The Splash

The Pursuit of a Green Carbon Steel Corrosion Inhibitor: Laying the Groundwork - Part 2

Abstract
Many inhibitors considered for use in an environmentally friendly corrosion inhibitor program are organic in nature and may be subject to oxidation. This paper aims to expand on previous studies to determine acceptable green corrosion inhibitors for use in open system cooling waters by examining the effect of oxidizing biocide on their performance. Pilot-scale corrosion testing is used to generate results from corrosion coupons and corrator electrodes under chlorinated synthetic water conditions. Several carbon steel corrosion inhibitors are examined and their performance monitored under conditions of continuous chlorination over five days. The results are used to further validate and select the most appropriate and cost effective product for use as a green corrosion inhibitor.

Introduction
Organic and inorganic corrosion inhibitors and combinations thereof have been used for many years to reduce corrosion of mild steel in industrial heat exchange equipment. It is important that the inhibitors used for mild steel corrosion protection be as safe to use as possible and be environmentally friendly. In previous work, the pursuit of a green carbon steel corrosion inhibitor led to examination of several commercially available inhibitors under synthetic laboratory conditions.1 This paper is a continuation of that quest by concentrating on many of these same inhibitors and the effect of oxidizing biocide, such as sodium hypochlorite, on their performance.

It is known that oxidation of inhibitors can affect their performance. For instance, reversion of some phosphonates to orthophosphate is quite common under the right oxidation conditions.2 Some manufacturers have suggested that their phosphorous based organic inhibitors be protected with amine to prevent this phenomenon.2 It is thought that combined chlorine (e.g. chloramines) is less destructive of inhibitor structures than free chlorine. Another example of a performance issue for inhibitors is the oxidation of azoles and the resultant increase in yellow metal corrosion if too much azole is lost in the affected system. This can lead to a serious secondary problem of increased mild steel corrosion caused by plating of the copper onto the mild steel.3 Some water technology providers have recognized this serious problem and used various methods to combat the loss of azole due to oxidizing biocides.4

Oxidizing biocides like sodium hypochlorite are used to reduce biological problems in cooling systems. They also minimize loss of heat transfer and minimize health related issues like Legionella pneumophila. Biological slimes can lead to under-deposit corrosion and efficiency loss due to a combination of organic and inorganic scale deposits.5 Legionella is a serious health concern and has even lead to the death of affected individuals.6 Aside from possible negative health consequences, poor biological control can also be a great liability to facilities and management for unprotected systems. Although oxidizing biocides perform the very necessary function of minimization of biological problems, they are also known to reduce the efficiency of some scale and corrosion inhibitors as previously mentioned. One of the main purposes of this current research is to identify green inhibitors that are least affected by oxidation. The relationship between inhibitor choice, mild steel corrosion rates, and copper corrosion rates was also investigated in this work.

Methodology
As was the case in previous work, the goal is to be as green as possible while performing adequately for the purpose intended. In the first paper, green criteria were identified as high biodegradability, low ecological toxicity, favorable NPDES status, minimal heavy metals, good safety profile, and status as a drinking water additive. Certain inhibitors were identified to be more favorably green than others, including: aspartic acid polymer (AAP), phosphono-carboxylic acid mixture (PCM), hydroxyphosphonic acid (HPA), polyamino-phosphonate (PAP), and stannous chloride. Corrosion inhibition mechanisms of these inhibitors are discussed in more detail in previous work.1 This work concentrates on testing the organic inhibitors and their possible reaction with an oxidizing biocide such as sodium hypochlorite. An enhanced phosphono-carboxylate (EPOC) has been added to the list of organic inhibitors investigated in this work while stannous chloride has been omitted from the testing because it is inorganic.

Experimental Procedure
The conditions of the test were identical to those found in our previous work, except for the automatic addition of a bleach solution based on an ORP probe. After 24 hours of a test run, the ORP set point was increased by 100 mV. The free and total chlorine levels were intentionally high at 0.5–1.5 and 1.0–2.5, respectively, to simulate a system that did not have good control, as can be commonly found in field conditions. This also provided circumstances that were conducive to comparing inhibitors to a control. Test runs were five days long during which mild steel and copper corrator data was collected. Appearance of corrosion coupons was also observed. Most of the tests were run using a synthetic water that was both corrosive and scaling, shown in Table 1. Later tests runs were done using a water that would be considered low hardness to simulate a soft water system, shown in Table 2.Figure1

Test Apparatus
Figure 1 shows the test apparatus, consisting of a circulation loop with the return water line aerated before returning to the sump. This provided the necessary oxygen to simulate cooling tower water. The flow rate was 7.0 gallons per minute in 1” clear PVC piping for ease of visual inspection. This corresponds to a linear velocity of 3.2 feet per second, which is in the range of accepted flow rates for corrosion coupon racks.7 The temperature for each run was maintained at 95 degrees Fahrenheit; the heat was provided by the circulation pump. Oxidizing biocide was also added to the sump using an ORP monitor and relay set-point. This feature allowed for automatic oxidant addition for the five day test runs.

Results and Discussion
HARD WATER
Initial testing was performed on synthetic high hardness water. The composition of this water is shown in Table 1.

Pursuit of a Green Carbon Steel Corrosion Inhibitor: Tables 1, 2, 3 & 4Corrosion Coupons
Due to the shortened five day exposure time of each trial, coupon analysis was limited to qualitative observations. The results, shown in Table 3, provide a visual comparison between corrosion inhibitors. Under the test conditions outlined in the experimental procedure, the carbon steel inhibitor performance can be ranked as follows:
EPOC ≈ PCM > PAP ≈
HPA/MEA >> AAP

Under test conditions, residual orthophosphate and azole levels were measured at the conclusion of each five day test. Results are shown in Table 4. There was little phosphate in the system at the start of each run and any orthophosphate at the end was due to the reversion of organic phosphonate to orthophosphate. The azole level at the beginning of each run was 3 ppm. The reduction of azole is due to its susceptibility to oxidation under higher sustained ORP levels.4 The AAP that was used contained a small amount of phosphate resulting from the manufacturing process.

Reversion of an organic phosphate inhibitor to orthophosphate represents a change from an organic program toward an inorganic program, which could affect the overall green status of the program or its performance. Orthophosphate is not as good at inhibiting mild steel corrosion as organic phosphate, especially at low levels.8 Performance of an orthophosphate program may be reduced due to increased scaling potential of calcium phosphate, which could introduce new mechanisms of corrosion such as under deposit corrosion.9 This can be addressed by lowering the pH or increasing the phosphate dispersant polymer, however this can often be difficult to control.

Changes in the ratio of organic phosphate to orthophosphate can also introduce system testing errors and control difficulties that can lead to an increase in the total phosphorous in the system. More phosphorous in a water system can lead to increased fish mortality caused by eutrophication, algae growth, and loss of oxygen.

Minimizing the loss of azole in a system is also beneficial to the green status of an inhibitor because azole is known to increase the aquatic toxicity of an inhibitor.10 For a system that loses azole due to oxidation, more must be fed to maintain yellow metal corrosion protection, possibly increasing the overall toxicity of the system water.

Although the system containing EPOC shows the least deterioration of azole, the reversion of organic phosphate to orthophosphate was quite high. Conversely, PCM shows minimal reversion to orthophosphate, but coincides with a high loss of azole in the system. HPA/MEA and PAP coincided with less azole deterioration than PCM, but this may be due to the higher ratio of total chlorine to free chlorine provided by the amine functionality of these inhibitor combinations. Even though MEA was added to the HPA to minimize reversion to orthophosphate, the reversion over the five day test period at continuously elevated oxidation levels was substantial.

Corrator Analysis
This type of corrosion analysis yields graphical results that provide
a quantitative representation for the full five day test run. The two-channel corrator output provided continuous results on general corrosion and the pitting potential, which is called the imbalance.11 Addition of oxidizing biocide led to large variability in the data sets. Graphical smoothing of the data was performed for ease of comparing the different corrator data sets. Examples of the raw and smoothed data are shown in Figure 2. The raw data shows the copper corrosion spikes corresponding to hypochlorous acid additions.

Pursuit of a Green Carbon Steel Corrosion Inhibitor: Figure 2, 3 and 4Binary combinations of organic inhibitors were also investigated in this work. In one example, the apparent combinative effect of AAP and PAP was observed. While AAP and PAP performed modestly as individual mild steel corrosion inhibitors over the course of the five day test, Figure 5 shows there is a combined positive effect of having both inhibitors in the system. Table 3 shows each inhibitor, AAP and PAP, was tested at a concentration of 15 ppm as active product. For the combined inhibitor testing, the total treatment concentration was held constant. Hence, AAP and PAP were each added to the system at a concentration of 7.5 ppm as active product.

Copper corrosion rates can also be improved when treating with combinations of organic inhibitors. Figure 6 shows the effect of adding both AAP and PAP to the system at a concentration of 7.5 ppm as active product for each inhibitor. The sharp spikes observed at 24 hours are a result of increasing the ORP set point by 100 mV. Each inhibitor starts the test with a corrosion rate below 0.5, but rises to 2.0 by the end of the five day test. The combined inhibitor, however, sustains a corrosion rate below 0.5 for the entire duration of the run. It should be noted that each run, including the control, had a background level of 3 ppm tolyltriazole added at the start of the test, as shown in Table1. As previously discussed, this again shows the susceptibility of azole degradation under oxidizing conditions.

Pursuit of a Green Carbon Steel Corrosion Inhibitor: Figure 5, 6 & 7

SOFT WATER
Secondary testing was performed on synthetic low hardness water. The composition of the water is shown in Table 2. The aim of this testing was to compare a traditional soft water open recirculation treatment against a greener option, both of which are shown in Table 5. Both inhibitors programs, as well as the control, were used at a dosage rate of 150 ppm. Hence, the values listed in Table 5 are the active ingredient concentrations in the system at a dosage rate of 150 ppm.

The greener option was constructed based on the results from the hard water testing discussed previously. Although there is an increase in the total organic phosphorous of the product, it was thought that the reduction in zinc and removal of molybdate makes the overall product greener than the traditional open recirculation treatment program. Table 5 also shows the traditional treatment program is 40 percent more costly than the greener option, although this was not the goal in developing the greener product.

Pursuit of a Green Carbon Steel Corrosion Inhibitor: Figure 8, Table 5 & Table 6Corrosion Coupons
Table 6 shows the mild steel corrosion coupons for the synthetic soft water system. The control coupon for the soft water system does not have the same copper plating shown in the control coupon for the hard water system. This is likely an artifact of the higher amount of hypochlorous acid used in the hard water control test. The higher amount of oxidant deteriorates the azole faster, which leads to higher copper corrosion. The free copper in the system then plates the mild steel corrosion coupon. A lesser amount of continuous chlorination was used in the soft water simulation as the investigation to a greener product choice was narrowed. The free and total chlorine levels during this testing were at 0.25–1.0 and 0.75–1.5, respectively, to simulate a relatively well-controlled cooling water system found in field conditions.

Corrator Analysis
Figure 7 shows mild steel corrosion rates under the synthetic soft water conditions for the two different treatments. As before, corrosion rates match up well with qualitative corrosion coupon observations. Figure 8 shows the copper corrosion rates for these inhibitors, which also agrees with the results shown in Figure 7 that the greener treatment performs better than the traditional treatment. Along with Table 6, Figures 7 and 8 demonstrate that moving toward greener alternatives for open recirculation treatments does not necessarily lead to sacrificing inhibitor performance or increasing usage cost.

Conclusions
A pilot test apparatus is capable of screening potential corrosion inhibitors and combinations of inhibitors to move toward greener products. This may include reducing the concentration of less desirable ingredients, eliminating certain ingredients altogether,
or even combining ingredients in new ways to take advantage of combinative effects. All inhibitors tested provided some level of carbon steel corrosion protection, even under the conditions of high continuous oxidation provided by sodium hypochlorite. Under the experimental conditions, PCM and EPOC performed the best at inhibiting mild steel corrosion as single component inhibitors. There is an apparent combinative effect provided by combinations of certain inhibitors, as was shown for AAP and PAP, particularly in the inhibition of copper corrosion.

Copper plating unto mild steel becomes more prevalent as the concentration of oxidant increases and the level of azole copper inhibitor diminishes. Reducing the oxidation degradation of azole and other inhibitor components is important in seeking greener product offerings because this can reduce the total amount of inhibitor used while still maintaining good scale and corrosion protection. This work has shown that under pilot conditions, it is possible to design a greener soft water make-up product that does not sacrifice performance or increase usage cost. More work is necessary to further compare pilot results with field trials.

Acknowledgments
The authors wish to thank the Association of Water Technologies for allowing this paper to be presented and to U.S. Water for the resources necessary to conduct the research.

Contact U.S. Water for more information.


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