Electrochemical measures against corrosion

The recognition that corrosion, with the exception of physical erosion, has an electrochemical mechanism, should allow one to devise measures to reduce the rate at which corrosion occurs. Of course, this expectation has been confirmed in practice and it is the purpose of this section to review the principal procedures based on electrochemical concepts. 

In many earlier chapters, the key to the success of an application of electrochemistry has been the ability to control the potential of,a metal surface, either with a potentiostat or, more likely, by passing a calculated and controlled current density. 

Clearly this concept can be applied to reducing the rate of corrosion by fixing the potential of the surface to a value where the current for metal oxidation is very low. The complete log / versus E characteristic for a metal in a medium where it passivates at positive potentials is shown in Fig. 9.13 it suggests two potential regions where the potential could usefully be held, at negative potentials or in the passive region. The former is known as cathodic protection and, in principle, can give an extremely low rate of corrosion. The second is known as anodic protection and its effectiveness depends on the passivation current density. 

Both these forms of protection, based on an imposed current, require careful positioning of the counterelectrode to ensure that the whole surface is protected. 

For anodic protection this only requires that all the surface has a potential within 

---

The direct electrochemical measurement of such low corrosion rates is difficult and limited in accuracy. However, electrochemical techniques can be used to establish a database against which to validate rates determined by more conventional methods (such as weight change measurements) applied after long exposure times. Blackwood et al. (29) used a combination of anodic polarization scans and open circuit potential measurements to determine the dissolution rates of passive films on titanium in acidic and alkaline solutions. An oxide film was first grown by applying an anodic potential scan to a preset anodic limit (generally 3.0 V), Fig. 24, curve 1. Subsequently, the electrode was switched to open-circuit and a portion of the oxide allowed to chemically dissolve. Then a second anodic... 

As a protective measure against the electrochemical cell formation described above, it is recommended to install a local cathodic corrosion protection (see Figure 1) (14,15]. 

Among the alloying elements used to improve the corrosion resistance of passivated alloys, molybdenum plays a central role in stainless steels. Indeed, stainless steels (iron-chromium or iron-chromium-nickel alloys) that contain molybdenum offer much better corrosion resistance (especially against pitting) than those without molybdenum. Despite the enormous amount of research work carried out on the process involved, using surface analytical methods combined with electrochemical measurements, the exact mechanism of the effect of molybdenum is not fully understood, and is still a matter of debate. However, all the data indicate that the improved corrosion resistance brought about by alloyed molybdenum is due to different phenomena, which may be rationalized in the following way ... 

The research on corrosion, started in this institute in the 1950s, continued successfully further. The intergranular corrosion of steels was measured by an electrochemical potentiodynamic reactivation method [310-312]. Since the 1960s, the passivity of brass was further studied, the rates of corrosion were measured by polarization resistance, the effect of deformation on anodic dissolution of steels was followed, and the surface roughness of metals was measured other subjects of research were, e.g., the behavior of passive films on steel, the effect of compositirai and motion of electrolyte on corrosion of passivated aluminum, the cathodic protection of passive metals against corrosion, the anodes for cathodic protection of steels, etc.[313-316]. Measurements of polarization resistance in the system iron—concentrated sulfuric acid or boiling nitric acid, of corrosion and matter... 

Analytical Oxygen concentration in process stream Useful where oxygen control against corrosion using oxygen scavengers, such as bisulfite or dithionite, is necessary. Electrochemical measurement Moderate... 

Corrosion of steel is known by engineers as the result of electrochemical reaction when different potentials are developed by electrically connected metal parts in contact with a solution containing free ions. The so-called electrode potential is dependent on the particular metal and the nature of the solution. Comparative values of electrode potentials may be measured against a standard electrode-electrol)de system. For example, if hydrogen is considered of zero V electrode potential, then lead, iron, zinc and aluminum potentials are 0.13,0.44,0.75 and 1.66 V, resp>ectively. 

Reference Electrode an equilibrium (reversible) electrochemical half-cell of reproducible potential against which an unknown electrode potential can be measured. Examples of those commonly used in corrosion are the Pt, H /H (the hydrogen electrode), Hg/Hg Clj/Cl" (the calomel electrode), Cu/CuS04/Cu, Ag/AgCl/Cl", all with fixed activities of the dissolved ions. 

The time of wetness can be calculated either by meteorological measurements of temperature and relative humidity or by electrochemical cells. For practical purposes, meteorological measurements are used when the relative humidity is 80% at temperatures >32°F/0°C to determine the time of wetness. However, the time of wetness determined in this manner may not be the same, as the actual time of wetness because wetness is also influenced by the type of metal, pollution of the atmosphere, presence of corrosion products, and the degree of coverage against rain. Even so, the results from these measurements usually show a good correlation with corrosion data from field tests under ordinary outdoor conditions. 

---