The corrosion monitoring sub-committee of the Japan Society of Corrosion Engineering started at an urgent request from the Society of Chemical Engineering of Japan in 1976.
The guiding principles as well as the activities of the committee in the past two years are out-lined. Recent international trends in researches and meetings on corrosion monitoring are also referred.
It is pointed out that corrosion testings and monitoring techniques have been developed along with a progress in corrosion sciences and technologies of metals. Recently corrosion monitoring techniques have attracted attention as one of powerful means for reduction of corrosion loss in technological societies. The monitoring techniques are, however, still at a developing stage and will await intensive studies not only by corrosion scientists and engineers but also by outsiders.

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Objects of and principles in various techniques of corrosion monitoring are presented.
Some typical monitoring techniques practically applied to equipments and plants such as thickness-, weight loss-, eddy current- and electric resistance-measurements, and electrochemical (linear polarization)-, corrosion sound- and hydrogen probe- methods are also given.

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Three methods of measuring corrosion rate, two polarization resistance methods and an electric resistance method, were discussed in this chapter. At first, the principle of the d. c. and a. c. polarization resistance methods based on the Stern-Geary equation for instantaneous rates of corrosion was described and the error of the methods was briefly discussed. Two measuring technique based on the polarization resistance, the PAIR method and the Marsh method, were described, respectively, and the characteristic feature of the instruments available on the market was outlined and probe materials applicable to the instruments were described for user's convenience. The polarization resistance meter using a rectangular wave technique was also illustrated. The equipment for measuring corrosion by the electric resistance technique and available probe materials were described. Some new types of electrochemical corrosion measurements and the mesuring instruments were also introduced.

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This paper deals with some examples of application of methods for monitoring of corrosion of metals to engine cooling systems. In service tests of a simulated automobile cooling system linear polarization methods in-situ were applied to the measurements of corrosion rates of aluminum specimens and aluminum water pumps immersed in inhibited ethylenglycol coolant under operating conditions. It was found that rates of corrosion of Al estimated from the polarization measurements correlated well with the actual weight losses. Periodic linear polarization measurements made in service tests continued for 7 months (running distance: 11, 300kms) under cooling conditions similar to practical ones showed that the estimated corrosion rates of the aluminum were in a good agreement with the average corrosion rate calculated from the weight losses of aluminum specimens removed from the test cars. The linear polarization resistance method for monitoring of corrosion of metals in diesel locomotive cooling system and an electronic sensor for detecting of the corrosivity of automobile antifreezing coolants are also described.

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Three examples of application of corrosion monitoring methods to petroleum refining facilities and oil field systems reported in Literatures are outlined. In the first case, electrical resistance probes were used in the overhead system of a crude distillation unit in the field to investigate corrosion of metals in the system. The electrical resistance probes proved useful to monitor corrosion rates of metals in actual refinery operation and contributable to the findings of factors affecting corrosion behaviours of metals and to corrosion control programs.
Polarization resistance probes were employed in laboratory studies of the corrosion in sour water stripper in the second case. Linear polarization was measured under various experimental conditions such as different concentrations of KCN and different ratios of NH₃ to H₂S which affect the corrosion of metals in simulated sour water stripper solutions. In the third case, polarization resistance probes, especially of a flush mounted type, were used for gas well flow lines. The flush mounted probes can monitor fluctuation in corrosion rates of pipe walls under a great care of the sensitivity of the probes.

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Applications of electrochemical methods for measuring of corrosion rates to the corrosion of mild steel and stainless steel in cooling water systems were reviewed. Some remarks in the use of the electrochemical methods were pointed out.
For cooling water with chromate inhibitors or nonchromate inhibitors, estimated corrosion rates of mild steel were in a good agreement with measured rates from weight loss. Electrochemical methods allow emergency control action to be taken speedily, for the methods can give instantaneous corrosion rates of steel specimens and monitor change in the corrosion rates. Corrosion probes have to be placed in the most corrosive parts in the systems.
Besides corrosion rate measurements, control of cycles of concentration by additional measurements of conductivity of cooling water is required for operations under better corrosion protection.

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This paper deals with some topical examples of application of corrosion monitoring by electrochemical means to chemical plants, cooling water systems of a nuclear reactor and atmospheric corrosion fields. The corrosion monitoring with the linear polarization resistance method has an advantage in instantaneous measuring of corrosion rates of metals over conventional weight loss measurements. However, this method always requires the correlation between the corrosion rates calculated from the weight losses of immersed test specimens and those obtained from the readings of a corrosion meter.
Deviations from the correlation are sometimes found for corrosion occurred in slurry scrubbers equipped in desulfurization plants and for corrosion in plants with processes operated at high pressures and high temperatures. The deviations can often be interpreted in terms of deposits, scales and accumulation of corrosion products formed on the surfaces of monitoring probes.
Potential measurements are also useful for a better understanding of specific corrosion phenomena in a cooling water system of a nuclear reactor. The electrodes of stainless steel and platinum used as the corrosion monitoring can detect a large drop in corrosion potential due to the increase in temperature of the water and change in water chemistry.
The application of monitoring techniques developed recently to practical fields is also described.

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Corrosion control methods in chemical plants both in operation and during inspection periods have been proposed from the standpoint of corrosion rate measurements.
During the operation of the plants, a spool test of steel specimens, chemical analysis of dissolved ions in reaction media, weight loss measurements by a corrosion probe method and measurements of the electrical resistance of the corroded specimens have been made.
During the inspection periods, the state of corrosion progress in chemical plants has been estimated with a supersonic thickness meter, radioactive tracer techniques, a roughness meter, and an eddy current corrosion detector.
Finally, several suggestions on practical corrosion monitoring in the plants have been made.

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Examples of corrosion monitoring in chemical industry are presented. Discussions are made on the feasibility of the application of corrosion monitoring techniques to corrosion problems in the industry, such as caustic stress cracking, carbonate stress cracking of steels, Cl^- stress corossion cracking of austenitic stainless steels, hydrogen embrittlement of titanium, hydrogen blistering of steels, corrosion of stainless steels in reducing ammonium sulfate and carburization of some heat resisting materials.

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The monitoring of localized corrosion of stainless steels in chemical plants by measuring the spontaneous corrosion potential of the equipment itself is discussed concerning the monitoring principle and points to be remarked in the measruements. Variations in corrosion potential with time measured for fermentors and evaporators under the operating conditions are shown to monitor the possibility of occurrence of localized corrosion such as pitting and crevice corrosion.

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An electrochemical monitoring of corrosion of a high pressure stainless steel decomposes in a urea plant, one of corrosion hazards often taking place in various synthesis processes of urea, was made with a newly developed electrochemical test apparatus.
The process liquid in the decomposer was composed of NH₃, H₂O, CO₂, urea, ammonium carbamate and some other minor elements. The decomposer was operated at a temperature of 120-160°C and a pressure of 15-10kg/cm². Test of corrosion of the stainless steels for the decomposer were conducted for both the process liquid in the high pressure decomposer and simulated solutions prepared from pure chemicals.
Anodic polarizations showed that 316L stainless steel in the process liquid had two current peaks and that the first peak around -0.7V and the second peak around +0.3V represented a primary passivation current and a rapid transpassive dissolution current, respectively.
It was found that potentials kept between the two peak potentials could prevent 316L S. S. from corroding and that under normal operating conditions, 316L S. S. of the decomposer had potentials in its passive region, -0.1 to 0 volt.
Monitoring of the potentials of electrodes put in an autoclave set connected to the decomposer allow an early detection of the corrosion of the decomposer 316L S. S. taking place under unusual poerating conditions.

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Cathodic protection is effetively applied to systems by well-controlled corrosion monitoring.
A brief review is given for different kinds of corrosion monitoring methods such as direct observations of protected parts or test specimens, potential monitoring methods of equipments and plants placed in corrosion media, measurements of corrosion current passing through protected metals, measurements of protective current, and automatic potential control.
Recent developments in potential monitoring are also presented.

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The development of electrochemical measurements in high temperature and high pressure environments was briefly reviewed. Reference electrode systems available in high temperature water were discussed based on corrosion principles. Electrochemical corrosion monitoring systems were classified into two categories which include corrosion rate monitoring and the prediction of enviromental corrosivity. Possible developments in high temperature elctrochemistry were also discussed.

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The recent stage and future problems in corrosion monitoring in chemical industry are reviewed with respect to each monitoring technique and its developing trend. The importance of corrosion education for engineers is also pointed out to develop a high level monitoring system and to apply it to practical fields successfully.

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Laboratory and field tests of corrosion rates of mild steel in cooling water for heat exchangers and reactors were made by the linear polarization resistance methods. In the laboratory tests, intermittent measurements of polarization resistances (R_p) of rod specimens were made with 0.1Hz galvanostatic pulses for solutions with zinc-phosphate inhibitors and an organic inhibitor. Gravimetrical corrosion rates (v_w) were also calculated from weight loss of the specimens at the end of each polarization experiment. The value of K in the equation of corrosion rate (v), v=K/R_p, for each specimen was determined with the measured v_w and the mean value of 1/R_p over the whole period of the polarization test. The average K obtained from values of K for the solutions with 0-200ppm zinc-phosphate inhibitors was used for the estimation of corrosion rates (v_p) in field tests from measured R_p. Mean values of v_p were in a good agreement with v_w within 2mdd for cooling water treated with zinc-phosphate inhibitors and without inhibitor. It was found that solution resistance should be corrected for R_p with comparable solution resistance.

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