Monitoring of Corrosion Inhibitors

Monitoring of Corrosion Inhibitors Absorption by Films Using Physical Fields... 

J. P. Broomfield, Results of long term monitoring of corrosion inhibitors applied to corroding rein-forced-concrete structures , Corro-sion/2000, paper 0791, NACE International Houston (TX) USA, 2000. 

Broomfield, J.P. (2000). Results of Long Term Monitoring of Corrosion Inhibitors Applied to Corroding Reinforced Concrete Structures. Corrosion 2000. Paper No. 791. 

The freezing point of the coolant should be monitored for coolants in all types of service. Additionally, maintenance of the corrosion inhibitor levels is requited of the heavy-duty service coolants and the stationary engine coolants. Because corrosion inhibitors and combinations of corrosion inhibitors work most effectively at given concentrations and specific ratios to the other inhibitors, appropriate concentrations must be maintained to maximize corrosion protection. Many manufacturers of coolants for stationary engines, and manufacturers of SCAs, provide an analytical service to monitor the effective inhibitor concentrations in the system periodically. Recommendations can then be made for proper maintenance and inhibitor replenishment. 

Monitoring can also be used to optimize the chemistry and level of corrosion inhibitors used. If too little inhibitor is used, enhanced corrosion can result and failure may follow. If too much is used, costs will increase without providing any additional protection. Optimization of the addition of inhibitor in terms of time, location in the process, and method of addition can also be evaluated through the use of carefully placed probes. 

Oil and gas streams require additional consideration regarding the phase to be monitored, i.e. water or oil stream, which in turn, will depend on the objectives of the monitoring, i.e. metal loss and/or optimisation of corrosion inhibitor addition. Factors to be considered are ... 

When the bottom plates are welded together, the coating is partially destroyed. Research and field work showed that protection can be achieved using volatile corrosion inhibitors under the tank [688]. This works alone or in combination with cathodic protection. Double tank bottoms for leakage monitoring are often specified for new tanks. However, the same problem of coating destruction occurs. Volatile corrosion inhibitors are an excellent solution from both a technical and an economic standpoint. This type of corrosion inhibitor... 

For this reason, it is recommended that addition of corrosion inhibitor to previously uninhibited systems be closely monitored. Initial low level addition followed by a gradual increase in the corrosion inhibitor treat rate will help to minimize the removal of pipeline rouge and filter plugging problems. 

Polarization resistance of the corroding sample may also be monitored over an extended duration. Thus, AC impedance may be used for online monitoring of a corrosion system such as on-line determination of corrosion inhibitor performance, as depicted in Figure 1.31. 

A systematic evaluation of corrosion inhibitors is now underway at the UK Building Research Establishment preliminary results may be available in 1997-8, SHRP undertook field trials of two inhibitor applications on bridge decks and substructures in the USA, These will be monitored from 1994 to 1999, Other trials are underway in Europe on. i pecLfic materiaLs. 

In summary, all electrochemical methods used and proposed for corrosion monitoring and eveduation of corrosion inhibitors serve useful purposes and certainly have their place in the industry. However, careful interpretation of the results is always called for. In this context, it must be emphasized that all commercial instruments employing one or the other technique are sold with exaggerated claims of accuracy, reliability, or usefulness. 

The relationship between a corrosion product layer and inhibitors has also been discussed, among others, by Lorenz [66], and Lorenz and Mansfeld [67]. These authors point out that in many practical systems, for instance aerated water and carbon steel, an interaction occurs between, in this case, iron oxide and the inhibitor to the point where the inhibitor is not only adsorbed on the oxide surface, but actually incorporated into the three-dimensional oxide layer. Clearly, three-dimensional or interphase inhibition caimot be achieved in short tests or by filming procedures. Furthermore, measuring techniques have to take into consideration the altered chemical and electrochemical conditions across such bulk interphase layers. It is unfortunate that this aspect of corrosion and corrosion inhibition has not received more attention, and it is suggested that this lack of attention has seriously held back all aspects of corrosion inhibitor applications and monitoring of effectiveness. 

Suppliers usually provide the laboratory evaluation (using conventional test methods) of corrosion inhibitors, scale inhibitors, bactericides, and oxygen scavengers. User companies generally provide evaluation by field monitoring with corrosion specimens. Crevices may be incorporated in specimens to evaluate the possibility of localized corrosion, such as that related to oxygen contamination. 

The corrosion potential can be easily measured by recording the potential difference between a corroding electrode and a stable reference electrode. It can be used in conjunction with Pourbaix diagrams as a basic indicator of corrosion thermodynamic status, such as active, passive or immune behavior with and without the presence of corrosion inhibitors. Corrosion potential monitoring is often useful in understanding the effects of a corrosion inhibitor on corrosion thermodynamics. Unfortunately, like any thermodynamic quantity, the potential value on its own does not provide information on the rate of corrosion. 

Y. J. Tan, S. Bailey, and B. Kinsella, The monitoring of the formation and destruction of corrosion inhibitor films using electrochemical noise analysis (ENA) , Corros. Sci., 38,1681 (1996)... 

Differential weld corrosion has been found to be particularly problematic in oil and gas flow lines. Even minor differences in composition and microstructure have been found to result in severe preferential galvanic dissolution of pipeline weldments. The selection of welding consumables and welding procedures to minimize this risk is critical. However, even with these precautions, operating conditions can induce severe preferential weld corrosion. On-line corrosion monitoring programs have been conducted in oil and gas pipelines to identify these operating conditions and to optimize the application of corrosion inhibitors to control the problem. 

One of the maj or ways of protecting oil and gas production and operating systems against corrosion is by applying corrosion inhibitors. The corrosion inhibitors are evaluated in order to determine if the corrosion preventive measures applied are necessary, and if the required lifetime can be achieved with a particular inhibitor, as the effective life of corrosion inhibitors varies with the quantity of water intrusion. The purpose of this chapter is to evaluate the on-line monitoring of corrosion and corrosion inhibitor effectiveness under different conditions. 

Finally it deals with monitoring internal corrosion. It provides guidance for on-line monitoring of internal corrosion in plants associated with the oil, gas and chemical industries, and guidance on laboratory monitoring and evaluation of corrosion inhibitors. The book also covers experiments on the corrosion behaviour of high-alloy tubular materials iu inhibited acidizing conditions. 

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