmospheric conditions. For that reason, the corrosion behavior of zinc and the formation of reaction products has been intensively investigated over decades. The recently published critical review by Wallinder and Leygraf [2] very impressively summarizes current and future research activities. The standard potential of zinc is rather negative E 0 Zn=Zn 2þ ¼ À0:76 V SHE , being reason why zinc usually dissolves in acidic solutions (Eq. (1)) under hydrogen evolution (hydrogen corrosion):This reaction occurs spontaneously (ΔG 0 < 0) and expresses the high reactivity of zinc. However, according to Kruse [3] the hydrogen corrosion of zinc in water at low temperatures is kinetically inhibited and can be neglected. Only the oxygen corrosion under formation of hydroxides has to be regarded (Eq. (2)):

Electrochemical Corrosion Investigations on Binary and Ternary Zinc Alloy Coatings using Gel Electrolytes

“…R L determined by linear polarization does not equal the pure R p as determined by fitting of EIS results. As a sum of all resistances, R L contains the electrolyte resistance, the different layer resistances, charge transfer resistance, and the resistance of the corrosion product film according to Langklotz et al [2] Thus, R L is in accordance with the sum of all resistances determined by EIS fitting since an analogous behavior in dependence of coating type and artificial aging time can be seen for all investigated samples. Valet et al have shown that R L values correspond well to EIS results and to the overall protective ability of zinc coatings and their respective corrosion products.…”

“…A Gamry Instruments Interface 1000 potentiostat/galvanostat was used as the measuring system. The influence of the iR drop in the gel electrolyte can be neglected for the measurement setup, according to the results of Langklotz et al [2] Table 1. Types of zinc coating, information on the type of post-treatment, and coating thickness.…”

“…To fully describe the temporal change of these processes, special attention is paid to the initial stage of corrosion product formation. [1,2] The formed corrosion products still show high reaction kinetics in this time period and can quickly transform or dissolve under changed corrosion conditions.For years, a significantly accelerated development of new zinc coatings and alloys for ensuring corrosion protection under service conditions has been observed. In particular, ZnAl, ZnMg, and ZnAlMg coatings are being extensively researched and their corrosion behavior is described.…”

“…To fully describe the temporal change of these processes, special attention is paid to the initial stage of corrosion product formation. [1,2] The formed corrosion products still show high reaction kinetics in this time period and can quickly transform or dissolve under changed corrosion conditions.…”

“…[23] A comprehensive introduction to advantages and drawbacks of gel electrolytes for utilization on zinc and zinc coatings was carried out in previous work. [2,23] Monrrabal et al also have also successfully used gel electrolytes for galvanized zinc and have been able to highlight the differences compared to a liquid electrolyte. It was shown that gel electrolytes inherit a lower aggressiveness than a liquid electrolyte with the same saline composition.…”