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STRESS CORROSION CRACKING IN FUEL ETHANOL: A NEWLY RECOGNIZED PHENOMENON R.D. Kane and J.G. Maldonado InterCorr International, Inc. 14503 Bammel North Houston Road, Suite 300 Houston, Texas 77014 USA L. J. Klein BP - Cherry Point Refinery 4519 Grandview Rd. Blaine, WA 98230 ABSTRACT During the past decade, there has been evidence of stress corrosion cracking of steel storage tanks and associated piping used in fuel ethanol service. While SCC has not been wide spread, it has produced failures in some user facilities. No failures have been identified in facilities that produce fuel ethanol. This paper describes a review and survey conducted under an API funded study of this newly recognized phenomenon. It summarizes the basis of SCC in fuel ethanol and related environments and documents service experience. Further, more detailed information and survey results can be found in API publication 939D. It includes over 70 pages, 22 figures, 17 tables, 42 references, a bibliography of 15 related references, and a comprehensive summary table detailing 16 case histories involving ethanol SCC and non-SCC experiences. INTRODUCTION An extensive survey of published literature, service experience and previously unpublished studies on stress corrosion cracking (SCC) of carbon steel in fuel grade ethanol and related topics was conducted by InterCorr International, Inc. (Houston, Texas) for The American Petroleum Institute and a consortium of fuel ethanol producers which includes the Renewable Fuels Association. The results of this study along with detailed information and survey results can be found in API publication 939D. [1] It includes over 70 pages, 22 figures, 17 tables, 42 references, a bibliography of 15 related references, and a comprehensive summary table detailing 16 case histories involving ethanol SCC and non-SCC experiences. BACKGROUND Ethanol has been used in automobile fuels to a certain extent for more than 25 years. In the early 1990's the U.S. Congress passed the Clean Air Act that required an oxygenate in gasoline supply in specific regions of the country. Oxygenates used are either ethanol or MTBE. Recently, MTBE has been found Russell Kane - Invoice INV-106095-ASLF5Q, downloaded on 7/9/2008 11:34:22 AM - Single-user license only, copying and networking prohibited. Administrator Stamp as a contaminant in groundwater and 17 states have banned its use. Additionally, the federal government is considering new energy legislation that among other things, would phase out the use of MTBE, eliminate some oxygenate requirements and phase in a modest but increasing requirement to use renewable fuels like ethanol and biodiesel. Consequently, the use of ethanol as an additive/extender to gasoline is expected to increase. The present study was the first part of a multi-part plan developed by the API Refining Committee, Subcommittee on Corrosion and Materials, to address the needs of industry regarding potential problems associated with SCC of carbon steel in fuel ethanol. The initial direction taken was to expeditiously develop a white paper to provide a concise and accurate review of the currently available information on SCC in fuel grade ethanol. This work also included documentation of the experience from companies involved in fuel ethanol supply, transportation, storage and distribution, and an initial assessment of the potential economic impact of this problem to the petroleum industry. What is Fuel Ethanol vs. Ethanol? Ethanol is an alcohol that can be produced from a variety of sources. In the United States the most common source is from corn and grain. However, ethanol can also be produced naturally (fermented) from any carbohydrate source, such as wheat, cane, beet and fruits like grapes and apples. While grain and synthetic alcohols are technically the same (the molecule is identical), there are differences in the amounts of contaminants (butanol, acetone, methanol, organic acids) in each. Fuel ethanol is not sold with zero water content, where it would be referred to as anhydrous ethanol. Denatured alcohol typically contains up to 1 percent water and other constituents. Fuel ethanol with less than 0.5 percent water is considered “anhydrous ethanol”. Ethanol with higher water contents is usually referred to as “hydrated ethanol”. Such hydrated ethanol is uncommon in the United States but is used as a fuel in Brazil. In the United States, denaturants are also added to fuel alcohol in accordance with the Bureau of Alcohol, Tobacco and Firearms. According to Federal Regulation Title 27 Parts 19, 20 and 21 (including CFR 19.1005, 27CFR 21.24 and C.D.A. 20), a denaturant is to be added to alcohol in order to make it unfit for beverage or internal human medical use. The API research report 936D provides more background related to the manufacturing processes and range of denaturants used in fuel ethanol. There are several standards that govern fuel grade alcohol, related analyses and its use as a fuel. The primary standard for fuel ethanol can be found in ASTM D4806 – Standard Specification for Denatured Fuel Ethanol for Blending with Gasolines for Use as Automotive Spark-Ignition Engine Fuel – which gives the compositional and physical limits for fuel ethanol. [2] These are summarized in Table 1. A parameter that is used in evaluation of fuel ethanol is the pHe as defined by ASTM D6423. [3] The pHe value is a measure of the acid strength of high ethanol content fuels. It is applicable to fuels containing nominally 70 volume percent or more ethanol, or higher. pHe is similar to the pH parameter used in aqueous solutions. An extremely important point is that pH 7 is considered neutral for aqueous solutions, whereas a pHe value of 9.55 is the neutralization point for ethanol. Therefore, environments that have a pH of 6 in aqueous solutions may be considered only mildly acidic, whereas in ethanol pHe 6 represents a solution of significantly higher acidity (as defined as the magnitude of the reduction from the neutralization value). Russell Kane - Invoice INV-106095-ASLF5Q, downloaded on 7/9/2008 11:34:22 AM - Single-user license only, copying and networking prohibited. Table 1 – Quality Specification for Fuel Ethanol per ASTM D4806 Property Units Specification ASTM Designation Ethanol %v min 92.1 D5501 Methanol %v max 0.5 -- Solvent-Washed Gum mg/100 ml max 5.0 D381 Water Content %v max 1.0 E203 Denaturant Content %v min %v max 1.96 4.76 D4806 Inorganic Chloride Content ppm (mg/L) max 40 (32) E512 Copper Content mg/kg max 0.1 D1688 Acidity as acetic acid %m (mg/L) 0.007 (56) D1613 pHe -- 6.5-9.0 D6423 Appearance Visibly free of suspended or precipitated contaminants (e.g. clear & bright) CORROSION AND SCC IN ALCOHOLIC MEDIA Corrosion in Alcoholic Environments Most engineers and researchers are more familiar with corrosion chemistry of aqueous solutions. For the most part, the similarities between organic and aqueous media dominate. Figure 1 shows that the electrode potentials of various materials in water, ethanol and methanol are very similar and the potential for the hydrogen electrode is similar, as well. It is primarily in the region of Hg and Ag, which have a very high electrode potential in water, where a significant reduction in the electrode potential is observed. Additionally, water, methanol and ethanol are all protic media capable of sustaining electron transfer and ionization of the hydrogen atom. Therefore, in most cases, corrosion processes and galvanic interactions would be expected to be thermodynamically similar in water, methanol and ethanol. [4] Comparison of the physical properties of water, methanol and ethanol are also revealing. Ethanolic solutions have lower conductivity than either methanol or water. The oxygen solubility in methanol and ethanol are similar; however, they are both an order of magnitude higher than that of water. Therefore, the availability of oxygen for participation in the corrosion reaction is expected to be generally greater in ethanol and its solutions as well. Another important aspect of ethanol with potential relevance to its corrosivity is its hygroscopic nature relative to other fuels. Data show a radical increase in water content of ethanol after 30 days exposure to a humid environment. [5] Russell Kane - Invoice INV-106095-ASLF5Q, downloaded on 7/9/2008 11:34:22 AM - Single-user license only, copying and networking prohibited. Figure 1 – Comparison of electrode potentials of metals in different solvents: Including water, methanol and ethanol. [1’] Figure 2, shows the corrosivity of zinc, iron and nickel in water and in alcohols versus their chain length (for C1 through C8) [4]. The relationship displayed indicates that there is an increase in aggressivity going from water to methanol (C1) which then decreases with increasing carbon number. This relationship leaves ethanol with approximately the same general corrosivity as water. The reason for the increase in corrosivity going from water to methanol is generally considered to be the effect of the increased oxygen solubility in methanol. Since it has been shown that the oxygen solubility of methanol and ethanol is similar (both higher than that of water), the decrease in corrosion rate from C1 through C8 is likely to be the result of the increased chain length and, in turn, its impact on molecular or ionic mobility in the media. SCC in Ethanol A major finding of this study was that only limited data existed in the published literature on SCC of steel in ethanol. [5, 6] The first of the two references cited indicate that SCC of steel may be possible in ethanolic solutions as evidenced by examination of surface features of slow strain rate (SSR) test specimens exposed to ethanol with additions of LiCl and H2SO4. The cracking in ethanol appeared qualitatively to be less severe than found for methanol but no ductility loss data (elongation or reduction in area versus air properties) was presented. The cracking of steel in methanolic and ethanolic environments was comparable in many ways to SCC of steel in liquid ammonia where susceptibility can be affected by minor impurities of water. The second reference describes the SSR testing of steel in ethanolic solutions with formic and acetic acids, and water at 60 C. Additions of 0.10 to 25 percent formic and 0.1 percent water in ethanol did not produce SCC. However, steel bend specimens produced SCC in a solution of 0.01 percent acetic acid and 0.1 percent water that was less severe than found in methanolic solutions. Cracking was basically surface fissuring of less than 0.01 mm in depth. Russell Kane - Invoice INV-106095-ASLF5Q, downloaded on 7/9/2008 11:34:22 AM - Single-user license only, copying and networking prohibited. Figure 2 – Corrosion rate (icorr) of zinc, iron and nickel in primary alcohols Plotted versus carbon number of solvent: Methanol (C =1); Ethanol (C = 2) Corrosion and Pitting in Ethanolic Environments Despite the limited data on SCC of steel in ethanolic solutions, there was, in fact, substantial information found on the general and pitting corrosion behavior of steel in ethanol and ethanolic solutions that were relevant to the present concerns for corrosion of steel tanks and piping in fuel ethanol. Since SCC of steel likely involves corrosion to a certain degree and, in particular, the initiation of local anodic sites, it was felt that this review should attempt to characterize corrosion in ethanolic solutions. In overview, the literature was found to be consistent in the representation that ethanol solutions are generally less corrosive than those of methanol. Many of these studies were conducted on fuel grade hydrated ethanol in Brazil with the impurities shown in Table 2. [7] Table 2 – Characteristics of Brazilian Fuel Grade Ethanol Characteristics Result Specific Gravity at 20 C 0.8093 + 0.0017 Alcohol (%) 93.6 + 0.6 Total Acid (mg/100 ml), max. 3.0 Aldehydes (mg/100 ml), max. 6.0 Esters (mg/100 ml), max. 8.0 Higher alcohols (mg/100 ml), max. 6.0 The literature also indicates that ethanol containing certain impurities can sustain corrosion in carbon steel and other materials. Furthermore, these impurities and other additives can quite commonly promote tendencies toward localized corrosion, as shown in Table 3. Russell Kane - Invoice INV-106095-ASLF5Q, downloaded on 7/9/2008 11:34:22 AM - Single-user license only, copying and networking prohibited. Table 3 – Corrosion in Brasilian Fuel Ethanol Fuel Ethanol Ethanol Test Morphology Corrosion Morphology Corrosion 1 Pitting Density – 35 pits/inch2 Rate – 1.9 mpy General 0.006 mpy 2 General Rate – 0.36 mpy General 0.009 mpy 3 Pitting Rate – 0.09 mpy Table 4 – Parametric Study of Selected Variables on Corrosion in Hydrated Ethanol Solution Variables Mass Loss Ethanol (%) SO4-2 (mg/L) Cl- (mg/L) pH (g/m2) (g/m2) ABCD 92.6 4 2 4 13.600 12.500 ABD 92.6 4 0.5 4 11.000 11.300 ACD 92.6 1 2 4 10.200 8.000 BCD 93.8 4 2 4 9.710 11.200 CD 93.8 1 2 4 9.010 9.530 BD 93.8 4 0.5 4 8.750 9.200 AD 92.6 1 0.5 4 5.800 4.120 D 93.8 1 0.5 4 5.130 4.580 ABC 92.6 4 2 8.5 1.470 1.270 B 93.8 4 0.5 8.5 1.010 0.958 A 92.6 1 0.5 8.5 0.955 1.00 AB 92.6 4 0.5 8.5 0.950 1.110 BC 93.8 4 2 8.5 0.928 0.960 Base 93.8 1 0.5 8.5 0.885 0.744 C 93.8 1 2 8.5 0.870 0.900 AC 92.6 1 2 8.5 0.724 1.090 The results in Table 4 from a corrosion study in hydrated ethanol were sorted in descending order based on the extent of mass loss corrosion during the exposure. Most notable is the influence of pH. All of the environments in the top half of the listing (most corrosive) were run at low pH (pH 4). These environments have about an order of magnitude higher mass loss than those at pH 8. The next most important factor in producing high corrosivity appears to be high sulfate level as evidenced by the Russell Kane - Invoice INV-106095-ASLF5Q, downloaded on 7/9/2008 11:34:22 AM - Single-user license only, copying and networking prohibited. 7 position in the top two slots at the top of the list. However, the actual effect of this variable has not been independently evaluated. The three most corrosive environments (at the top of the list) were those with the higher amount of water (lower ethanol). Additionally, high chloride concentrations existed in all but one of the top five most corrosive environments. One shortcoming of the abovementioned study, however, was that it did not measure susceptibility to localized pitting attack on the steel coupons. Still, while these studies do not match directly with compositional range for fuel ethanol (per ASTM), they do indicate that pH (pHe), sulfate (sulfur) and chloride may also be important variables and may also be involved in the SCC process as well. In another Brazilian study involving additions of a 20 percent HCl solution to ethanol, the corrosion rate and morphology were found to change according to Table 5. [7] These data indicate a change in corrosion rate from high to low to high, and a change in corrosion morphology from uniform to pitting back to uniform, over the range of 1 to 80 percent water content. This illustrates the complex relationship of acidic water in ethanol. At low concentrations, it leads to a dramatic reduction in the general corrosion and the onset of pitting at around 4 percent water in ethanol. However, the trend is reversed with supplementary additions of acidic water. Table 5 – Impact of additions of 20% HCl to Ethanol on Corrosion Rate Water (%) Duration (hrs) Corrosion Rate (mmpy) Corrosion Morphology 0 44 4.5 Uniform 1 300 2.5 Uniform 4 300 0.53 Pitting 6 300 0.01 Slight Pitting 32 140 0.01 Uniform 64 140 2.5 Uniform 80 140 2.8 Uniform No comprehensive conclusions of corrosion and SCC in fuel ethanol have been reported in the published literature. In general, the tendency expected in purer ethanol solutions typical of fuel grade ethanol may be similar to that found for steel in methanolic solutions where the steel tends to exhibit passivity or very low corrosion rates (pseudo-passivity) in very mild environments. With increasing aggressivity of the environment, there is an initiation of local anodic attack (local loss of passivity), followed by general corrosion at still higher levels of solution aggressivity. Most of what can be inferred has come from investigations involving steel in methanolic solutions with the assumption that these environments are mechanistically similar. In solutions of methanol and water in the range of 200 to 460 ppm, the natural (air-formed) oxide film on the surface is not stable and partially dissolves leading to the following reaction in the protic medium [8]: 2 MeOH => MeOH2+ + MeO- The oxide free areas on the steel surface will rapidly dissolve according to the following reaction: Fe2+ + H20 => FeOH+ + H+ Russell Kane - Invoice INV-106095-ASLF5Q, downloaded on 7/9/2008 11:34:22 AM - Single-user license only, copying and networking prohibited. followed under aerated conditions by: Fe2+ + MeOH => FeOMe+ + H+ This leads to local anodic attack under aerated conditions which, in turn, sets up the conditions conductive for SCC. If sufficient corrosive causing agents are added to the system, the local anodic attack changes to a more generalized form of corrosion over the complete surface thus reducing susceptibility to SCC. The role of acidity in partially hydrated methanolic environments has also been evaluated in terms of its role in the corrosion mechanism. The presence of H+ in methanol contributes to destruction of the normally protective oxide film. An anodic Tafel slope of 30-35 mV per current decade has been measured. At higher currents, loss of linearity in the potential versus log current relationship was also observed as a result of diffusion controlled transport of the Fe2+ ions. At still higher anodic potentials, absorbed oxygen is formed and the growth of a passive oxide layer takes place as Fe + H2O (ads) => Fe-Oads +2H+ = 2e- n Fe-Oads => iron oxide (Fe2O3 or Fe3O4; corrosion product layer) The region of potential that corresponds to this transition from active (anodic) to passive behaviors is precisely where pitting and SCC can occur. However, these processes depend on the level of acidity and water content in the environment through specific (and possibly competitive) adsorption on the metal surface. Therefore, there maybe differences in the exact behavior of water and acidic species in other alcohols such as ethanol. SCC in methanol Studies have been conducted to evaluate the phenomenon of SCC of steel in methanolic environments. [6, 9, 10] Typically, these studies have included the use of plastically deformed and/or dynamically strained tests such as found in U-bend (ASTM G30) and slow strain rate (ASTM G129) specimens. In aerated solutions at 20 C, it has been shown that there is a critical amount of water that produces SCC in steel that ranges from somewhere above 0 but less than 0.05 volume percent to just below 1 percent water by volume, based on reported trends in multiple tests (See Figure 3). The highest probability of failure by SCC was reported to be around 0.20 percent water in methanol. Studies of iron and a low alloy steel (Fe-0.94Cr-0.98Ni-0.16Mo) with a yield strength of 932 MPa, [10] showed that conditions of slow oxide growth were observed in methanolic solutions containing impurities in the following range: acidity (0 to 10-3 M with formic acid), chlorides (10-4 to 10-3 M) and water (0.01 to 0.5 percent) when the p