Types of Corrosion

The electrolytic corrosion is most often referred to as electrolysis and galvanic corrosion is known as two metals or dissimilar metal corrosion. Even through the nature is quite similar to these types of corrosions, yet the cause of the corrosion is different or dissimilar in nature. Electrolytic corrosion arises from the action of an imposed direct current. The dissimilar metal corrosion arises from the electric potentiality between the two dissimilar metals, which are in contact. These kinds of corrosions have always posed a problem for the engineers since metal corrosion, in some situations are extremely serious which can pose a threat to human lives (Zhang 2013).

This report primarily deals with two cases studies which revolves around dissimilar metal corrosion and what are the possible failures that revolves around them. This report further investigates the possible solution, which can be used to mitigate such a crisis. The mechanism of corrosion and dissimilar metal crisis has been further explained with the help of diagrams and figures to explain the phenomenon.

The purpose of this report is to deal and investigate the situations that arise due to dissimilar metal corrosion and how this situation poses as one of the major threats to engineers. This report further helps understand and analyze the possible solutions to both the case studies, which has been cited in this report.

Corrosion mostly occurs in metals. However, degradation can also take place in different other mediums. Metal corrosion has been one of the major issues that has been taking place for many years, probably centuries. This is one of the threats that has been causing serious problems for engineers for quite a long period. Nonetheless, corrosion usually takes place under eight different forms and situations. The eight different forms of corrosion are uniform attack, dissimilar metal corrosion, and crevice-corrosion, intergranular corrosion, pitting corrosion, selective leaching, erosion corrosion and stress corrosion (Shreir 2013).

The first case study is related to the identification of the mode of failures and the potential causes of the failure to the metal (Steel) rods that are used for the liquid fills in a manufacturing plant. The rods have been breaking down prematurely while they are being placed in an assembly line machine. This has posed to be one of the major concerns since the premature breaking of the steel rods is certainly a matter of serious concern (Modern Microscopy 2017).

First Case Study: Premature Breaking of Steel Rods

Based on the analysis, which has been done with the help of an electron microscope it can be found that the central field of view shows the fracture face of one of the rods which have been bought for the examination (Shreir 2013). The upper hand corner of the left field of view illustrates the walls of the rod, and the lower corner of the right hand corner is the exterior of the rod. It can be demonstrated that the rough frozen fracture face moves all the way across the full thickness of the metal rod. As the image is focused to a closer magnification, it can be found the lower right hand side of the frame shows the crystal grains having crystal boundaries, also in most of the areas it can found that the crystal boundaries are slowly dissolving away and loosening in the process (Turk et al. 2013). As a result, the entire structure is eventually losing its strength and integrity, eventually breaking off. The crystals have begun to pull apart and the face of the fracture being exposed for most of the part. On the other hand, the fracture face has a relatively similar appearance (Mouanga et al. 2013). It can also be found that some of the darker areas, which comprises of the circular regions appear in form of pits within the crystals. This can be thus, considered as one of the severe corrosion attacks which has been in the process (Palani et al. 2014).

 

Figure 1: Secondary Electron Images (SEM) of fracture face

(Source: Palani et al. 2014)

With the analysis that has been done with the help of energy dispersive X-ray spectrometry (EDS), the elemental composition of the materials have been identified. The EDS is an attachment is more of the secondary electron images to look into the fracture face of the materials. Based on the images, analysis has been done within the purple rectangle, which has been drawn, with the purpose of examining the elementary composition within the section. The spectrum in the upper right demonstrates the presence of elements such as iron, chromium and nickel with a minor amount of manganese, with a typically 300 series of stainless steel substance. The content of the oxygen is around 13 percent by weight and which is high in composition (). Thus, it can be assumed that it is the oxygen, which is responsible for the oxidization of the metal, which in turn stimulates the corrosion process. Under the visuals of a microscope, a significant different in the brightness and contrast between the elements can be observed which is due the difference in the atomic number of the elements. The base metal in between the nice convoluted river like pattern can be found all the way through the thickness of the metal, which can be considered as a typical intergranular corrosion. A higher magnification in the interior section of the metal rod illustrates the separation of the grains, which has begun to separate and pull apart, with the metal losing its integrity and falling apart from each other ().

Second Case Study: Corrosion in Boeing’s Aircrafts

Nonetheless, towards the bottom of the image it can be found that the darker areas in the grainy borders have not started to be separated from each other. So based on the analysis of this area, the corrosive agents are likely to be considered as the excess presence of chromium in the corrosive products in comparison to the base metal. The presence of minor amount of sulphur, with little traces of chlorine is responsible for the corrosion, since based on the chemical composition and analysis. It can be presumed that sulphur and chlorine are themselves identified as corrosive agents. Chlorides and Sulphides along with all the other halides are considerably extremely corrosive in nature, for steel alloys, in particular.

 

Figure 2: Data gathered through energy dispersive x-ray spectrometer

(Source: Kamruzzaman et al. 2014)

So based on the analysis, it can be assumed that traces of corrosive agents in the components of the steel rods are mainly responsible for their break down during their mechanical processes and functioning. The area ratio can also be considered important of the probability of the dissimilar metal corrosion. The larger the cathode is in comparison to the anode, the more oxygen reduction occurs hence the higher galvanic current results in the corrosion (Kamruzzaman et al. 2014).

The second case study, which has been selected for the analysis is Boeing. Boeing has been selected for the analysis since it is one of the companies that holds a high integrity and assures a superior quality in the designs of it offerings. The part of their approach, which shows their utmost dedication in the understanding, and improvement of the products related to the failure of the products is also one of the reasons for selecting the company for the case study and the analysis. The main part selected for the analysis is the part, which joins the bearing and the shaft, in the aircrafts, which needs to be analyzed to understand the reason of corrosion. Based on the case study, the part joining the bearing and the shaft critically fails to perform properly as it was designed, thus resulting in operational malfunctioning (Eigner et al. 2013). The part is mainly responsible for the achievement of strength in the tension along with the compression. The part rotates on the bearing in a lateral motion. However, during the cyclic load, the part deliberately fails to deliver the tension in the vertical direction. The reason for the malfunctioning is the result of a crack in the part.

Elemental Composition and Corrosive Agents

 

Figure 3: Optical Microscopic view of the corrosion

 

Figure 4: Optical Microscope examining the crack

(Source: Eigner et al. 2013)

Based on the investigation, the first area has been explored with the help of an optical microscope to explore into the grain structure of the part. On the left of the image, the void or the defect can be seen with a magnified view as depicted in figure 3. The grain structure in the figure shows the components to be 4340 steel in figure 3 and 4. In the figure 4, the picture of the fracture is clearly illustrated with a magnified view of ten times. In image 3, the corrosion has been evidently demonstrated, which has penetrated the surface, which is adjacent to the initiating point of the (Eigner et al. 2013).

 

Figure 5: SEM examining the corrosion with a 350-x view

(Source: Sastri 2012)

In the figure 5, the specimen, which has been examined, demonstrates the detachment due to oxidation and corrosion attack. In the figure 4, the exact spot of the crack has been examined along with traces of Cadmium-Titanium base. This shows clear contact of two metals, which can be considered as one of the reasons for the corrosion. The failure mainly occurred due to the corrosion, which penetrated into the surface, thus creating a huge flaw in the part, imparting enough time for the progressive deformation, ultimately leading to the crack near the steel bearing. In this image, a trait depicting the galvanic or dissimilar metal corrosion has been exhibited. On further investigation, it can be assumed that being isolated to a specific area in the 4340 steel, which itself is in contact with the stainless steel has escalated the corrosion to a crack. The point of contact of the two dissimilar metals is thus the considerable origin point of the galvanic corrosion.

Solution for case study 1

Based in the case study and the existing bimetallic corrosion, the counter measures need to be taken. Ideally, the metals should be selected which are close to each other in the galvanic cycle (Caunter and Maxwell 2014). However, engineering requirements needs different material and their properties, which are not necessarily close to each other. The other objective is to maximize the area of the anodic metal and minimize the cathodic metal (Carter 2013). Experience in similar situations should be sought wherever possible, since it is not uncommon for the dissimilar metals to be coupled with adverse effects, specifically when the electrical conductivity or the oxygen content of the electrolytic component is low in count (Sastri 2012).

Countermeasures against Dissimilar Metal Corrosion

 

Figure 6: Effect of area ratios under bimetallic corrosion

(Source: Carter 2013)

Another most common way to break or prevent the frequency of the bimetallic corrosion is to break or intercept the electrical path in the metallic or the electrolytic part of the systematic framework, which excludes oxygen form the electrolyte by the adding of inhibitors in the electrolyte (Peratta and Adey 2013).

Apart from these solutions, another easy strategy to prevent corrosion between bimetallic dissimilar elements is to apply paint. The metals such as zinc or iron should be applied with paint, but where it is impracticable. The cathodic elements should be coated in preferably, keeping it in higher predilection (Cicek 2013). However, treating only the anodic element increases the risk of severe localized bimetallic corrosion at any defect in the coating. The process of electroplating, dripping or spraying to give a close identification with the second metal can be applied for the metal coating (Xie et al. 2014).

Solution for case study 2

The corrosion, which took place in this case study, displayed a crack in the place of contact of the two dissimilar metals. This is evident from the fact that it was isolated only in the specific area of the contact where the 4340-steel meets the stainless steel, which can be elevated as the primary cause for the galvanic corrosion, which took place. However based on this type of corrosion, three different solutions can be recommended (Popov 2015). The first solution involves the application of a zinc ring pressed against the two reactive metals. This would in turn impart a protection against the galvanic corrosion, by leaving both 4340steel and stainless steel non-reactive to each other. However, the zinc ring itself can be susceptible to corrosion, which might need regular replacement (Hassani-Gangaraj, Moridi and Guagliano 2015). The second solution can be the placing of plastic sheath beneath the 4340-steel and the stainless steel to prevent reaction and contact with the bearing (Bockris 2013). The third solution is application and the usage of circuit testing to see if there is electric conductivity between the two metals, if the results are positive then those parts need to be removed and examined for corrosion or damage.

Conclusion

With the help of these case studies and the report, it can be concluded that bimetallic corrosion is one of the severe consequences of keeping two different metals in the galvanic table close to each other. This in turn can cause a huge impact on both the metals giving rise to disasters like breaking, crackling and withering. In order to prevent such catastrophe, the metals can be treated in such a manner, so that they do not contribute to the corrosive process. Methods of electroplating and painting the metals are one of the common methods to avoid corrosion and malfunctioning of the technical parts. 

References

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Carter, V.E. ed., 2013. Corrosion Testing for Metal Finishing: Institute of Metal Finishing. Elsevier.

Carter, V.E., 2013. Metallic Coatings for Corrosion Control: Corrosion Control Series. Newnes.

Caunter, N.J. and Maxwell, I., Crown Packaging Technology, Inc., 2014. Bimetallic corrosion mitigation. U.S. Patent 8,807,374.

Cicek, V., 2013. Cathodic protection: Industrial solutions for protecting against corrosion. John Wiley & Sons.

Eigner, M., Ernst, J., Roubanov, D., Sindermann, S. and Eickhoff, T., 2013. Information exchange along the product development process using the example of bimetallic corrosion. In DS 75-6: Proceedings of the 19th International Conference on Engineering Design (ICED13), Design for Harmonies, Vol. 6: Design Information and Knowledge, Seoul, Korea, 19-22.08. 2013.

Hassani-Gangaraj, S.M., Moridi, A. and Guagliano, M., 2015. Critical review of corrosion protection by cold spray coatings. Surface Engineering, 31(11), pp.803-815.

Kamruzzaman, M., Jumaat, M.Z., Ramli Sulong, N.H. and Islam, A.B.M., 2014. A review on strengthening steel beams using FRP under fatigue. The Scientific World Journal, 2014.

Modern Microscopy. (2017). Case Studies of Corrosion Failures. [online] Available at: https://www.mccrone.com/mm/case-studies-corrosion-failures/ [Accessed 26 Dec. 2017].

Mouanga, M., Puiggali, M., Tribollet, B., Vivier, V., Pébère, N. and Devos, O., 2013. Galvanic corrosion between zinc and carbon steel investigated by local electrochemical impedance spectroscopy. Electrochimica Acta, 88, pp.6-14.

Palani, S., Hack, T., Deconinck, J. and Lohner, H., 2014. Validation of predictive model for galvanic corrosion under thin electrolyte layers: An application to aluminium 2024-CFRP material combination. Corrosion Science, 78, pp.89-100.

Peratta, A. and Adey, R., 2013, March. Modeling Galvanic corrosion in multi-material aircraft structures. In CORROSION 2013. NACE International.

Popov, B.N., 2015. Corrosion engineering: principles and solved problems. Elsevier.

Sastri, V.S., 2012. Green corrosion inhibitors: theory and practice (Vol. 10). John Wiley & Sons.

Shreir, L.L. ed., 2013. Corrosion: corrosion control. Newnes.

Turk, M.C., Rock, S.E., Amanapu, H.P., Teugels, L.G. and Roy, D., 2013. Investigation of percarbonate based slurry chemistry for controlling galvanic corrosion during CMP of ruthenium. ECS Journal of Solid State Science and Technology, 2(5), pp.P205-P213.

Xie, Z., Luo, Z., Yang, Q., Chen, T., Tan, S., Wang, Y. and Luo, Y., 2014. Improving anti-wear and anti-corrosion properties of AM60 magnesium alloy by ion implantation and Al/AlN/CrAlN/CrN/MoS 2 gradient duplex coating. Vacuum, 101, pp.171-176.

Zhang, X.G., 2013. Corrosion and electrochemistry of zinc. Springer Science & Business Media.