Corrosion inhibitors test procedures

Testing procedures for corrosion inhibitors in heat flux conditions are discussed below. 

The ASTM Corrosion Test procedure by Total Immersion Method requires that all specimens in a test series should have the same dimensions when comparisons are to be made. In these experiments, carbon steel was used representing the pipeline material. The coupons were cleaned, polished and weighed. Coal-water slurry, 10 to 40 weight percent, was used in the corrosion tests. For the coal-water slurry the intitial pH of the medium varied from 2.3 to 2.6 and the test series containing the nutrient media, the microorganisms and the inhibitors the pH varied from 2 to 2.5. Specimens were immersed in the reaction vessel maintained at a constant temperature of 86°F for 72 hours or the specified time. The coupons were removed, washed with deionized water, dried and weighed. The loss in weight of the specimen, before and after the test was attributed to corrosion. 

As a valuable step toward rationalizing the approval procedure for aviation fuel additives, guidelines are available (ASTM D-1655, ASTM D-4054).Tests are available for measuring or specifying additives such as color dyes (ASTM D-156, ASTM D-2392, ASTM D-5386, IP 17), corrosion inhibitors (often measured by the corrosivity of the fuel— ASTM D-130, ASTM D-5968, IP 154), lubricity (ASTM D-5001), fuel system icing inhibitors (ASTM D-910, ASTM D-4171, ASTM D-5006, IP 277), and static dissipator additives (ASTM D-2624, ASTM D-4865, IP 274). 

The industry has always attempted to standardize materials and their applications. Despite great and protracted efforts to standardize test procedures, and there are many standard corrosion tests there are no standard tests for corrosion inhibitor evaluation. The reasons for this are complex as may become clear below. However, basically it had been thought that inhibitors, once evaluated under a specific set... 

Qualifying Oilfield and Refinery Corrosion Inhibitors in the Laboratory. Such a guide, rather than specific standard test procedures, has become necessary for all the reasons spelled out in this chapter. 

It had been demonstrated [39] early on that in hydrogen sulfide corrosion the inhibitor has to be in contact with a sulfided (precorroded) surface in order to show inhibition. This observation rendered useless all those test procedures in which the inhibitor is applied to a clean metal surface in so-called film persistency tests. While it is well established that certain chemicals, such as high molecular weight fatty acids, have an affinity for metal surfaces and can form a water repellent film, they are readily replaced by other compounds that have a stronger affinity, for instance H2S. Fatty amines, on the other hand, adsorb strongly on iron sulfide, and their desorption from sulfided surfaces takes much longer them the desorption of fatty acids from bare metal surfaces in the presence of H2S. 

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. 

Any specimen preparation procedure leaves the surface in some state of roughness. It has been observed that the corrosion on poUshed specimens tends to increase in the first stages of a test, while on rough surfaces the rate decreases. Eventually, both specimens will reach a similar steady state. However, a large real surface area requires more inhibitor for inhibition than a polished one. Thus, surface preparation will affect the determination of the effective inhibitor concentration, particularly in constant inventory tests with a small liquid volume to surface area ratio. 

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