Galvanic Corrosion Study in Ethylene Glycol

CORROSION ENGINEERING SECTION

Galvanic Corrosion Study in Ethylene Glycol P. Rostron*

Abstract The potential for galvanic corrosion between carbon steel and duplex (22Cr) stainless steel in impure ethylene glycol (CH2OHCH2OH) is assessed. The temperature dependence of the corrosion is determined, as well as the effect of temperature on the conductivity of dry, wet, and impure wet ethylene glycol. The temperature dependence of the corrosion can be explained by the change in electrical resistance of the glycol with temperature. The authors also show the importance of testing corrosiveness by using the actual working fluid rather than a simulation fluid. KEY WORDS: conductivity, ethylene glycol, galvanic corrosion, glycol recovery unit corrosion, temperature dependence

Introduction Petroleum gas is dewatered using an adsorption tower and a counter-current flow of ethylene glycol (CH2OHCH2OH). When the ethylene glycol approaches a 30% (w/w) water solution, the glycol is sent to a recovery heat exchanger where the water is distilled off at approximately 115°C to regenerate dry glycol. The heat exchanger consists of duplex stainless steel (22Cr) tubes in a carbon steel (CS) shell. Because of the length of the heater tubes (6 m), there is potential for the tube to bend and make metallic contact with the carbon steel shell. This could lead to galvanic corrosion because the two metals are far enough apart Submitted for publication September 27, 2010; in revised form, February 15, 2011. * Chemistry Department, The Petroleum Institute, Abu Dhabi, United Arab Emirates. E-mail: [email protected].

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in the galvanic series for this to be an issue. In this paper we review the effects of the fluid on the corrosion rate, as well as report the effect of temperature on the corrosion rate.

Experimental PROCEDURES Corrosion Studies Measurement of the galvanic corrosion potential was performed using a potentiostat in the zero-resistance ammeter mode, according to the ASTM G711 protocol. Equal area (3.56 cm2) samples of CS and 22Cr were cut from stock bar and a small hole (6 mm) was drilled in one end to a depth of 10 mm. An insulated lead wire was electrically connected to the sample in the hole using a drop of molten solder, and the hole was filled flush to the top with a fast cure epoxy resin. This design eliminates any loss of area in addition to eliminating any galvanic effects from the copper lead wire, or any heat effects as a result of the soldering process. Since solder does not stick to 22Cr, the solder was included merely to ensure good electrical connection between the electrode and the lead wire. Mechanical strength as well as insulating the electrode from the wire was provided by the epoxy resin. Each electrode was supported in a 700-mL flange flask using a 5-hole adaptor lid. The electrodes were supported on glass tubes and held apart at a distance of 50 mm. The lid also contained dry nitrogen inlet, thermocouple, and reflux condenser. The flange flask was stirred at a constant rate (400 rpm) using

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FIGURE 1. Corrosion current as a function of temperature for recovery unit glycol.

a hotplate stirrer and polytetrafluoroethylene (PTFE)coated magnetic stirrer bar. Prior to each experiment the flask was flushed with dry nitrogen to displace oxygen. Samples of the ethylene glycol from the recovery unit were used one time only, approximately 400 mL in each experiment, as well as dry distilled ethylene glycol (boiling range: 190°C to 200°C), and a 30% (w/w) solution of water in laboratory-grade ethylene glycol.

FIGURE 2. Effect of temperature on the resistance for recovery unit glycol as supplied.

To convert current density to penetration rate, the following calculation was used: PR( mmy –1 ) = 0.00327 × equivalent weight × curre rent density (µA/cm A/cm 2 ) (1) ρ ( g / cm 3 )

Conductivity Measurements The same experimental setup as for the corrosion studies was used, with the electrodes replaced by platinum foils with a separation of 1.0 cm. Resistances were measured using a multimeter with 10 MΩ internal resistance.

rEsults Recovery Unit Ethylene Glycol The effect of temperature on the galvanic corrosion rate is given in Figure 1. The corrosion current increased in an exponential fashion from 314 µA, equivalent to 88.2 µA/cm2, or 1.02 mmy–1 (40.4 mpy) at 25°C to 9,800 µA, equivalent to 2,750 µA/cm2, or 31.9 mmy–1 (1,270 mpy) at 115°C. This was a very significant and unexpected increase in corrosiveness.

e.g., a corrosion current of 88.2 µA/cm2 in carbon steel, a density of 7.87 g/cm3 equivalent weight of 27.92 gmol electrons–1 equates to a penetration rate of: PR =

0.00327 × 27.92 × 88.2 = 1.02 02 mmy –1 or 40.0 mpy y (2) 7. 8 87

The resistance of the solution as a function of temperature is shown in Figure 2. From the figure, it can be seen that the resistance of the fluid decreases from 40 KΩ/cm at 25°C to 1.2 KΩ/cm at 115°C.

Dry and Wet Lab-Grade Ethylene Glycol The same experiments were performed in both wet and dry ethylene glycol and a summary of the corrosion rate is given in Table 1.

TABLE 1 Summary of Corrosion Rate Data for Wet, Dry, and Field Samples Corrosion Rate 25°C

Corrosion Rate 115°C

314 µA = 1.02 mmy–1 (40.4 mpy)

9,800 µA = 31.9 mmy–1 (1,270 mpy)

Wet (30% w/w) ethylene glycol (bpt 115°C)

5.0 µA = 0.0130 mmy–1 (0.514 mpy)

107 µA = 0.349 mmy–1 (13.8 mpy)

Dry glycol (bpt 190°C to 200°C)

3.2 µA = 0.0104 mmy–1 (0.411 mpy)

74.2 µA = 0.242 mmy–1 (9.54 mpy)

Fluid Used Recovery unit fluid

Recovery unit after 5 h reflux at 115°C

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174 µA = 0.567 mmy–1 (22.4 mpy)

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results AND Discussion There is a significant, nonlinear increase in the corrosion rate shown in all forms of ethylene glycol as the temperature increases. We believe that this is because of the increase in conductivity found as a function of temperature. As the solution becomes a better electrolyte, the solution is able to support a higher corrosion current. There is insufficient data available in the literature on this subject, and so we use other factors to explain by inference. The conductivity of pure water, in equilibrium with its saturated vapor, increases from 0.0550 mS/cm at 25°C to 0.765 mS/cm at 100°C.2 Literature data on the conductivity of ethylene glycol at 20°C is reported3 as 1.07 S/cm. It is reasonable to assume that since both ethylene glycol and water are both hydrogenbonded covalent molecules, they should behave similarly. Since no data on the effect of temperature on conductivity of ethylene glycol is reported, it is therefore assumed that the conductivity of ethylene glycol should increase as a function of temperature. Since the solution becomes more conductive at higher temperatures, the corrosion rate would be expected to increase also, as observed. We postulate that this is a result of the increasing extent of ionization occurring in the ethylene glycol. The only dissociation constant available in the literature4 for ethylene glycol is a pKa of 15.1 at 298 K. It appears that there are no studies on the effect of temperature on the dissociation of ethylene glycol. It is to be expected, however, that the percentage of dissociation and hence the conductivity should increase as a function of temperature since ionization is an endothermic process; as the temperature increases, the extent of ionization should also increase according to le Chatelier’s Principle.

Fluid Composition The effect of the type of fluid used is significant. Wet glycol was found to be more corrosive than dry glycol, primarily because of the lower electrical resistance of the fluids. However, we were not able to replicate the corrosion rate of the recovery unit fluid. We believe that the sample contains more than just glycol and water. There is an unidentified reactive chemical in the fluid. This can be seen from the 5-h reflux data in Figure 3. The corrosion rate dropped significantly over time, indicating that a reactive species had been consumed. This is tentatively assigned to a possible thiol present in the solution. The evidence for this is based on the observation that the corrosion product on the carbon steel electrode was black. Treatment with hydrochloric acid (HCl, 5 M) yielded a faint hydrogen sulfide aroma, a characteristic positive test for iron sulfide. The hydrogen sulfide probably formed by thermal decomposition of the thiol, liberating

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FIGURE 3. Long-term reflux at constant 115°C, sample glycol, showing decrease in corrosion rate over time. (Corrosion current is denoted).

hydrogen sulfide, which then reacted with the steel. Alternatively, the thiol could have reacted directly with the steel, forming a sulfide. The mechanism of sulfide formation was not a part of our study.

Conclusions v The initial question, namely, “Is it OK for the 22Cr and the CS to make electrical contact?,” has been resolved. Clearly, the corrosion potential in the conditions in the heat exchanger is such that, should this happen, significant corrosion is to be expected. It is recommended that spacer bars of a nonconductive material, able to withstand the environment, such as PTFE, be added to the design to prevent contact between the tube and shell. When installing them, care should be taken to prevent crevice corrosion by ensuring that a caulk is used to seal the gap. v Corrosiveness of ethylene glycol is temperaturedependant, and this tentatively is ascribed to the change in conductivity of the fluid as a function of temperature—data that is missing from the literature. v When performing laboratory investigations of phenomena, it is critical that wherever possible the actual conditions are met. In this case it was clear that simulation fluids would not have been representative of the galvanic corrosion potential in the recovery unit, and the wrong recommendation would have been given. v When designing simulation fluids for laboratory measurements, the factors that might be present and might affect the corrosion rate are taken into account first. Factors that are thought not to affect the corrosion rate are eliminated from the laboratory protocol. In this case, an assessment would have concluded that the important factors to consider in a laboratory study are temperature and water content. It is pos-

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sible that the turbulence may have an effect, but as long as a reasonable (and constant) amount of turbulence is supplied (namely stirring at 400 rpm), this effect would be negated. Also, the oxygen regime is assumed to be 0 ppm oxygen, so this was removed by dry nitrogen. In this case there were factors that normally would not have been considered, namely, a contaminant in the sample that changed the measurements completely, because it was not expected to be present. v This highlights the difficulty in laboratory measurements of field conditions, and explains why so many companies still rely on field evaluations. Laboratories need to work with more accurate and representa-

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tive samples if the amount of field trials is ever to be reduced. Owners of equipment need to understand that accurate and representative samples need to be prepared for testing, so that they can obtain relevant data from which informed decisions can be made. References 1. ASTM G71-81(2009), “Standard Guide for Conducting and Evaluating Galvanic Corrosion Tests in Electrolytes” (West Consho hocken, ASTM International, 2009). 2. W.L. Marshall, J. Chem. Eng. Data 32 (1987): p. 221. 3. Dow Chemical Company, “Physical Properties: Dow Ethylene Glycols,” accessed September 2010, http://www.dow.com/ ethyleneglycol/about/properties.htm. 4. D.R. Lide, CRC Handbook of Chemistry and Physics, Section 8, 86th ed. (Boca Raton, FL: CRC Press, 2005), p. 42.

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