As a result of satisfactory performance, cathodic protection gained worldwide recognition as the most effective and economical method to protect coated steel pipelines, structures, canal gates, condensers, and others against corrosion together with stress corrosion cracking. The effectiveness of cathodic protection in practice can be established in more than one way, and several criteria have been used in the past to prove whether protection is complete. This paper reviews the criteria for cathodic protection of coated steel pipelines and structures in the environments, including seawater, soil, and concrete. As for criteria for cathodic protection of steels in seawater and soil, the action of bacteria is considered. A criterion that indicates degree of protection, including overprotection, is obtained through measuring the potential of the protected structure. The potential of a cathodically protected structure is obtained ideally by placing the reference electrode as close as possible to the structure to avoid an IR drop through coatings and the electrolyte such as soil and concrete. In the case of potential measurement of pipeline, the instant-off method, with steel probes simulating coating defects, is recommended as an effective method to avoid an IR drop. A particular emphasis is placed on the necessity for establishment of the criterion to assess the effect of induced AC voltage on a cathodic protection system for buried pipeline.
The galvanic anode system by aluminum alloy anode is popular as cathodic protection system for harbor steel structures. An example of cathodic protection construction work which is presented this paper, is constructed for Aomi wharf of Port of Tokyo on 1999. The purpose of this construction is for a renewal of aluminum alloy anodes. Four hundred eighty (480) aluminum alloy anodes are installed for the corrosion control of the steel pipe piles wharf of 370m long submerged in sea water and sea mud.
The cathodic protection system together with coating on the pipe is general, however asphalt coating and/or coal-tar enamel has been substituted by polyethylene lining, which is good quality, has very high electric insulation characteristic and therefore the mainstream now. As the result, high coating resistance has minimized the protective current required to the pipe. In the meantime the tendency to increase the high voltage transmission line has resulted in A/C corrosion, A/C induction and further the damage of lightning are getting bigger and the countermeasures to than are now highlighting. This article is to introduce the points which have to be paid attention in design, example and judgement of the some protection systems for effect suitable to the site. Finally we introduce some examples which have been very efficiently controlled.
Cathodic protection has been expected to be one of the most useful methods in corrosion protection for reinforcement of concrete structure. On prestressed-concrete (PC) structure, however, there are only a few special examples for the adoption of the cathodic protection because of a serious problem that must be resolved. That is, hydrogen embrittlement of PC tendons due to hydrogen evolution around the tendon in the over-protected condition. This report will present the applicable range of cathodic protection to chloride attacked PC structures.
In recent years, the progress in the electronic technology and software has led to computerization of measuring instruments and diversification of communication technology. As for the monitoring technology progress has been made by leaps and bounds especially in the field of remote monitoring technology. Unless a system is on a large-scale, the manager of the system is able to construct a remote monitoring system easily. An inexpensive Windows-PC can be used for the main station, while a user can select communication means suited for his needs such as the conventional telephone network, cellular network. Also for terminal stations, a terminal data-logger incorporating functions of measurement/control/communication can be easily developed. This report explains requirements of the remote monitoring systems for cathodic protection and reviews an example of systems for cathodic protection of rebars in reinforced concrete structures and underground pipelines, respectively.
A new corrosion resistant Ni-Cr-Mo-Ta alloy, MAT 21 has been developed. This new alloy, which is composed of 19wt% chromium, 19wt% molybdenum, 1.8wt% tantalum and balance nickel, shows excellent corrosion resistance to pitting and crevice corrosion as well as to general corrosion compared to the existing Ni-Cr-Mo alloys.
A channel flow double electrode was applied to the investigation of a disproportional reaction of Cu in acidic solution containing chloride ions. The disproportional reaction rate could be determined by the collecting electrode, on which the Cu(I) emitted from the working electrode was oxidized to Cu(II). It was found that the disproportional reaction was divided into three regions depending on the chloride concentration: (1) the region without chloride ion or at low concentration of chloride ions, (2) the region in the presence of chloride ions, and (3) the region at high concentration of chloride ions. The disproportional reaction in the regions (2), (3) and is consist of the following anodic and cathodic reactions involving the intermediate CuCl. Anodic reaction: Cu+Cl-↔CuCl+e- CuCl+Cl-→CuCl2- Cathodic reaction: Cu2++Cl-+e-→CuCl CuCl+Cl-=CuCl2-
Titanium nitride (TiN) coatings for bearings and seals used in rotating machinery have been developed by the dynamic ion beam mixing (DM) process, in which Ti vapor deposition is combined with simultaneous nitrogen ion beam irradiation. Pinhole defects of TiN coatings on a stainless steel were investigated electrochemically in a 0.5kmol·m-3 H2SO4+0.05kmol·m-3 KSCN solution at 298K. A TiN film with a compositional ratio of N/Ti=0.8 has a single phase TiN structure and the preferred orientation of TiN (111) plane. The critical current density for passivation, icrit, of TiN-coated SUS 304 stainless steel decreased with increasing film thickness. The defect area ratio of TiN coatings was evaluated by comparing the icrit of TiN-coated specimen with that of non-coated specimen, and the ‘true’ defect area ratio was also evaluated by optical microscopy after anodic polarization tests. Such electrochemical methods was proved to be very effective for the quantitative evaluation of pinhole defects in corrosion-resistant coatings.
The pitting corrosion of copper tubes in tap water has been investigated by electrochemical measurements. Measurements of corrosion potentials, potentiodynamic polarization tests and potentiostatic corrosion tests were carried out in two types of tap water: one type of water was taken in the area where the moundless pitting has occurred (called as ‘pitting area's water’) and another type was taken in the area where the pitting has not occurred (called as ‘pitting-free area's water’). The corrosion potential in pitting area's water was higher than that in pitting-free area's water and shifted to a nobler value gradually. Current densities of pitting area's water also indicated higher values than those of pitting-free area's water in both polarization tests and corrosion potentiostatic tests. Many visible pits existed on the surface of the specimen after the potentiostatic tests at 200mV (Ag/AgCl) in pitting area's water.
For strategic maintenance of the plant components against stress corrosion cracking (SCC) which is a time-dependent degradation phenomenon, the SCC prediction technology is indispensable. This paper presents a kinetic slip dissolution SCC growth model for the technology, which was developed for the nickel-base alloys and stainless steels used in high temperature water of nuclear power plants. The general expression derived from the present model seems to be applicable to all the process from SCC initiation to crack propagation, because both data of the dependency of stress intensity on SCC crack growth and the dependency of stress on SCC initiation life could be rationally explained by the equation based on the SCC model.
The influence of water quality factors such as silica (SiO2) and chloride ion (Cl-) on a stability index of water and a corrosion rate of steel was investigated by corrosion weight loss tests and analyses of numerical values. SiO2 had an ability of corrosion inhibition, and then, tended to change the stability index at chemical equilibrium state. The stability index and corrosion rate were not affected by Cl- in solution with SiO2.