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66 MAY 2015 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 54, NO. 5 compounds, such as FeS. These begin to complicate the calculation. Appropriate Use and Relevance of the Ratio Since the ratio calculation assumes the coexistence of FeCO 3 and the mackinawite form of FeS, the conditions where the ratio is valid is shown as Line 16 of Figure 3. At the lower temperatures shown by Line 8, the ratio calculation is not valid because the equilibrium shown by this line involves the equilibrium between Fe 2+ and FeS cor- rosion products, not FeCO 3 -FeS. Although it might be possible to extrapolate the ratio for Line 16 to the range defined by Line 8 for rough guidance purposes, such actions could be fraught with danger because other considerations, such as pH and the total dissolved solids, could introduce signifi- cant errors. It should also be remembered that the ratio calculation assumes that the form of FeS that is precipitated is mackinawite. This assumption is valid for low concentra- tions of H 2 S and temperatures below ~100 to 120 °C. At higher temperatures and con- centrations of H 2 S, the form of FeS that is produced will usually be either troilite or pyrrhotite. Both of these compounds are more thermodynamically stable. Changing from mackinawite to pyrrhotite using Dun- lop's original calculation would mean that the value of the ratio would need to be adjusted from 500 to at least 2,500. Appropriate use of the ratio therefore requires the consideration of a number of factors, such as: • The sensitivity of the ratio to as- sumed thermodynamic inputs, which can alter the calculated ratio to val- ues as low as 9 to more than 2,600. • C on si d eration of th e pro du ction conditions to det ermin e w h eth er FeCO 3 would form in the absence of H 2 S as opposed to Fe 2+ ions in solu- tion. These conditions usually exist for temperatures in excess of 65 °C or where sufficiently high concentra- tions of bicarbonate exist in the pro- duced water to exceed the super - saturation value for FeCO 3 . FIGURE 2 Calculated effect of temperature upon the CO 2 /H 2 S ratio. FIGURE 3 Relationship between oilfeld corrosion products. 9 MATERIALS SELECTION & DESIGN

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