Corrosion adsorbate-catalyzed

In this paper, two studies are presented to showcase the power of UHV-electrochemical STM. The first illustration involves the dissolution of Pd that occurs only when a monolayer of iodine was present on the smface [4]. The structural features of the halogen-metal interface that accompany the adsorbate-catalyzed corrosion were first explored with low-energy electron diffraction (LEED) initial- and final-state structures were readily determined. However, questions such as the mechanism for dissolution required an in-situ technique for answers it was in this regard that electrochemical STM work was pursued [5]. 

Electrocatalysis in metallic corrosion may be classified into two groups Adsorption-induced catalyses and solid precipitate catalyses on the metal surface. In general, the bare surface of metals is soft acid in the Lewis acid-base concept and tends to adsorb ions and molecules of soft base forming the covalent binding between the metal surface and the adsorbates. The Lewis acidity of the metal surface however may turn gradually to be hard as the electrode potential is made positive, and the bare metal surface will then adsorb species of hard base such as water molecules and hydroxide ions in aqueous solution. Ions and molecules thus adsorbed on the metal surface catalyze or inhibit the corrosion processes. Solid precipitates, on the other hand, are produced by the combination of hydrated cations of hard acid and anions of hard base forming the ionic bonding between the cations and the anions on the metal surface. 

For example, the corrosion rate of steel in 0.01 % Na2Cr207 is <0.0001-in. penetration per year (i.p.y.), but on addition of NaCl to make a 3.5% solution, the rate increases to 0.0017 i.p.y., which is still a low rate. In absence of dichromate, the rate is 0.024 i.p.y. Halide ions catalyze reduction of dichromates probably by introducing imperfections in the adsorbed film (through competitive adsorption), at which areas metal ions not only enter solution but also HjO+ can discharge and hydrogen atoms can adsorb. It is probably the adsorbed hydrogen atoms that reduce adsorbed chromate, or dichromate. The situation is similar to that described by Langmuir... 

It is generally assumed that ions which can accelerate either or both partial reactions in a corrosion process are capable of being adsorbed on the iron surface. Thus it is known that hydrogen sulfide ions which accelerate both partial reactions of acid corrosion (although predominantly the anodic one), and formic acid molecules which catalyze the cathodic partial reaction but inhibit the anodic one, as well as commercial inhibitors which reduce both partial reactions, are in fact adsorbed on the iron surface. As a consequence the mere fact that adsorption takes place cannot be used to predict an expected change in corrosion rate as it is also known that halide ions cat-alize the anodic dissolution of indium, while hydroxyl adsorption catalyzes the anodic dissolution of iron. Furthermore, it is also known that certain ions can act either as a catalyst or an inhibitor when adsorbed on the metal surface depending on the type of metal considered. Kolotyrkin (18) observed that the adsorp-... 

In both mechanisms the first step is the reduction of hydrogen ions and the formation of adsorbed hydrogen atoms. At higher pH values the direct reduction of water is possible. Metals with a strong affinity to hydrogen, like the platinum group metals, catalyze the reduction and the corrosion. 

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