RANKING CORROSION INHIBITORS BY PERCENT PROTECTION MISLEADING

Frederick H. Walters, James D. Garber, Scott J. Garber
University of Southwestern Louisiana
Lafayette

Tests indicate that ranking of corrosion inhibitors by the percent of protection may be incorrect. Other conditions need to be considered because the low weight loss of a high percent of protection inhibitors may be insufficient for accurate measurement.

Sooner or later most petroleum engineers will hear or read statements saying a certain corrosion inhibitor is the best and has 95% protection on the wheel test. Is that better than a 91% inhibitor?

In fact both inhibitors may be equivalent. For low weight losses there is a wide degree of variability in the 95% confidence interval because of two problems:

Weight loss determination near the accuracy limit of the analytic balance is plus or minus 0.1 mg.
Cleanup procedures used before weighing.

One solution is to use more corrosive medium to obtain greater weight loss so that the inhibitors have a greater difference. A 20-80% weight loss range would yield more accurate results than a range above 85%.

WHEEL TEST

The wheel test is one of the oldest methods available for evaluating corrosion inhibitors.1-3 In spite of the movement toward electrochemical methods, many companies still use this gravimetric test. A number of investigators4-6 have shown that statistics can be applied to this test and that discrimination between inhibitors can be difficult.

A spectrophotometric error curve, from analytical chemistry, is believed to apply also to gravimetric corrosion tests.78 The error results in a U-shaped curve with minimum error between 20 and 70% T or 0.2 to 0.7 absorbance (Fig. 1). The percent relative error in absorbance or concentration is high because:

n the low percent transmittance range, the detector has problems sensing small amounts of light.
In the high percent transmittance range, the detector has problems discriminating between high levels of light going through the sample and reference cells.

In the wheel test, a metal strip is weighed, immersed in a sealed environment, and placed on a wheel in a thermostatted box. After rotating for 24 hr, the strip is removed, cleaned, and weighed. The weight loss is compared to a blank without the inhibitor.

The percent protection is calculated by Equation 1 in the equation box. A high percent of protection indicates low weight loss. Other factors involved-such as sample volumes, size of coupons, temperature, time, and environment-affect weight loss. In each run, several blanks are included to correct for fluctuations in operating conditions.

Normally, a series of different inhibitor concentrations are run to determine the plateau level. Fig. 2 shows a typical plot of percent protection-vs.-inhibitor concentration.

In screening inhibitors, one desires a high percent protection. However, how much better is a 93% inhibitor compared to 90% or even 88%. Without more stringent conditions, such as the weight loss exceeding 10 mg, the ranking of these three inhibitors might be inaccurate.

The low weight loss achieved by high percent protection inhibitors leads to uncertainty about the real effectiveness of the inhibitor.

EXPERIMENTAL PROCEDURE

The tests used 1010 cold rolled, 0.005 gauge steel shim stock with 1/4 in. x 6 in. dimensions. The coupons were degreased with acetone before being pre-corroded for 30 sec in 15% hydrochloric acid.

Each coupon was then placed in a 6 1/2-oz bottle, previously purged with carbon dioxide, containing 164 ml of NACE solution. Kerosine (18 ml) was added to the bottle, and it was capped after being purged with carbon dioxide. The NACE solution consisted of distilled water and the following salts: 1.77 moles/l. NaCl, 0.0983 moles/l. MgCl2, and 0.0295 moles/l. CaCl2. The kerosine was pretreated by filtering it through Fuller's earth powder.

The bottles, attached to a 3-ft diameter wheel, rotated at 30 rpm for 24 hr. Temperature for the pipeline inhibitors was 30 C., and for downhole inhibitors it was 70 C.

The inhibitor concentrations were 25, 50, 100, and 200 ppm on a total volume basis. Duplicate tests at each concentration were run and triplicate blanks were used.

The strips were retrieved after 24 hr and cleaned to bare metal with inhibited 15% hydrochloric acid and steel wool. Weight loss was calculated after drying and weighing the strips. Table 1 shows the results for 22 pipeline and 30 downhole inhibitors that were supplied by 19 major companies.

The designations are code names and not abbreviations of trade or manufacturers' names.

RESULTS

In these tests, the accuracy of the analytical balance was O.1 mg. Therefore, there would be a limitation on accuracy only for very minor weight losses. Many of the tested inhibitors had greater than 90% protection. This provides an ideal situation to demonstrate error.

The weight loss in solutions with inhibitor concentration greater than 25 ppm was essentially constant because adding more inhibitor did not significantly lessen weight loss (Fig. 2).

The standard deviation of the weight loss at 25, 50, 100, and 200 ppm divided by the overall average is called the relative standard deviation (RSD). This measures experimental error.

An RSD close to 1 represents a high degree of error while values close to zero have the least error. Figs. 3a and b plot RSD-vs.weight loss for the inhibitors studied. Both curves show large errors for small weight losses.

Pipeline inhibitors averaged 28.97 mg of weight loss and a 0.15 RSD, while downhole inhibitors averaged 51.83 mg of weight loss and only a 0.098 RSD.

Because of normal scatter, the curves are jagged. For downhole inhibitors with less than 5 mg of weight loss (90% protection) the RSD is 0.4-1.0 which indicates a 40-100% error whereas at a weight loss value of greater than 15 mg, the error is about 15%. This trend is also true for pipeline inhibitors, but because of smaller weight losses, it is difficult to get below 30% error.

Figs. 3c and d plot RSD-vs.-Percent protection for pipeline and downhole inhibitors. To obtain RSD ratios of 0.30, it is necessary to be below 65% protection for pipeline inhibitors while the corresponding value is about 80% for downhole inhibitors.

Both of these percent protection values correspond to a weight loss of about 10 mg, as seen in Figs. 3a and b. These results mean that it is more difficult to discriminate between pipelines than downhole inhibitors.

In an attempt to determine the 95% boundary limits (two standard deviations) for various percent protection values, it was necessary to model Figs 3c and d using the Number Cruncher statistical software (NCSS).

Equations 2 and 3 were developed to determine the percent protection as a function of RSD for both pipeline and downhole inhibitors.

The output from this program also gives upper and lower 95% mean values for each measured percent protection. Table 2 shows these mean values for various measured percent protection values for pipeline and downhole inhibitors, respectively. The upper and lower limit indicates the uncertainty range of the percent protection of the inhibitor.

For example, in Table 2 measured percent protection of 92% could actually be any number between 97.1 and 89.3%.

Eight of the 21 pipeline inhibitors used in the study fall into this range, and because of the large error value (RSD), the inhibitors cannot be ranked against each other.

The same percent protection in downhole inhibitors gives a similar range of 96.2-87.6.

Eighteen inhibitors were within that range. Note that as you go down in the table to lower percent protection values, there are fewer inhibitors in these ranges and the ranges become tighter because of lower RSD ratios.

These results show that the accuracy of gravimetric test methods is highly dependent on the weight loss. In this test, Increasing the temperature from 30 to 70 C. approximately doubled the weight loss of the blanks.

This allows for better discrimination between inhibitors.

In both the pipeline and downhole tests, it took a weight loss of about 10 mg before the percent error would drop below 30%.

These results are consistent with those of Dawson, et al., who stated that weight loss test methods had a high coefficient of variation of around 40%.9 The large variation found in the study is most likely because of the cleaning procedures and accuracy in weighing.

The results do show that more stringent conditions that shift the weight loss to greater than 10 mg would help reduce the percent error.

This weight loss corresponds to a percent protection of 80% for downhole inhibitors and of 65% for pipeline inhibitors.

ACKNOWLEDGMENTS

The authors would like to acknowledge the consortium of 19 oil field companies that supported a study of CO2 corrosion inhibitor testing between October 1988 and December 1990. The companies included are: Amoco Corp., ARCO Oil & Gas Co., Arkla Inc., Baker Chemical Inc., Betz Process Chemical Co., Chevron Corp., Chemlink Co., ChemRich Co., Energy Chemical Co., Kerr McGee Corp., KMCO Co., Marathon Oil Co., Nalco Chemical Co., Oryx Energy Co. (Sun E&P), Petrolite Corp., Texaco Inc., Texas Gas Co., Unichem Inc., and Welchem Corp.

REFERENCES

Caldwell, J.A., and Lytle, M.L., "A Laboratory Method for Screening Oil Well Corrosion Inhibitors, Corrosion, Vol. 9, pp. 186-89, May 1953.
Greer, E.C.; and Spalding, J.C., "Laboratory Methods for Evaluation of Inhibitors for Use in Oil and Gas Wells," Corrosion, Vol. 10, pp. 103-109, 1954.
Raifsnider, P.J., Trisider, C.S., and Wachter, A., "Laboratory Evaluation of Inhibitors for Sweet Gas-Condensate Wells," Corrosion, Vol. 11, p. 19-21, 1955.
Nathan, C.C., and Risener, E.,"Statistical Concepts in the Testing of Corrosion Inhibitors," Corrosion, Vol. 14, pp. 47-53, August 1958.
Nestle, A.C., "Simulated Field Usage Testing for Organic Inhibitors, NACE 22nd Conference," 1967, Los Angeles.
Nathan, C.C., and Dulaney, C.L., "How Statistical Concepts Facilitate Evaluation of Corrosion Inhibitors," Material Protection, February 1971, pp. 21-25.
Dennis, G.P., Hayes, J.M., and Hieftje, G.M., Chemical Separations and Measurements, Saunders, Philadelphia, 1974, pp. 637-41.
Harris, D.C., Quantitative Chemical Analysis, W. H. Freeman, New York, NY, 3rd Edition (1991), pp. 573-75.
Dawson, J.L., Miller, R.G., John, D.G., Gearcy, D., and King, R.A., "Inhibitor Evaluation Methodology for Oil Field Applications," Paper No. 391, NACE '88, 1988.