Corrosion interference

Stray currents are produced in the electrolyte during the operation of cathodic-protection systems and part of the protection current may traverse nearby immersed structures which are not being cathodically protected. The resultant corrosion produced on the unprotected structure is referred to as corrosion interaction or corrosion interference. 

In order to be able to properly examine the inherent activity of minute amounts of OER catalysts, one needs a substrate with minimal interference, extremely slow OER kinetics of its own and extraordinary stability at high positive electrode potentials. The unique featiues of 3M s Pt-NSTF (nanostructured thin film) catalyst [12] such as superior durability, electrochemical inertness at high potentials, and the absence of corrosion interference due to exposed carbrui, made it a logical choice as a support [13, 14]. It is well known that pure platinum has a high overpotential for OER. For instance, at a current density of 1 mA/cm, the OER on platinum proceeds at a potential that is 0.47 V higher than oti single crystal ruthenium oxide [15]. Thus, the OER partial current density oti the Pt-NSTF substrate wiU be orders of magnitudes lower than on ruthenium, iridium, and other similar OER-active materials. 

Secnre compatibihty and prevent corrosive interference within their design between items supplied from various sources. 

The kinetics of reactions in which a new phase is formed may be complicated by the interference of that phase with the ease of access of the reactants to each other. This is the situation in corrosion and tarnishing reactions. Thus in the corrosion of a metal by oxygen the increasingly thick coating of oxide that builds up may offer more and more impedance to the reaction. Typical rate expressions are the logarithmic law,... 

Theoretically, controUed deposition of calcium carbonate scale can provide a film thick enough to protect, yet thin enough to allow adequate heat transfer. However, low temperature areas do not permit the development of sufficient scale for corrosion protection, and excessive scale forms in high temperature areas and interferes with heat transfer. Therefore, this approach is not used for industrial cooling systems. ControUed calcium carbonate deposition has been used successhiUy in some waterworks distribution systems where substantial temperature increases are not encountered. 

Clear-bright and blue-bright chromium conversion colors are thin films (qv) and may be obtained from both Cr(III) and Cr(VI) conversion baths. The perceived colors are actually the result of interference phenomena. Iridescent yellows, browns, bron2es, oHve drabs, and blacks are only obtained from hexavalent conversion baths, and the colors are Hsted in the order of increasing film thickness. Generally, the thicker the film, the better the corrosion protection (see Eilmdepositiontechniques). 

For many years the usual procedure in plant design was to identify the hazards, by one of the systematic techniques described later or by waiting until an accident occurred, and then add on protec tive equipment to control future accidents or protect people from their consequences. This protective equipment is often complex and expensive and requires regular testing and maintenance. It often interferes with the smooth operation of the plant and is sometimes bypassed. Gradually the industry came to resize that, whenever possible, one should design user-friendly plants which can withstand human error and equipment failure without serious effects on safety (and output and emciency). When we handle flammable, explosive, toxic, or corrosive materials we can tolerate only very low failure rates, of people and equipment—rates which it may be impossible or impracticable to achieve consistently for long periods of time. 

For conducting tests in pipe lines of 75-mm (3-in) diameter or larger, a spool holder as shown in Fig. 28-21, which employs the same disk-type specimens used on the standard spool holder, has been used. This frame is so designed that it may be placed in a pipe line in any position without permitting the disk specimens to touch the wall of the pipe. As with the strip-type holder, this assembly does not materially interfere with the fluid through the pipe and permits the study of corrosion effects prevailing in the pipe line. 

For corrosion protection in soils, the anodes can be brought close to the object to be protected in the same construction pit so that practically no further excavations are needed. By connecting anodes to locally endangered objects (e.g., in the case of interference by foreign cathodic voltage cones) the interference can be overcome (see Section 9.2.3). 

Measurement of the cable sheathing/soil potential can be used to assess the corrosion danger from stray current interference (see Section 15.5.1). Since the measured values vary widely and the stray currents cannot be switched off, IR-free potential measurements are only possible with great effort. In order to keep the IR term of the potential measurement low, the reference electrode must be placed as close as possible to the measured object. With measurements in cable ducts (e.g., underneath tramway tracks), the reference electrodes can be introduced in an open duct. 

In the Verband Deutscher Elektrotechniker (VDE) regulations [1,4], no demands are made on the accuracy of the measured or calculated voltage drops in a rail network. An inaccuracy of 10% and, in difficult cases, up to 20%, should be permitted. A calculation of the annual mean values is required. If the necessary equipment is not available, a calculation is permitted over a shorter period (e.g., an average day). Voltage drops in the rail network only indicate the trend of the interference of buried installations. Assessment of the risk of corrosion of an installation can only be made by measuring the object/soil potential. A change in potential of 0.1 V can be taken as an indication of an inadmissible corrosion risk [5]. 

Corrosion by Anodic Interference (Cell Formation, Stray Currents)... 

It should be clearly pointed out that with anodic interference according to the data in Fig. 2-6 in Section 2.2.4.1, the corrosivity of the electrolyte for the particular material has no influence on the current exit corrosion. On the other hand, the conductivity of the electrolyte has an effect according to Eqs. (24-102) and (20-4). Chemical parameters have a further influence that determines the formation of surface films and the polarization resistance. 

A secondary seal loop is provided for water withdrawal during major blows when turbulence at the downstream overflow connection to the primary seal loop interferes with normal drainage. Extending the base of the flare stack 3 diameters below the sloped inlet line provides vapor disengaging for the secondary seal leg. The bottom of the stack and inlet line up to 1.5 m above the seal water level are gunite lined for corrosion protection. 

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