Passive Corrosion Protection

Cables with a copper sheathing are used only seldom. The protective cover is the same as with a corrugated steel-sheathed cable. If a cable with copper sheathing is connected to a lead-sheathed cable (A-PMbc) (see Table 13-1), the copper sheathing acts as a cathode in a galvanic cell and is therefore cathodically protected. 

Cable type Construction and material Rest potential Protection potentials Aerobic Anaerobic  

A-PMbc Lead sheath, steel strip armoring -0.40 to -0.52 -0.85 -0.95 -1.7 

A-PWE2Y Corrugated steel sheath, PE protective coating -0.65 to -0.75 -0.85 -0.95 — 

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Passivation/corrosion protection in pre-molded plastic packages. 

EIS is the response of corrosion systems to ac excitations. It has been used in corrosion research to estimate corrosion rates or study the metal passivation, corrosion protection using inhibitors, sacrificial barrier properties, and polymer coating performance on metals. EIS only estimates polarization resistance. As with Hnear polarization, the corrosion current is calculated using the Stem-Geary equation for known values of the anodic and cathodic Tafel slopes. 

Beck, R, V. Haase, and M. Schroetz. 1996. Polyheteroaromatic layers on commodity metals (CIPL)— passivation, corrosion protection, AIP Conference Proceedings 354 (Organic Coatings), 115. 

Corrosion protection of metals can take many fonns, one of which is passivation. As mentioned above, passivation is the fonnation of a thin protective film (most commonly oxide or hydrated oxide) on a metallic surface. Certain metals that are prone to passivation will fonn a thin oxide film that displaces the electrode potential of the metal by +0.5-2.0 V. The film severely hinders the difflision rate of metal ions from the electrode to tire solid-gas or solid-liquid interface, thus providing corrosion resistance. This decreased corrosion rate is best illustrated by anodic polarization curves, which are constructed by measuring the net current from an electrode into solution (the corrosion current) under an applied voltage. For passivable metals, the current will increase steadily with increasing voltage in the so-called active region until the passivating film fonns, at which point the current will rapidly decrease. This behaviour is characteristic of metals that are susceptible to passivation. 

An especially insidious type of corrosion is localized corrosion (1—3,5) which occurs at distinct sites on the surface of a metal while the remainder of the metal is either not attacked or attacked much more slowly. Localized corrosion is usually seen on metals that are passivated, ie, protected from corrosion by oxide films, and occurs as a result of the breakdown of the oxide film. Generally the oxide film breakdown requires the presence of an aggressive anion, the most common of which is chloride. Localized corrosion can cause considerable damage to a metal stmcture without the metal exhibiting any appreciable loss in weight. Localized corrosion occurs on a number of technologically important materials such as stainless steels, nickel-base alloys, aluminum, titanium, and copper (see Aluminumand ALUMINUM ALLOYS Nickel AND nickel alloys Steel and Titaniumand titanium alloys). 

The second class of anodic inhibitors contains ions which need oxygen to passivate a metal. Tungstate and molybdate, for example, requke the presence of oxygen to passivate a steel. The concentration of the anodic inhibitor is critical for corrosion protection. Insufficient concentrations can lead to pitting corrosion or an increase in the corrosion rate. The use of anodic inhibitors is more difficult at higher salt concentrations, higher temperatures, lower pH values, and in some cases, at lower oxygen concentrations (37). 

The active and passive electrochemical processes on which present-day corrosion protection is based were already known in the 19th century, but reliable protection for pipelines only developed at the turn of the 20th century. 

Corrosion protection measures are divided into active and passive processes. Electrochemical corrosion protection plays an active part in the corrosion process by changing the potential. Coatings on the object to be protected keep the aggressive medium at a distance. Both protection measures are theoretically applicable on their own. However, a combination of both is requisite and beneficial for the following reasons ... 

Rectifiers working according to the control diagram in Fig. 8-6 are used for anodic corrosion protection in passivatable systems that go spontaneously from the passive to the active state when the protection current is switched off [12]. The predetermined nominal voltage between reference electrode and protected object is compared with the actual voltage f/j in a differential display unit D. The difference AU = is amplified in a voltage amplifier SV to VqAU. This... 

The fundamentals of this method of protection are dealt with in Section 2.3 and illustrated in Fig. 2-15. Corrosion protection for the stable-passive state is unnecessary because the material is sufficiently corrosion resistant for free corrosion conditions. If activation occurs due to a temporary disturbance, the material immediately returns to the stable passive state. This does not apply to the metastable passive state. In this case anodic protection is necessary to impose the return to the passive state. Anodic protection is also effective in the unstable passive state of the material but it must be permanently switched on, in contrast to the metastable passive state. 

It is known that thin (-20 A) passive films form on iron, nickel, chromium, and other metals. In s ressive environments, these films provide excellent corrosion protection to the underlying metal. The structure and composition of passive films on iron have been investigated through iron K-edge EXAFS obtained under a variety of conditions, yet there is still some controversy about the exact nature of... 

Corrosion products formed as thin layers on metal surfaces in either aqueous or gaseous environments, and the nature and stability of passive and protective films on metals and alloys, have also been major areas of XPS application. XPS has been used in two ways, one in which materials corroded or passivated in the natural environment are analyzed, and another in which well-characterized, usually pure metal surfaces are studied after exposure to controlled conditions. 

Evans, U.R. (1937) Metallic Corrosion, Passivity and Protection (Edward Arnold, London), 2nd edition, 1945. 

Metals which owe their good corrosion resistance to the presence of thin, passive or protective surface films may be susceptible to pitting attack when the surface film breaks down locally and does not reform. Thus stainless steels, mild steels, aluminium alloys, and nickel and copper-base alloys (as well as many other less common alloys) may all be susceptible to pitting attack under certain environmental conditions, and pitting corrosion provides an excellent example of the way in which crystal defects of various kinds can affect the integrity of surface films and hence corrosion behaviour. 

Full chromate passivation (Section 15.3) improves the corrosion resistance of both zinc and cadmium towards all environments and is applied for a wide range of applications. Clear and olive-coloured chromate coatings can also be applied for certain purposes. The highest degrees of corrosion protection... 

See also Corrosion Potential, Electrode Potential, Equilibrium Potential, Flade Potential, Open-circuit Potential, Passivation Potential, Protection Potential, Redox Potential.)... 

In these simpler types of boiler plants, corrosion protection generally is provided solely by the use of chemical treatments containing anodic passivators and alkali. Under conditions in which the (oxygen-containing) MU water requirement is expected to be very low, no specific oxygen scavenger is typically employed. This is often not the case in practice, however, for various reasons such as inadequate preventative maintenance, poor BD practice, the infiltration of air or oxygen, or other such reason. 

Chemical conversion coatings are applied to previously deposited metal or basis material for increased corrosion protection, lubricity, preparation of the surface for additional coatings, or formulation of a special surface appearance. This operation includes chromating, phosphating, metal coloring, and passivating. 

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