Organic inhibitors adsorption

In modern practice, inhibitors are rarely used in the form of single compounds — particularly in near-neutral solutions. It is much more usual for formulations made up from two, three or more inhibitors to be employed. Three factors are responsible for this approach. Firstly, because individual inhibitors are effective with only a limited number of metals the protection of multi-metal systems requires the presence of more than one inhibitor. (Toxicity and pollution considerations frequently prevent the use of chromates as universal inhibitors.) Secondly, because of the separate advantages possessed by inhibitors of the anodic and cathodic types it is sometimes of benefit to use a formulation composed of examples from each type. This procedure often results in improved protection above that given by either type alone and makes it possible to use lower inhibitor concentrations. The third factor relates to the use of halide ions to improve the action of organic inhibitors in acid solutions. The halides are not, strictly speaking, acting as inhibitors in this sense, and their function is to assist in the adsorption of the inhibitor on to the metal surface. The second and third of these methods are often referred to as synergised treatments. 

Direct measurements on metals such as iron, nickel and stainless steel have shown that adsorption occurs from acid solutions of inhibitors such as iodide ions, carbon monoxide and organic compounds such as amines , thioureas , sulphoxides , sulphidesand mer-captans. These studies have shown that the efficiency of inhibition (expressed as the relative reduction in corrosion rate) can be qualitatively related to the amount of adsorbed inhibitor on the metal surface. However, no detailed quantitative correlation has yet been achieved between these parameters. There is some evidence that adsorption of inhibitor species at low surface coverage d (for complete surface coverage 0=1) may be more effective in producing inhibition than adsorption at high surface coverage. In particular, the adsorption of polyvinyl pyridine on iron in hydrochloric acid at 0 < 0 -1 monolayer has been found to produce an 80% reduction in corrosion rate . 

It was thought that adsorption onto charcoal might prove a useful step in purification of the organic inhibitor(s). Accordingly, an extract was mixed with activated charcoal, the mixture was filtered, and the charcoal was washed with water and eluted with acetone. Both filtrate and eluate proved to be inhibitory. Either the amount of charcoal used was insufficient to adsorb all of the inhibitor, or two types of inhibitors were present, only one of which was adsorbed by charcoal. 

Thus, the filtrate, after charcoal adsorption, contains a mixture of at least two inhibitors an inorganic component which is inhibitory after ashing and an organic component (organic inhibitor A) which is responsible for most of the total inhibitory activity and is destroyed by ashing. 

Organic inhibitors in the nickel bath also influence the texture of nickel deposits. The inhibition effects are related to their molecular structure [6.69]. In the presence of brightners with unsaturated ethylenic or acetylenic compounds, the [110] texture is preferentially formed. With aryl-sulfonic compounds used as leveling agents, the [100] or [211] textures are favored. The modification of the crystal growth has been interpreted by an adsorption-hydrogenation-desorption model. The nature and the strength of a bond between a metallic surface and an adsorbed species depend on the... 

One other inhibitor type that will prevent cavitation-corrosion is the soluble oil type, which incorporates a light mineral oil plus emulsifiers and adsorption-type inhibitors, such as organic amines. Unfortunately, although effective in controlling cavitation, (possibly by cushioning effects of the adsorbed oil reinforced film) they soften and damage rubber connectors and seals, cause leakage and water loss. When emulsifiers are exhausted, the oil emulsion breaks, and allows oily films to form on heat transfer surfaces. 

The consensus is that organic compounds inhibit corrosion by adsorbing at the metal/solu-tion interface. Three possible types of adsorption are associated with organic inhibitors n-bond orbital adsorption, electrostatic adsorption, and chemisorptions. A more simplistic view of the mechanism of corrosion inhibitors can be described as controlled precipitation of the inhibitor from its environment (water and hydrocarbons) onto metal surfaces. During the past decade, the primary improvements in inhibitor technology have been the refinement of formulations and the development of improved methods of applying inhibitors (Totlani and Athavale 2000 Farquhar et al. 1994). 

Inhibition of corrosion (through deKber-ate addition of organic inhibitors) has been studied on Ni [95] and Fe [50] in many respects this aspect is an adsorption problem, and indeed Landolt and coworkers used the EQCM to test different isotherms for the selected inhibitors. 

ADSORPTION OF ORGANIC INHIBITOR MOLECULES ON METAL AND OXIDIZED SURFACES STUDIED BY ATOMISTIC THEORETICAL METHODS... 

F ure 12.29 Adsorption of an organic inhibitor onto a metal surface in aqueous environment... 

F ure 12.35 Measured adsorption isotherms of an organic inhibitor on iron oxide powder in water, 0.01 M NaC104, Na2S04 and NaCl [25]. 

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