Corrosion inhibitors are often the first line of defense against internal corrosion, and effective mitigation relies on proactive monitoring and management of these inhibitors to allow for regular feedback and dose adjustment. There have been recent developments in this field, for example in greener chemistries (NACE 2016-7738) and treatment methods (Achour et al. 2008) and a growth in the number of papers published in OnePetro. We found an 18% increase in the number of publications in the last 10 years, compared with the previous 10, when searching the terms “corrosion AND inhibitor”.
Nevertheless, there is room for improvement. Common industry opinion is that current residual monitoring methods, such as colorimetric-complex methods or liquid chromatography mass spectrometry are either not sufficiently reliable for effective inhibitor dosage management or too complex to apply in the field.
Yet there is a significant commercial driver, as dosing corrosion inhibitors is an expensive undertaking and the potential cost improvements from improved management are significant. One North Sea operator reported that it believed it was overdosing inhibitor at an extra cost of pound 400,000 per year. An alternative corrosion inhibitor monitoring approach, first published in 2011 (NACE 11071), exploits the formation of corrosion inhibitor micelles and poses the questions:
Does micelle detection have a place in the inhibitor qualification process?
Can it help inform chemical management in the field?
The most common class of corrosion inhibitor used in the oilfield is amphiphilic surfactant molecules, which form a barrier on the pipe surface to protect from corrosion. Above a certain concentration, nanoscale aggregates called corrosion inhibitor micelles are formed. This point is called the Critical Micelle Concentration (CMC) and is specific to the chemical and unique physical conditions found in each system.
The CMC has been shown to be an important factor in establishing an effective inhibitor dose, with the optimum dose being equivalent to the CMC (e.g. NACE 10326). Below the CMC, there is an opportunity to dose additional inhibitor to further reduce corrosion while above the CMC, surplus inhibitor may be present.
Earlier studies looking at CMC and its relation to optimal inhibitor dose were relatively simplistic laboratory studies. Out of the 43 CMC studies reviewed, only one study used formulated inhibitors, with the majority looking only at single components, e.g. an imidazoline. Importantly, all 43 studies used model systems to test the corrosion inhibitors, rather than complex field fluids. It is therefore pertinent to address the question of whether micelles and the significance of the CMC are still relevant in real field conditions.