Corrosion Science in Aerospace Industry
We have seen corrosion in the classroom window iron bar, rust in the old exposed and unused iron nails, and corrosion in the car and bus everywhere around us. Have we ever considered it to be a big issue? No! Even when I was taught in class 10 about corrosion science on a basic level, stating that corrosion is one of the major economic problems, I did not understand its seriousness. Now, in regards to aeronautical and space missions, corrosion has to be considered very important to avoid serious disasters in space or flight missions. During the early periods, many flights crashed or failed due to the unnoticeable minute corrosion in the parts of the flight. It is your task to find many such real-life incident cases and associate them with what we are going to learn today. Let's begin!
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| Photo by Jimmy Ofisia on Unsplash |
Corrosion:
Till our grade 12, we have this basic definition of corrosion:
"Corrosion is wearing and tearing away of metals due to the reaction with environmental agents"
In Engineering aspect, corrosion can be defined in more precise manner as follow:
"Corrosion is a electrochemical phenomenon which cause the destruction of metal by the electrochemical species"
What does this actually mean? Corrosion is simply an electrochemical reaction in which materials and agent responsible for corrosion react electrochemically, during which the material start decaying in this reaction. Generally electrochemical reaction requires both anode, cathode and electrolyte. Anode corrode and give the electron, which is used by the cathode to form the byproducts like rust. In most basic engineering level, this can be the best definition of corrosion. We are so much worried about this corrosion, now we have dealt a proper definition. We need to understand why corrosion occurs? Is it inevitable? Lets take a freshly prepared and processed new iron nail. How does iron actually exist in nature? It exist as an iron ore, which has lot of impurities in it. We have processed so much on it to obtain the pure iron nail. Why nature choose iron to be in the form of iron ore? Because nature always prefer the minimum energy to be associated with any material. When we process the ore into pure metal, we actually increase the energy of the system and make it unstable, so nature always tries to bring it stable state by bringing down the energy associated with the material, so the pure iron is been brought nearly to initial stage of the ore through the process of corrosion. So corrosion is inevitable! See the chart given below.
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| CS.01 Depiction of why corrosion is inevitable |
We say that the corrosion is inevitable, but why does not it happen instantly and take long time to happen? To answer this question, we need little knowledge of kinetics and thermodynamics. We said corrosion is inevitable that is it sure to occur without our intervention, so thermodynamically, we say corrosion is spontaneous reaction (meaning reaction is favored and can happen without any intervention). A reaction is spontaneous, when the Gibbs free energy of the reaction is negative. But the rate of reaction is very slow, which is given by the rate constant of the reaction k(corr).
Mathematically,
k (corrosion) = A exp (-ΔG_t/RT)
In the above formula, ΔG_t is not the overall Gibbs energy of the reaction, it the difference in Gibbs energy between the initial state to the transition state, so it is is usually referred as Gibbs free energy barrier which is of course positive
Q. Given two material P and Q with the corresponding Gibbs Free Energy barrier for corrosion as 200 J and 400 J respectively. Which material corrodes faster at room temperature, assuming A is constant?
Ans: k(P) = A exp (-200/ (8.314 * 298)) using the equation given above.
k(Q) = A exp (-400/(8.314 * 298)) using the equation above.
k(P) / k(Q) = 0.922/0.850 = 1.084
k(P) = 1.084 k(Q)
Technically, P corrodes faster than Q. So from this question for under same condition, the metal with higher Gibbs free energy barrier corrodes slower than the metal with lower Gibbs free Energy barrier. So, among gold and iron, iron corrodes faster than gold. So, we can tell the barrier energy of gold is high compared to that of the iron.
Classification of corrosion: A) Dry corrosion (due to gases) B) Wet corrosion (electrochemical agent) C) Concentration cell corrosion
All three types are of primary importance, but in this section we discuss mainly about the Wet Corrosion.
Wet corrosion or galvanic corrosion is primarily due to the electrochemical reaction between the cathode and anode. When we have two different metals joined or hinged, for example, iron nails in copper plate, one metal acts as cathode and the other metal act as anode on basis of their tendency to reduce or oxidize based on the position in galvanic series shown below:
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| CS.02 galvanic series |
So copper is more cathodic than iron, or equivalently, iron is more anodic than copper. So iron acts as the anode and copper acts as the cathode for the corrosion to take place. So, Fe becomes Fe2+ + 2e- and these two electrons flow towards the cathode, constituting the corrosion current. So a byproduct like rust is formed near the cathode. As long as the cathode keeps on eating up the electron, the corrosion keeps on happening since the anode keeps on decaying and there is a process of rust formation near the cathode. We will not get too deep into it. So on the ground level, the cathode size must be small compared to the anode size so that corrosion ceases as soon as the cathode gets used up. Secondly, the more distant the two metals on the galvanic series, the greater the corrosion current.
Q. A iron plate is made into a metal cube using aluminum nails. As an engineering do you think its optimal to use this combination? Justify.
Ans: No, it is not correct to use this combination. On basis of galvanic series, Aluminum is less cathodic than iron. If we use iron plate (more cathodic and large area) with aluminum nails (more anodic and smaller area), corrosion happens till the large cathodic iron plate is exhausted. So for optimal use we prefer, aluminum sheet and iron nails as the combination
Corrosion Science is large module. In this section, we have discussed regarding the basics of Galvanic corrosion type. What about the other types of corrosion ? That is for an other interesting posts. Stay curious!