Inhibitors performance

The principles and practice of corrosion inhibition have been described in terms of the factors affecting inhibitor performance and selection (principles) and the more important practical situations in which inhibitors are used (practice). For the latter a brief account is given of the nature of the system, the reasons for inhibitor application and the types of inhibitor in use. 

Quaternary ammonium iodides were tested alone and in combination with propargyl alcohol with several steels in 15% HCl. The quaternary ammonium iodides showed superior inhibitor performance to that of propargyl alcohol (propargyl = —CH2-C=CH) at identical dosage levels. Mixtures of propargyl alcohol and quaternary anunonium iodide showed a synergistic effect [1330], as did formic acid [246] and thiols [1808]. 

Monitoring When ezetimibe is coadministered with an HMG-CoA reductase inhibitor, perform liver function tests at initiation of therapy and according to the recommendations of the HMG-CoA reductase inhibitor. At the time of hospitalization for an acute coronary event, take lipid measures on admission or within 24 hours. Liver enzymes Wnen ezetimibe is coadministered with an HMG-CoA reductase inhibitor, perform liver function tests at initiation of therapy and according to the recommendations of the HMG-CoA reductase inhibitor. 

The cooling tower, which is an efficient air scrubber can easily become a catchall for contaminants resulting from the location of the tower or from the industrial process. In arid areas, ingress of sand contributes to fouling, which reduces efficiency and contributes to biofilm and under-deposit corrosion. In coastal areas, sand laden with chlorides can cause corrosion of stainless steel components and impair chemical corrosion inhibitor performance. Heavy industries, such as steel or aluminum manufacture, produce severely contaminated cooling water resulting from direct contact with metal slags and lubricants. 

Oil is another problem. Even small traces of oil can reduce chemical inhibitor performance and impede heat transfer. Therefore it must be eliminated. It is always preferable to remove microbiological organisms, oil, and other forms of organic contaminants from a water source prior to its use as cooling system makeup. 

Each formulation is required to be present in the cooling water at a certain minimum concentration before any inhibitor effect is observed. With increase in concentration there is an increase in inhibitor performance until a certain maximum concentration is reached, beyond which no increase in performance is apparent. The improvement in performance with increase in material concentration is seldom linear. 

Polarization resistance of the corroding sample may also be monitored over an extended duration. Thus, AC impedance may be used for online monitoring of a corrosion system such as on-line determination of corrosion inhibitor performance, as depicted in Figure 1.31. 

Originally it had been intended to find a correlation between inhibitor constitution and inhibitor activity. The approach taken in evaluating the data essentially followed the thinking of other investigators in the field. It was assumed that the parameters of an adsorption isotherm would be a means to correlate chemical structure with inhibitor performance (25). 

EIS examines the response of the corroding system to ac excitations at frequencies from 100 MHz to 100 KHz. The current or voltage is measured when a small amplitude voltage or current is applied to the working electrode. The method was initially used by DoHn and Ershler [50] and developed later by Randles and Somerton [51] and Grahame [52]. In corrosion research, EIS was extensively used to study the passivation of metals [53-57] to determine corrosion rates and to study the inhibitor performance, sacrificial and barrier coating performance, and the disbondment of polymer-coated metals [9,23,58—66]. Macdonald and Me Kubre pubHshed an excellent review on the use of EIS [67]. 

Figure 3. Testing device diagram of the inhibitor performance.

We can know from Fig. 4 that under the same temperature conditions, the CO released coal samples processed by the gel foam is much lower than the raw coal samples. The inhibitor rate Eco is 45% at 100°C. We can know from the data of temperature changing (data omitted) temperature of the coal sample processed by the gel foam and CaClj is lower than the raw coal sample in the same heating time, and the inhibitor performance of gel foam excels significantly than CaClj. When the temperature of furnace is 180°C, the inhibitor performance of gel foam is the best (FT = 25%). 

Hodgekiess and Vassilion (2003) once stndied the corrosion behavior of copper-10% nickel-based alloy in NaCl solntion. They condncted the tests at 19°C and exposure time of 4 h. It was concluded that the corrosion behavior of this particnlar alloy is a function of the impingement velocity. Craddock et al. (2006) stndied the inhibitor performance of some formulated and unidentified inhibitors in sonr conditions. The inhibitors were evaluated at a dose of 50 ppm generally and at typical worst-case temperatnres in 5-h test dnration. They observed in their investigation that more rapid redaction in corrosion rate after 5 h is possible when 50-ppm formulation A was injected. This shows that the chemistry is effective in the presence of dissolved H2S, reducing the corrosion rate of brine on mild steel and maintaining it at a low level. 

Da) and high molecular weight (6000-25,000 Da) of PAA or a blend oflow molecular weight PAA and organophosphonates, giving exceUent dispersive and inhibitor performance. 

Several different terms are used in the industry to cover curing agents catalyst (not technically accurate, but widely used), hardener , or initiator , Activators (also called promoters or accelerators ) are used to speed up the cure, Inhibitors perform the opposite function. 

Many of the measurements involved in corrosion tests are defined in standards to establish precision, accuracy, representativeness, and comparability [10-14], Analytical techniques and measurements used to characterize the environment and corrosion product chemistry, and to evaluate coatings properties and inhibitor performance, are largely covered by good laboratory practices" standards [45]. In addition, the requirements for certain analytical and environmental measurements used in regulatory efforts have been established in the Code of Federal Regulations (CFR) [47]. The corrosionist should become familiar with and use these sources of information as they apply to the goals and objectives of the study. Table 5 lists some of the ASTM standards that are more widely used by corrosionists. 

Experience has shown that the third option is viable, provided that inhibitor performance has been assessed predictably. Experience has also shown dramatic failures where these conditions had not been met. Selection of a corrosion inhibitor, therefore, boils down to quantitative risk assessment and cost performance, which can only be achieved on the basis of reliable/beUevable performance data. 

FIG. 1—Inhibitor performance in constant pH kettie test note dashed iine represents Langmuir behavior. 

The world, however, is not unidimensional, and there are many more parameters that affect inhibitor performance, including their interactions. One parameter that has caused workers in this field much concern and consternation is the flowrate under which the inhibitors are tested. Table 1 below summarizes some results obtained under controlled constant pH and various degrees of agitation in Kettle Test arrangements. Even though the experimental design was not complete, the data show that with the exception of Inhibitor D the other three are either pH or flow sensitive or both. 

Furthermore, because the inhibitor performance characteristic (protection versus concentration)... 

If experiments of the above nature are carried out at different concentrations, an inhibitor performance cheu-acter-istic can be obtained. A typical example is shown in Fig. 4. In most instances, one finds that double logarithmically again a straight line is obtained. Indeed, if Eqs 1 and 2 2kre combined, one obtains a relationship between the corrosion rate and the inhibitor concentration as expressed in Eq 4. 

FIG. 9—Effect of precorrosion on inhibitor performance. Constant pH kettle test, 55°C, 1 bar CO2,100 ppm inhibitor. Corrosion rates monitored by Fe-counts or weight loss corrected LPR measurements. 

Transport of gasoline and other refined products in steel pipelines may result in corrosion products that can create a product contamination problem. Internal corrosion of the pipeline can also have an adverse effect on pipeline capacity. Corrosion results from condensation of a water film on the pipe wall plus dissolved air or SRB in the product. Corrosion control is commonly achieved by adding a corrosion inhibitor. Evaluation of inhibitor performance can be done using NACE Test Method for Antirust Properties of Cargoes in Petroleum Product Pipeline (TM0172). This test method is a modification of ASTM D 665, Test Method for Rust-Preventing Characteristics of Inhibited Mineral Oil in the Presence of Water. 

Further reference to TFE/P and corrosion inhibitor performance is given in a paper by Hull. Results for Aflas type lOOH in NACE A and B are given in Table 10. 

Aromatic nitrogen salts were extensively investigated. Halides of n-alkyl pyridine, n-aikylbenzylpyridine (Katapin in the Soviet Union)," n-alkylisoquinoline, and alkylbenzylquinolines" are all highly effective steel corrosion inhibitors. Most of these compounds are not only excellent inhibitors for iron dissolution, but also for hydrogen absorption, and they appear to work well even in the presence of HjS. Recently, Schmitt and Bedbur" systematically tested several pyridinium and quino-linium derivatives on iron and steel in deaerated 10% HCI the best corrosion inhibitor was found to be f-naphthylmethylquinolinium chloride. Interestingly, none of the inhibitors performed as well on carbon steel as on iron. Phosphoniums can also perform well, as evidenced by Sanyal and Srivastava s work on phenyl benzylphosphonium chloride. ... 

Horner et al. have studied the mechanisms by which onium compounds inhibit steel corrosion. Kichigin et al. studied tetra-n-butylammonium cation (TBA ) and other quaternary nitrogen compounds in the presence of iodide, and Aramaki et al. studied TBA+ in the presence of halides, SOj", SCN , SH , NOJ, and N3. In all these cases, halides were shown to be necessary for good inhibitor performance and the observed behavior was consistent with competitive co-adsorption of ion pairs (side by side). Sulfonium derivatives also require the presence of halide ions and, likewise, apparently co-adsorb to form ion pairs on the surface. As discussed below, unsaturated oxygen compounds, such as aldehydes and acetylenic alcohols, also require a halide ion for good performance. 

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