Visualizing Corrosion Cells

The existence and location of anodes and cathodes in a corrosion cell can be demonstrated by the changes in color of certain reagents. Such color changes have been very helpful in the early days of corrosion science to study the local interplay of local anodes and cathodes on apparently homogeneous steel surfaces exposed to a corrosive environment. As noted by Cushman and Gardner in their 1910 textbook, it is a matter of common observation that iron usually corrodes rapidly at certain weak points in an effect known as pitting [24]. 

As explained in much detail in Chaps. 2 and 3, the cathodic reaction in a corrosion process generally produces an increase in the concentration of hydroxyl ions as a result of removal of hydrogen 

Similarly, potassium ferricyanide is a reagent which produces, as described in Eq. (7.3), a blue color by reaction with ferrous ions (Fe II) as they form at the anodic areas when iron corrodes. The appearance of this blue color, therefore, demonstrates the existence and location of anodes on iron. 

The combination of these two reagents in a gelling agent to which sodium chloride is added is known as a ferroxyl solution. Cushman and Gardner made ample use of this medium to reveal local corrosion cells and supported their theories with many pictures in their 1910 

One classic example of a local cell revealed by the ferroxyl agent is the development and location of the anode and cathode in a cell established on a steel surface within a drop of ferroxyl gel. Oxygen from the air is more accessible to the periphery of the drop and sets up a cathode that becomes visible as a pink color. Simultaneously an anode that develops near the center of the drop which is less accessible to oxygen is revealed by the gel turning blue (Fig. 7.40). 

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The classic example to visualize corrosion is the galvanic cell. Figure 11.2 shows the typical galvanic cell, which consists of four fundamental components ... 

That mystery often surrounds the process of corrosion is probably because of the hard-to-recognize forms that the electrochemical cell takes. Persons accustomed to the laboratory will visualize an electrochemical cell as a beaker containing electrolyte in which to pieces of metal are immersed and joined externally with a wire. It is difficult to make the translation between this situation and that of a water pipe running through alternate marshy and sandy patches of soil, yet both are electrochemical cells—and both will be subject to the reactions that go to make up corrosion. 

The purpose of the detailed survey is to ensure a cost-effective repair in line with the client s requirements. This is done by accurately defining and measuring the cause, extent and severity of deterioration. In Chapter 7, we will discuss how test measurements may be used to model the deterioration rate, time to corrosion and life cycle costing. We will need to know how much damage has been done and what has caused the damage. Quantities for repair tenders will probably be based on the results of this survey, so a full survey of all affected elements may be required. Alternatively a full visual survey may be required, with a hammer (delamination) survey of all accessible locations. A number of representative areas may be selected for a detailed survey of cover depths, carbonation depths, chloride content or profile, half cell potentials and other techniques described in the following sections of this chapter. 

AFM is not Umited to only conductive surfaces like STM. AFM is extremely flexible. It allows visualization of conductive, nonconductive, or semiconductive materials, and even Uving cells under a variety of environments (air, aqueous, and even corrosive conditions). In addition, AFM is capable of spatial resolution sufficient to visualize individual atoms at its smallest range ( A) and is only limited by the scan-... 

Corrosion tests that are performed on lollipops are corrosion potential readings as described in ASTM C 876 (Test Method for Half-Cell Potentials of Uncoated Reinforcing Steel in Concrete), polarization resistance as described in ASTM G 59, using IR correction [3,9], and EIS [9]. Specimens are broken open to visually examine the bars for confirmation of the electrochemical results, and chloride analyses are performed. The chloride analysis correlates the chloride content to the corrosion activity. 

Linings are often evaluated in the type of cell described in ASTM C 868 [1S. In the test cell, two lined panels are exposed simultaneously to test liquid, to study the effect of chemical attack and permeation on the lining system. The tests are often conducted for six months, and visual observations may be conducted during the period. At the end of the test, the lined panels are given a thorough examination, and may also be destructively evaluated for loss of adhesion, substrate corrosion, and hardness change. 

The elements are inserted into cell containers of plastic or stainless steel. Plastic containers are made from polystyrene, polypropylene, or flame-retardant plastics. Important advantages of plastic containers over steel containers are that they allow visual control of the electrolyte level and they require no protection against corrosion. Also, they have lower weight and they can be more closely packed in the battery. The main drawbacks are that they are more sensitive to high temperatures and they require somewhat more space than steel containers. A plastic-bonded plate cell in a plastic container is shown in Fig. 26.3. 

The shear force constant distance mode SECM was later mounted onto an inverted optical microscope, in a so-called Bio-SECM configuration, in order to study individual living cells. The Bio-SECM instrument was notably used to detect nitric oxide released from single cells [78]. In parallel, the shear force setup was modified by replacing the optical detection system, used to monitor the tip vibrating motion, by a piezoelectric element [79]. Electrical detection of the tip vibration was shown to be much easier and more convenient than optical detection, partly because the delicate laser alignment on the tip was made unnecessary. Further developments to shear force SECM have seen the implementation of high-resolution constant distance mode AC-SECM, which was used for the visualization of corrosion pits on stainless steel samples [80,81]. 

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