Acid corrosion inhibition, time

Acid attack, propagation of wormholes, 619 Acid corrosion inhibition, time dependence, 644... 

Adsorption versus Polymerization. It is instructive to examine further the time dependence of the corrosion inhibition. In acid corrosion inhibition tests, steady state is customarily assumed to be reached within 10 to 20 min after initial exposure of the metal specimen. Since the inhibitors function by reducing the available active surface area, we expect an increase in and a corresponding decrease in P. The degree of corrosion protection the inhibitor provides is given by... 

Nasr-El-Din et al. (2002) have also investigated the corrosion inhibition of some aldehyde-based sulfide-snppression chemicals on steel conpons in a corrosion autoclave in acidic media. Formaldehyde was mentioned in the research, bnt other aldehyde-based chemicals were only tagged Aldehyde A, Aldehyde and so forth. Combinations of some of these aldehyde-based chemicals were also investigated. They condncted the tests at 0.5 mol% H2S with varying concentrations of sulfide scavengers at ambient, 150°F, and 275°F conditions of temperatures and at pressures of 3000 and 5000 psi. All the corrosion tests were completed at 4 h contact time. They concluded that all the chemicals are effective to some extent in controlling H2S and in corrosion inhibition. 

Next to sales contract specifications, coiTosion protection ranks highest among the reasons for the removal of acid gases. The partial pressure ol the acid gases may be used as a measure to determine whether treatment IS required. The partial pressure of a gas is defined as the total pressure of the system times the mole % of the ga,seous component. Where ( 02 is present with free water, a partial pres.sure ot. hi psia or greater would indicate that CO2 corrosion should be expected. If CO2 is not removed, inhibition and special metallurgy may be required. Below 15 psia, COt corrosion is not normally a problem, although inhibition may be required. 

This last effect may be an indication of adsorption of a small impurity in the electrolyte. The inhibited corrosion rates decrease with time and become essentially constant after about two hours. These slopes are not dependent on scan rate or on corrosion rate. The most interesting effect is observed when the inhibited hydrochloric acid solution is aerated the anodic Tafel slope increases while the cathodic Tafel slope decreases dramatically. As would have been expected from the resistance probe measurement the corrosion rate in the aerated inhibitor solution increases. 

Nitromethane can form azeotrope with many organic solvents. Additionally, it can dissolve certain polymers, nitro compounds, and nitrate compounds. Nitromethane is partially miscible with water at 20 °C, the solubility of nitromethane in water is 9.5 % (by volume) or 10.5 % (by mass), meanwhile, the solubility of water in nitromethane is 1.75 % (by mass). Generally, the miscibility of nitromethane increases with the rise of temperature. Dry nitromethane is a neutral substance, thus it possesses excellent storage stabihty. After being stored for certain period of time, wet nitromethane will become acidic, which making it capable of corroding metals, however, adding a small amount of phosphoric acid or phosphate can effectively inhibit corrosion. 

Table 3 represents an example of a comparative study of weight loss and LPR corrosion rates in inhibited hydrochloric acid at 200°F [44]. The corrosion rate was continuously monitored and plotted as a function of time. Subsequently, the area under the curve was integrated, converted to weight loss, and compared with the actual weight loss. In this manner a ratio, R, of actual weight loss versus LPR... 

The basic in vitro tests derived from teehnieal applications are the salt spray test (ASTM B-117) and the submersion test, performed in saline solutions (3.5% NaCl, ASTM G31-72 (2004) Standard Practice for Laboratory Immersion Corrosion Testing of Metals). Both tests are conducted at room temperature. By such tests the mass loss ean be determined according to equation (10.5). The mass loss is calculated after removing the corrosion products with chromic acid, which at the same time removes the corrosion products and inhibits further corrosion (Forking, 1964), which is suitable for technical applications but may raise problems in biological environments. 

The most widely used corrodent for testing precracked specimens is 3.5% NaCl solution applied dropwise to the precrack two or (usually) three times daily (Ref 43-46). This intermittent wetting technique accelerates SCC growth, but it also causes troublesome corrosion of the mechanical precrack. Less corrosive corrodents that have been used include substitute ocean water (ASTM D 1141) and an inhibited salt solution containing 0.06 M NaCl, 0.02 M sodium dichromate, 0.07 M sodium acetate, and acetic acid to pH 4 (Ref 45, 46, 50). Some investigators have tested 7000 series alloys in distilled water (Ref 47) and in water v or at 40 °C (104 °F) (Ref 54). Topical test durations thk have been used range from 200 to 2500 h. 

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