Corrosion testing continued inhibitors

In order to determine the effect of chemical additives on corrosion, an actual corrosion process must be taking place, so that the inhibitor test and the corrosion test are inseparable. The fact that many variables affect a corrosion process means that numerous different inhibitor tests are available. Although additive concentration is generally low, the type of system, whether once-through or recirculating, or the method of treatment, continuous or batch-wise, will determine, not only the test method, but also the inhibitor concentration required. 

The treatment of bilge water and emulsions resembles that of the treatment of oil field brines and produced water. Chen et al. [25], using ferric chloride and other chemicals to enhance the performance of Membralox 0.2, 0.5 and 0.8 pm membranes, describe permeate fluxes between 1400 and 34001/m h. Without pretreatment however severe fouling occurred as well as break-through of oil. Zaidi et al. [26] report about the continuation of this work. They quote fluxes between 800 and 12001/m h, but also mention substantial lower fluxes in long term pilot tests using 0.8 pm membranes. In addition they indicate a drop in permeate flux caused by conditions of low pH, the presence of sea water, corrosion inhibitor, oil slugs or flow variations. 

In other words, the quest for a standardized corrosion inhibitor evaluation test is as elusive as it ever was and continues to be so. 

The concentrate storage tanks for slurry pipelines are essentially large tanks with agitators (Figure 12-5). A smaU pump test loop near these tanks is used to test the concentrate before feeding it to the pipeline pumps. Feed is essentially from the thickeners, but continuous agitation in the tank and addition of viscosity control agents, corrosion inhibitors, and even some dilution water are part of the process. 

Most inhibitors are required to function in aqueous environments in various states of aeration. Furthermore, there may be a requirement for them to be added to an environment to help prevent the further development of corrosion. The data referred to above were obtained in quiescent NaCl solutions open to air. The performance of R salt inhibitors under various conditions of aeration, i.e. alternate immersion, constant immersion in a quiescent test solution and constant immersion in an aerated test solution, has been studied by Hinton (1989). The weight-loss data obtained for tests with 7075 Al alloy in 0.1 M NaCl containing 1000ppm CeCls are shown in fig. 6. The R salts are effective inhibitors even under the most severe corrosion conditions, some of which are typical of service conditions. For example, the inhibition efficiency is greatest in a continuously aerated solution. This is consistent with a mechanism of inhibition based on the formation of the R oxide on the metal surface (see sects. 2.6 and 2.8). Continuous aeration will favour the reduction of oxygen with the formation of hydroxyl ions, and consequently promotes the precipitation of the R oxide and its coverage of the metal surface. 

As with Al alloys, the efficiency of inhibition with R salts depends on the degree of aeration of the test solution (fig. 16). Hinton et al. (1988) obtained weight-loss data from tests in (a) quiescent tap water with restricted access of air, (b) quiescent tap water with unrestricted access of air, (c) continuously aerated tap water, and (d) tap water continuously recirculated using a pump and overhead spray. It was found that the CeCb inhibitor was most efficient (99.2%) in the continuously aerated water. Long term tests indicate that CeCb continues to inhibit the corrosion of mild steel over a period of several months in a recirculating water system (Hinton 1989), and that the rate of corrosion changes very little over this period of exposure. 

Loops No. 26 to 29 were run to study the effectiveness of Th, Th -f Zr, Ti -f Mg, and Ca - - Zr as inhibitors. It can be seen from these tests that these inhibitors were much inferior to Alg and Zr combinations. In tests No. 27 and 29 it was found that a slow but continuous loss of Zr, Th, and Ca occurred. Horsley [6] also ran Bi loops containing Ca and Zr as inhibitors. He reported similarly that the loops plugged, and that Ca and Zr were lost from the melt. The results of loop No. 28 indicate that Mg and Ti provide some inhibition, but arc not nearly as effective as Mg and Zr. Metallographic examination of this loop shows that the corrosion was uniform and that most of the attack took place 4 to 6 in. downstream from the "tee, in an area which is normally somewhat lower in temperature than the "tee. ... 

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