Corrosion protection process

Electrochemical corrosion is understood to include all corrosion processes that can be influenced electrically. This is the case for all the types of corrosion described in this handbook and means that data on corrosion velocities (e.g., removal rate, penetration rate in pitting corrosion, or rate of pit formation, time to failure of stressed specimens in stress corrosion) are dependent on the potential U [5]. Potential can be altered by chemical action (influence of a redox system) or by electrical factors (electric currents), thereby reducing or enhancing the corrosion. Thus exact knowledge of the dependence of corrosion on potential is the basic hypothesis for the concept of electrochemical corrosion protection processes. 

The principle of electrochemical corrosion protection processes is illustrated in Figs. 2-2 and 2-5. The necessary requirement for the protection process is the existence of a potential range in which corrosion reactions either do not occur or occur only at negligibly low rates. Unfortunately, it cannot be assumed that such a range always exists in electrochemical corrosion, since potential ranges for different types of corrosion overlap and because in addition theoretical protection ranges cannot be attained due to simultaneous disrupting reactions. 

More recently, workers in the field have recognized the probable need for a multiple parameter model to understand the corrosion protection process. Funke (12) proposed a model based on water... 

Table 5.9-16. Corrosion Protection Processes with Different Pigment Types.

The end product specification of a process may be defined by a customer (e.g. gas quality), by transport requirements (e.g. pipeline corrosion protection), or by storage considerations (e.g. pour point). Product specifications normally do not change, and one may be expected to deliver within narrow tolerances, though specification can be subject to negotiation with the customer, for example In gas contracts. 

Baeckman W v, Schenk W and Prinz W 1997 Handbook of Cathodic Corrosion Protection Theory and Practice of Electrochemical Protection Processes (Flouston, TX Gulf)... 

Corrosion Resistance. The environment to which a plated part is to be exposed should be a part of any definition of corrosion resistance. Problems arise in testing a part in its intended environment in part because of the long time period required. In many plating processes, corrosion resistance is direcdy proportional to the plate thickness, so a specification on plate thickness is a much faster method of indirectiy measuring corrosion resistance. In specifying a corrosion resistance requirement for the production of plated goods, accelerated tests are used especially if plate thicknesses caimot be related to corrosion protection. 

As an additional antifoulant measure, SHEs have been coated with a phenolic hning. This provides some degree of corrosion protection as well, but this is not guaranteed due to pinholes in the lining process. 

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. 

Electrochemical corrosion protection has the objective of reducing corrosion damage or removing it altogether. Three different processes are discussed in this work ... 

If the products of electrolysis favor other types of corrosion, electrochemical protection processes should not be applied or should be used only in a limited form. Hydrogen and OH ions are produced in cathodic protection according to Eq. (2-19). The following possible corrosion danger must be heeded ... 

Even with the superposition of the ac with a cathodic protection current, a large part of the anodic half wave persists for anodic corrosion. This process cannot be detected by the normal method (Section 3.3.2.1) of measuring the pipe/soil potential. The IR-free measurable voltage between an external probe and the reference electrode can be used as evidence of more positive potentials than the protection potential during the anodic phase. Investigations have shown, however, that the corrosion danger is considerably reduced, since only about 0.1 to 0.2% contributes to corrosion. 

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 ... 

Cement coatings are usually applied as linings for water pipes and water tanks, but occasionally also for external protection of pipelines [7]. Cement is not impervious to water, so electrochemical reactions can take place on the surface of the object to be protected. Because of the similar processes occurring at the interface of cement and object and reinforcing steel and concrete, data on the system iron/ cement mortar are dealt with in this chapter taking into account the action of electrolytes with and without electrochemical polarization. To ensure corrosion protection, certain requirements must be met (see Section 5.3 and Chapter 19). 

Buried steel pipelines for the transport of gases (at pressures >4 bars) and of crude oil, brine and chemical products must be cathodically protected against corrosion according to technical regulations [1-4], The cathodic protection process is also used to improve the operational safety and economics of gas distribution networks and in long-distance steel pipelines for water and heat distribution. Special measures are necessary in the region of insulated connections in pipelines that transport electrolytically conducting media. 

The electrolysis protection process using impressed current aluminum anodes allows uncoated and hot-dipped galvanized ferrous materials in domestic installations to be protected from corrosion. If impressed current aluminum anodes are installed in water tanks, the pipework is protected by the formation of a film without affecting the potability of the water. With domestic galvanized steel pipes, a marked retardation of the cathodic partial reaction occurs [15]. Electrolytic treatment alters the electrolytic characteristics of the water, as well as internal cathodic protection of the tank and its inserts (e.g., heating elements). The pipe protection relies on colloidal chemical processes and is applied only to new installations and not to old ones already attacked by corrosion. 

In applying electrolytic protection, galvanized tubes can be installed downstream from copper components in water boilers without danger of Cu " -induced pitting corrosion. The protection process extends the application range for galvanized tubes with respect to water parameters, temperature and material quality beyond that in the technical regulations [16, 17]. 

Internal Electrochemical Corrosion Protection of Processing Equipment, Vessels, and Tlibes... 

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