The history in Japan of adopting corrosion control technologies for cooling water systems is not so long as in European and American countries. In Japan the use of phosphate corrosion inhibitors for carbon steel was the first commercial corrosion protection for cooling water systems. This was followed by the use of zinc, chromate, organic phosphate and the like corrosion inhibitors, which contributed greatly to the development of corrosion control technologies. During 1980's technologies for stabilizing corrosion inhibitors by the use of polymer dispersants were developed, which permitted operating circulating cooling water at higher cycles of concentration. This was quite a technical innovation. More recently, environmental hazards associated with phosphates and heavy materials (chromate, etc.) have led to the development of “nonphosphate and non-metal” programs in which waters soluble polymers play key rules in controlling corrosion. This new direction of corrosion control the technologies has got to be fully established. On the other hand, “non-chemical treatment programs” which do not use any chemical are under consideration. In this paper, special mention is also made of treatment programs for closed recirculating cooling water systems and copper corrosion inhibitors.
The corrosion problems and the corrosion inhibitors for boiler systems are explained. In a boiler system, with places, water quality differs and the form and measure of corrosion change. Conventionally, as for the corrosion inhibitors for boiler systems, hydrazine and sodium sulfite have been used. Recently, the chemicals that improved the fault of the conventional corrosion inhibitors are developed and used. They are improved oxygen scavengers or film forming type corrosion inhibitors. Improving points are the toxicity of chemical, workability, etc. Volatile amines are used as the corrosion inhibitor for a steam condensation system. Volatile amine is classified into three sorts, a neutralized type, a film forming type, and a hybrid type. The contents of new chemicals are described. From now on, effective treatment is expected with the combination of water treatment equipment and chemical.
In order to increase the amount of middle distillate and to improve energy efficiency, there is a trend in atmospheric distillation tower operations to reduce the tower top operational temperature and pressure. Directionally, crude oil processing move towards heavier crude oils, higher amounts of salts and longer operational terms. Also, there will be a trend in increasing chlorides levels in tower tops caused by a reduction or termination of caustic soda usage due to processing of residual oil streams through hydrodesulfurization units to other cracking processes. It is desired to be able to understand and control this corrosion based on effective measurement in order to have stable long term operations in spite of this worstening corrosion environment in the tower overhead system. This review is about the various varification steps of applying this corrosion control technology on actual units in the field.
The availability of an electrochemical impedance spectroscopy (EIS) for an analysis of pitting corrosion has been discussed. In this analysis, the following problems must be worked out, i.e., a current distribution in the pitting and a time variation of the impedance. The current distribution was investigated by transmission line model using a glassy carbon and iron electrodes that have imitated pitting (cylindrical pore). On the other hand, the time variation of impedance amplitude cannot be neglected since the dissolution rate of metal is very large and the nature of the pitting changes with time. The time variation was compensated by the method using a spline function. In the present paper, the analytical method by EIS to solve the abovementioned problems was proposed.
Limitting current density ilim of oxygen reduction was measured under a thin NaCl solution film. The reciprocal value of the obtained ilim increased proportionally with an increase in the electrolyte layer thickness δ when the δ ranged from 20μm to 100μm. From the dependency of the ilim on the δ, the oxygen reduction mechanism under a thin electrolyte film was proposed. In the proposed model, the oxygen reduction consists of three consecutive processes; dissolution of oxygen into the electrolyte film from gas phase, diffusion through the electrolyte film and charge transfer at electrolyte/electrode interface. From the analysis on the basis of the model, the diffusion coefficients of oxygen in the electrolyte film were obtained to be 5.9×10-9 m2s-1 and 6.1×10-9 m2s-1 for 0.5kmol/m3 and 5.4kmol/m3 NaCl solution films, respectively, and the rate-determining rates of dissolution at gas/electrolyte interface were determined to be approximately 5Am-2 and 3Am-2 for 0.5kmol/m3 and 5.4kmol/m3 NaCl solutions, respectively.