Materials Performance

MAY 2017

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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44 MAY 2017 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 56, NO. 5 CHEMICAL TREATMENT F Application of nonideal solution chemistry to corrosion testing is not common, even though supercritical conditions and heavy brines have been encountered in oil and gas pro- duction for many years. This article reviews the concepts of nonideal chemistry that are relevant to corro- sion testing and illustrates the signifi- cant impact nonideality can have upon the test results in a given chem- ical environment. For many years, corrosion engineers have used ch emi str y-based model s to design test protocols that realistically reflect production conditions. With only a limited number of exceptions, these models have assumed ideal behavior for both the gas and the aqueous phases. In most cases, these assumptions have been suitable for the applications. However, with increasing interest and production in high-tempera- ture, high-pressure fields that involve super- critical gases as well as brine phases that either approach or exceed saturation, the assumption of ideality is no longer accept- able and can introduce significant errors that can cause test protocols to significantly deviate from actual conditions. Laboratory tests are used to evaluate the ability of materials to resist corrosion and cracking in these environments, espe- cially in high-temperature and high-pres- sure sour environments that frequently involve heavy produced brines. The design of these tests often requires compromises to accurately simulate field conditions while also considering the design limita- tions of the test equipment, the safety aspects of handling high-pressure gases that may contain hydrocarbons and hydro- gen sulfide (H 2 S), and the necessity of cleaning the equipment after the test to remove any precipitates that may have formed. If the compressibility of the gas phase is not considered for both the field and laboratory test conditions, a test that is based on simply matching the partial pres- sures of carbon dioxide (CO 2 ) and/or H 2 S can result in test conditions where the acid gas concentration in the test solution dif- fers from those the field by more than an order of magnitude. Similarly, a test solution that is based on a sodium chloride (NaCl) concentration to match the ionic strength of the field brine may address the effect of salt upon gas solubility, but avoids the formation of precipitates (e.g., barium sulfate [BaSO 4 ], strontium sulfate [SrSO 4 ], and calcium flu- oride [CaF 2 ]) that are difficult to remove. The substitution of NaCl for the other salts also prevents the formation of the various complexes and ion pairs that can form in the presence of divalent cations. Their for- mation can shift the pH and also alter the availability of species important to the cor- rosion reaction, such as Fe 2+ and HCO 3 – . Properties of Volatile Components In corrosive environments, the most important volatile components are CO 2 , H 2 S, nitrogen gas (N 2 ), oxygen (O 2 ), and sul- fur (S 0 ), various hydrocarbons, and water (H 2 O). The properties of these species are determined by their molecular structure. Effects of Nonideal Chemistry on Corrosion Testing s tep H en n . s mit H , F nace , Consultant, Pueblo, Colorado, USA a ndre a nderko, F nace , and p eiming Wang, OLI Systems, Inc., Cedar Knolls, New Jersey, USA

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