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Introduction to Corrosion Potential and Current in Dissimilar Metals

1. a) Name the three main constituents of a typical paint coating prior to application.

b) What two important functions do the pigments in the primer coat fulfill?

c)  What are the mechanisms by which these functions operate?

2. Using a sketch or schematic labeled Evans Diagram and mixed potential theory explain what happens with regards to Corrosion potential (Ecorr) and Corrosion Current (ICorr) when two dissimilar metals are joined as a couple in the presence of an electrolyte environment .

a) Using copper and steel as an example with free potentials of -0.35 and -0.72 respectively, estimate the Couple potential .

b) How does this relate to corrosion rates of the two individual metals?

c) List four factors affecting galvanic or dissimilar metal corrosion that make calculating exact corrosion rates difficult when data is extrapolated into large scale engineering situations.

d) Give two situations where mixed potential is used to advantage.

3. a) Why is -850 mV SSC (electrode potential) used for the cathodic protection potential for many steel structures?

b)  What are the dangers of using greater negative potentials with regards to the effects on the metal?

4. Compute the voltage (in V) at 25°C produced by an electrochemical cell consisting of pure copper (which has a standard electrode potential of -0.340 V) immersed in a 0.003 M solution of Cu2+ ions, and pure iron (which has a standard electrode potential of -0.440 V) in a 0.30 M solution of Fe2+ ions. Show all working and formula.

Hint:(This question is based around equation 4.18, it is a laboratory cell consisting of two different metals in their own ionic, solutions connected by a salt bridge. As such the products and reactants can be substituted by the Molar values, and Eo will be a mixed potential)

5. Describe in detail an electrochemical test procedure that could be used to assess an organic coating and compare it to another electrochemical or physical test. [You will need to look back at earlier learning packages and possibly do some extra research to answer this question fully].

Explain how the test is conducted, the parameters used and what you are actually measuring. What are the advantages and disadvantages compared to the other choice you have described. How can data be treated to obtain useful information? How does this relate to your understanding of the corrosion protection and degradation mechanisms of organic coatings used on metal substrates?

6. Explain the difference between pitting and crevice corrosion with regards mechanism? In your answer you should differentiate between the initiation stages and stable stages of these mechanisms using diagrams where appropriate.

Introduction to Corrosion Potential and Current in Dissimilar Metals

1.a) Solvent: A translucent and evaporative material which is readily volatile when exposed to airspace and hence forms no part of the dried coating. This substance enables the paint to perforate on materials of low permeability, thins down thick resin and wet-dry powder to help in manufacturing, improves the physical properties of paint such as drying, glossing and smelling and it also thins the viscosity of the paint to ease its application (Kadry, 2013).

Resin: It is the primary liquid part of a coating comprising of viscous, artificial or natural and sticky material that provides the coating with the capability to; make the paint shiny and more launder able, pliable to move with the expanding concealed substance, make the paint sticky and long lasting.

Pigment: It is the primary solid part of a paint that comprise one or various colored powders. Quality paints use good quality concealed pigments with other small pigments to give out color while lesser quality paints use extenders or fillers with a little number of colored pigments. Pigments give texture, hide the concealed substance, increases protection and provide a craved color (Kadry, 2013).

b) Primer is the first layer of paint applied to the surface.

The primer perforates in the porous surface to ensure perfect coating and adhesion.

The primer applied is waterproof to prevent any entrance of corrosive agents in the work piece.

c) The primer applied prevents any kind of corrosive agent such as hydrogen and oxygen in the substance. The pigments added in the primer coat makes the paint to have anticorrosive properties hence when a corrosive agent penetrates through the material it does not react with the content (Gudze and Melchers 2014).

The primer coat is compatible with the subsequent layers to clean and activate the surface of possible impurities by adhesion promoter purposed to ensure a perfect haven. Moreover, the primer is viscous to perforate and fill the valleys and ridges of the micro-roughness on the surface (Kadry 2013).

2. The Evans diagram is usually a graphical representation of the current densities along with potential in a corrosive process. However, this explains the metal corrosion (Kadry 2013). In that case, the anode and cathode have their potential, and they exchange the current the current density where the two half-cell reactions can't exist together on the surface of the same metal separately. Therefore, each of the potential changes to a typical intermediate value.

Estimation of Couple Potential and its Relationship with Corrosion Rates

When two dissimilar metals are joined as a couple, the Ecorr and Icorr usually decrease. The below is Evans diagram.

a) Given copper and steel with free potentials of -0.35 and -0.72 free potentials respectively, the couple potential will be the difference of both the free potentials. That is;

= -0.35-(-0.72) = -0.35 + 0.72

= 0.37 V

b) It relates to the corrosion rate of two individual metals where it has to be lower at one metal and high at the other because of the low current density reduction of hydrogen (Gudze and Melchers 2014).

c) The first factor is the difference in potential. As an illustration, the more significant the difference of potential between two metals, the higher is the magnitude of the galvanic corrosion. The second factor is small anodic area to a sizeable cathodic area. This is so because of the large density on the smaller anodic area (Kadry 2013). The third factor is deposition of the impurities along with deposits such as hygroscopic particles, salt or sand. This usually causes the formation of differential aeration cells which permits the absorption of moisture from the air that leads to corrosion. The last factor is conduction of the electrolyte (Gudze and Melchers 2014).

d) The first situation is that it is used for cathodic protection where it protects the buried pipelines along with marine equipment. The second situation is that it is used in the design of corrosion protection systems (Gudze and Melchers 2014).

3.a) The cathodic protection is a technique which controls the corrosions of steel. However, this shifts the potential of steel to a least probable range of corrosion.

b) The negative potential usually implies it is attractive. Conversely, the more significant negative potential means that a considerable number of atoms of metals within their ionic form has shifted to electrolyte thus leaving and an electrode which makes it profoundly negative and hence such metals great reducing agents. In that case, the more significant negative potential decreases the potential difference between the electrodes and thus the flow of current reduces (Kadry, 2013).

4. We first write the equations of the cells as follows

Fe2+ + 2e-          Fe                     E0 = -0.440V

Cu2+ + 2e-          Cu                  E0 = - 0.340V

The next step is to calculate the standard reduction potential of the cell

Ecell = Ecathode ­­­­– E0anode

E0cell = - 0.340-(-0.440)

= 0.100V

The next thing is to calculate the reduction potential of the cell at 250C as illustrated below:

Ecell = Ecell – 0.059V/n log [Fe2+]/ [Cu2+]

= 0.100V-0.059V/2 log 0.30M/0.003M

Ecell = 0.0410V

5. It is possible to deposit metals by many electrochemical methods like the potential dynamic polarization, linear sweep voltammetry, etc. (Gudze and Melchers 2014). However, the cyclic voltammetry is usually a common electrochemical method which is used to coat the organic materials on the working electrode. Conversely, the organic materials in this method will be reduced on the electrode in a convenient way by applying current (Kadry 2013). Compared to other methods, the cyclic voltammetry proves to be a suitable method of depositing the organic materials uniformly on the electrode which tightly integrate with the electrode. In that case, it reduces the number of active sites and hence increases the corrosion resistance properties of the deposited organic materials.

6. Pitting and crevice corrosion are usually forms of the localized corrosion which implies that carrion occurs within a limited area on the metal (Gudze and Melchers 2014). Conversely, the crevice corrosion mechanism is usually dependent on several parameters and hence may change according to the change in settings (Kadry, 2013). The attack usually happens within a restricted area usually a narrow crack and this often occurs where there are external agents like paint remains along with insulation which forms a crevice against the pipe surface. On the other hand, the pitting corrosion mechanism is often seen as the portent of crevice corrosion where it appears to have big pits.

References

Kadry, S., 2013. Corrosion analysis of stainless steel. Eur. J. Sci. Res, 22(4), pp.508-516.

Gudze, M.T., and Melchers, R.E., 2014. Operational-based corrosion analysis in naval ships. Corrosion science, 50(12), pp.3296-3307.

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