Corrosion inhibitors interphase

The vast majority of corrosion inhibitors in neutral environment as well as a number of acid corrosion inhibitors form protective 3D films on the metal surface ( interphase inhibition [4]). These films may consist of adsorbate multilayers, ox-ide/hydroxides, salts, or reaction products formed by interaction of the inhibitor with solution species on or near the corroding metal surface (e.g. dissolved metal ions). The type, structure, and thickness of the inhibiting films are strongly influenced by the environmental conditions. The interphase films act as a physical barrier that blocks or retards transport processes and the kinetics of the corrosion reactions at the metal surface. The inhibitive properties could, in some cases, be correlated with the chemical stability of the corresponding insoluble complexes as well as with the solubihty, adsorbabOity, and hydrophobicity of the inhibitor molecules [35]. Often, other ions from the electrolyte, such as... 

When corrosion inhibitors are present, the problems become infinitely more complex. First, inhibitors in most (perhaps all) practical systems adsorb on corrosion product layers. This notion hjis now been quite well accepted [39]. Second, most inhibitors that ire effective enough to be used in practical systems are organic in nature and more or less hydrophobic. A schematic representation of the Interphase forming in most practical systems is shown in Fig. 8. In order to visualize the effects of a potential perturbation and current flow across this interphase, a number of phenomena need to be considered. In the steady state of a corroding surface, many different transfer reactions must occur. From the basic corrosion reactions... 

The relationship between a corrosion product layer and inhibitors has also been discussed, among others, by Lorenz [66], and Lorenz and Mansfeld [67]. These authors point out that in many practical systems, for instance aerated water and carbon steel, an interaction occurs between, in this case, iron oxide and the inhibitor to the point where the inhibitor is not only adsorbed on the oxide surface, but actually incorporated into the three-dimensional oxide layer. Clearly, three-dimensional or interphase inhibition caimot be achieved in short tests or by filming procedures. Furthermore, measuring techniques have to take into consideration the altered chemical and electrochemical conditions across such bulk interphase layers. It is unfortunate that this aspect of corrosion and corrosion inhibition has not received more attention, and it is suggested that this lack of attention has seriously held back all aspects of corrosion inhibitor applications and monitoring of effectiveness. 

The starting point for such classification is the point of interference with the above sketched corrosion mechanism either in a phenomenological or in a mechanistic way, A simple system for classification, which will be discussed in more detail later, is based on whether the inhibitor interferes with the anodic or cathodic reaction. Thus inhibitors are classified as anodic or cathodic inhibitors. However, this distinction was shown to be too simplistic and a more complex classification was worked out by H. Fischer (JJ on the basis of where, instead of how, in the complex interphase of a metal-electrolyte system the inhibitor interferes with the corrosion reactions. The metal-electrolyte interphase can be visualized as consisting of (a) the interface per se, and (b) an electrolyte layer interposed between the Interface and the bulk of the electrolyte. On this basis Fisher distinguished as shown in Table 1, between "Interface Inhibition" and "Electrolyte Layer Inhibition."... 

In hydrochloric acid for instance at room temperature the corrosion rate at 95% inhibition is still several hundred mpy s. Therefore.under steady state conditions there is a flux of iron ions across the interphase which must be accommodated in the inhibition mechanism. This could be done more easily by assuming a corrosion product layer of discrete thickness composed of a complex formed from metal ions and inhibitor. Details of this model will be discussed below. 

---