Cathodic inhibitors galvanic corrosion

The concepts in Chapters 2 and 3 are used in Chapter 4 to discuss the corrosion of so-called active metals. Chapter 5 continues with application to active/passive type alloys. Initial emphasis in Chapter 4 is placed on how the coupling of cathodic and anodic reactions establishes a mixed electrode or surface of corrosion cells. Emphasis is placed on how the corrosion rate is established by the kinetic parameters associated with both the anodic and cathodic reactions and by the physical variables such as anode/cathode area ratios, surface films, and fluid velocity. Polarization curves are used extensively to show how these variables determine the corrosion current density and corrosion potential and, conversely, to show how electrochemical measurements can provide information on the nature of a given corroding system. Polarization curves are also used to illustrate how corrosion rates are influenced by inhibitors, galvanic coupling, and external currents. 

The effect of the phosphate layer as a cathodic inhibitor under atmospheric conditions is illustrated in Fig. 12. In this experiment, the Volta potential of a galvanized steel surface is measured as a function of time during a transition from air to Ar atmosphere, as indicated [51]. The measurement is performed with a Kelvin probe, and the Volta potential of the corroding surface is directly proportional to the corrosion potential with appropriate calibration [58]. The potential jump induced by the presence of air is a measure of the sensitivity of the surface to the oxygen reduction reaction. Here, we see that the galvanized steel surface shows a very large potential jump, on the order of 200 mV. However, the phosphated surface shows only... 

Use cathodic inhibitors to combat cathodic depolarization reactions, minimizing galvanic corrosion. 

One of the major issues facing any consultant or owner of a structure suffering from chloride or carbonation induced corrosion is what form of repair to undertake. As we have seen from the previous sections there are coatings, sealants, membranes and enclosures, specialized patch repair materials, options for total or partial replacement, impressed current and galvanic cathodic protection, electrochemical chloride removal, realkalization, electro-osmosis and corrosion inhibitors. These can be applied to structures suffering different degrees of corrosion due to chloride attack or carbonation or a combination of these two. Each treatment will have implications for the future maintenance requirements, time to next major intervention and ultimate service life of the structure. 

The main emphasis in power line corrosion control is to select materials resistant to a specific environmental attack and to galvanize ferrous materijils (carbon steel and cast iron). This approach is used in the design of new lines and is supplemented by selective use of coatings and cathodic (galvanic) protection. The in-service methods are hmited to apphcation of coatings, cathodic protection, juid inhibitors. 

Corrosion Inhibitors. A water-soluble corrosion inhibitor reduces galvanic action by making the metal passive or by providing an insulating film on the anode, the cathode, or both. A very small amount of chromate, polyphosphate, or silicate added to water creates a water-soluble inhibitor. A slightly soluble inhibitor incorporated into the prime coat of paint may also have a considerable protective influence. Inhibitive pigments in paint primers are successful inhibitors except when they dissolve sufficiently to leave holes in the paint film. Most paint primers contain a partially soluble inhibitive pigment such as zinc chromate, which reacts with the steel... 

Traditionally, metallic coatings serve only one or two functions. For example, zinc has excellent corrosion resistance and functions as a sacrificial anode (Tsura, 2005). Zinc galvanizing provides sacrificial cathodic protection and acts as a barrier (Jones, 1996) but does not nsnally supply inhibitor ions. The release of zinc ions during the sacrificial protection of galvanized steels (Tsuru, 2005 Pourbaix, 1974) only provides a small additional benefit compared with galvanic protection provided by the potential driving force. Metallic coatings used to protect Al alloys (Reddy et al., 2000 Walton et al, 1953) consist of a thin layer of nearly pure Al mechanically bonded to standard precipitation age... 

Another type of rate meter is one that uses the complete electrochemical circuit in the form of a galvanic cell. While the galvanic cell may not simulate the actual corrosion cell, it does generate its own current and voltage the cathodic and anodic reactants can be the same as those of the corrosion cell and the current generated is proportional to the corrosion rate. This system forms an inexpensive qualitative instrument for field use and can be used to monitor and evaluate corrosion inhibitors. 

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