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64 MAY 2015 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 54, NO. 5 MATERIALS SELECTION & DESIGN D During the past decade, there has been an increasing use of the assump- tion that there is a dividing line be- tween carbon dioxide (CO 2 ) and hy- drogen sulfde (H 2 S) corrosion that is determined by a CO 2 /H 2 S gas partial pressure ratio of 500. Most users of this ratio are not familiar with its ori- gins, the assumptions it implies, or its practical limits. This work investigates the history and the basis for the ratio. During the last 20 to 30 years, a greater understanding has developed regarding corrosion issues that exist in sour oil and gas systems that contain both hydrogen sulfide (H 2 S) and carbon dioxide (CO 2 ) vs. the sweet systems that contain just CO 2 . Since even small additions of H 2 S were recognized as having an impact, guidance was sought to help determine just how much H 2 S was required to turn a system f ro m sw e e t t o s o u r. O f t e n , a "r u l e of thumb" ratio of 500 for the CO 2 /H 2 S partial pressure has been utilized . This ratio is widely quoted and appears in a number of industr y documents, but its origin and technical basis do not seem to be well documented. There have been a number of docu- ments over the last few years that have ref- erenced a CO 2 /H 2 S ratio of 500 as the transi- t i o n p o i n t b e tw e e n s w e e t a n d s o u r corrosion. 1-2 The earliest identified men- tion of a value of 500 for the CO 2 /H 2 S ratio was found in an article by Dunlop, Hassell, and Rhodes 3 that was presented at CORRO- SION/83. This was only eight years after the original DeWaard and Milliams 4 article on CO 2 corrosion in May 1975, so it seems rea- sonable to assume that an earlier origin would not be likely. The topic of Dunlop's article was the fundamentals of sweet well corrosion, not sour or even slightly sour corrosion. The content of the article deal- ing with CO 2 /H 2 S is as follows: Iron Carbonate—Iron Sulfide: Here, a simpler order-of-magnitude guide is proposed. From the values tabulated by Latimer 5 for the appropriate solubility products and ionization constants, coexistence of siderite and iron sulfide (FeS) at 25 °C is indicated when the par- tial pressure ratio of CO 2 /H 2 S = 500. Thus, by ignoring temperature effects and solution imperfections, the conclu- sion is reached that siderite (iron car- bonate [FeCO 3 ]) should prevail when the partial pressure ratio CO 2 /H 2 S > 500; when it is <500, FeS can be expected. In order to re-derive Dunlop's ratio of 500, it was necessary to obtain a copy of the 1938 version of Latimer's publication in order to use the same solubility products and ionization constants. Assuming that this was Dunlop's only source of informa- tion, this was also the only way to deter- mine exactly what values were used and how the issue of gas solubility was handled. The data available and the method that was required to obtain a value of 500 are shown in the appendix of the presentation by Smith . 6 It i s assum ed that D unlop rounded the calculated value of 490 that was derived up to the reported value of 500 since this was common practice among engineers at that time, many of whom had started out using log tables. Calculators and desktop computers were still some- what new in 1983. The Carbon Dioxide/Hydrogen Sulfde Ratio—Use and Relevance StePhen n. SMith, FnaCe, Corrosion and Materials Consultant, The Woodlands, Texas

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