Content area
Full Text
During the past decade, there has been an increasing use of the assumption that there is a dividing line between carbon dioxide (CO^sub 2^) and hydrogen sulfde (H^sub 2^S) corrosion that is determined by a CO^sub 2^/H^sub 2^S gas partial pressure ratio of 500. Most users of this ratio are not familiar with its origins, 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 (H2S) and carbon dioxide (CO2) vs. the sweet systems that contain just CO2. Since even small additions of H2S were recognized as having an impact, guidance was sought to help determine just how much H2S was required to turn a system from sweet to sour. Often, a "rule of thumb" ratio of 500 for the CO2/H2S partial pressure has been utilized. This ratio is widely quoted and appears in a number of industry documents, but its origin and technical basis do not seem to be well documented.
There have been a number of documents over the last few years that have referenced a CO2/H2S ratio of 500 as the transition point between sweet and sour corrosion.1-2 The earliest identified mention of a value of 500 for the CO2/H2S ratio was found in an article by Dunlop, Hassell, and Rhodes3 that was presented at CORROSION/83. This was only eight years after the original DeWaard and Milliams4 article on CO2 corrosion in May 1975, so it seems reasonable 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 dealing with CO2/H2S is as follows:
Iron Carbonate-Iron Sulfide: Here, a simpler order-of-magnitude guide is proposed. From the values tabulated by Latimer5 for the appropriate solubility products and ionization constants, coexistence of siderite and iron sulfide (FeS) at 25 °C is indicated when the partial pressure ratio of CO2/H2S = 500. Thus, by ignoring temperature effects and solution imperfections, the conclusion is reached that siderite (iron carbonate [FeCO3]) should prevail when the...