Metallurgical Failure Analysis of Closed Water Circuit Containing Molybdate-Based Inhibitor
2. Materials and Methods
3. Results and Discussion
3.1. Fractographic Analysis
3.2. Chemical and Microstructural Analysis
3.3. Physical Model of the Observed Damage
- Steel with a low carbon content;
- A specific corrosive environment, usually weakly acidic or basic;
- Tensile stresses due to external forces or residual stresses.
- In Circuit n.1, initially, the oxygen present in the working fluid is in contact with the pipes’ inner surfaces. It can be assumed that it is present in the crevice as well. From a mechanical point of view, a tensile stress exists, mainly due to the residual welding stresses. The combination of these stresses and the crevice length can be described by the fracture mechanic parameter KI, the stress intensity factor, that allows a better understanding of the phenomena occurring in this zone.The initial value of the stress intensity factor did not exceed the critical value KIC; otherwise, unstable fracture would have occurred immediately.
- From time zero on, the corrosion of both the pipes’ inner surfaces and the crevice occurs because of the cathodic reaction of oxygen until, after a certain time, the oxygen at the crevice tip is totally consumed. The one in the remaining part of the system decreases too, but it is still present. In this moment, the conditions for crevice corrosion activation exist due to the different oxygen concentrations at the crevice tip and mouth. The pipes’ surfaces will show uniform or quasi-uniform corrosion, while the crevice depth will increase, leading to an increment of the applied stress intensity factor.
- In the crevice zone, the corrosion is more severe because of the potential difference between the tip and the outer areas. As time passes, the amount of free oxygen in the circuit decreases since it is consumed by the electrochemical reactions with the steel pipes. Since the oxygen content is decreasing, the corrosion intensity at the crevice tip will also reduce gradually.
- When the stress intensity factor increases, stress corrosion cracking can be activated. The technical literature  states that crack propagation can occur even when the applied KI values are lower or much lower than KIC if the environment is aggressive. In this condition, the crack propagation rate da/dt depends on the applied KI, as shown in Figure 10.
- KI ≥ KISCC. The stress corrosion phenomenon is activated and contributes to the final damage;
- KI < KISCC. No SCC damage occurs.
- When the oxygen is totally consumed by the reactions between the fluid and the pipe’s surface, the crevice effect is deactivated too. If the SCC phenomenon was started, the crack can grow according to the applied KI; otherwise, the corrosion phenomena stop. Finally, it is important to remark that the oxygen present in the circuit is consumed quickly because of the generalized corrosion occurring on the wide steel surfaces. This means that crevice corrosion can also work for a short time; hence, the onset of the SCC phenomenon is unlikely. The corrosion process can restart if new fluid is added in the circuit.
- At time zero, it can be assumed that Circuit n.2 and Circuit n.1 have the same amount of oxygen in the fluid and that both the oxygen and the inhibitor are present in the crevice. As said before, it is possible to state that a stress intensity factor KI is applied due to the tensile stress induced by the welding operation.Furthermore, in this case, this value is not over the critical stress intensity factor KIC; otherwise, unstable fracture would have occurred immediately;
- From time zero on, the corrosion process can start in both the pipes’ surfaces and the crevice because of the cathodic reaction of oxygen. When the passivation occurs due to the formation of FeMoO4 , the corrosion aggression stops;
- As described in the technical literature [26,27,28] and confirmed by the producer recommendations, the inhibitor’s efficacy decreases over time, and the amount present in the crevice cannot be replaced by that present in the fluid since it cannot circulate in the crevice freely [29,30,31]. When the protection finally disappears, crevice corrosion can start because of the different electrochemical conditions at the crevice tip and mouth. Differently from Circuit n.1, the outer oxygen content does not decrease or decreases very slowly, since it cannot react with the pipes’ surfaces that are now protected by the iron molybdate layer. The corrosion conditions can be worsened further because of the large potential difference between the pipes’ inner walls and the crevice tip. In fact, when a steel surface is protected by a molybdate layer, its nobility increases significantly , promoting critical corrosion phenomena;
- During this severe corrosion attack, the crevice depth extended continuously. In comparison with Circuit n.1, it is possible to state that the stress intensity factor applied in Circuit n.2 increases faster and for a longer time, since the adherent iron molybdate layer present on the pipes’ inner surfaces prevents the total consumption of oxygen. The ongoing crevice corrosion extends the crevice length until the applied stress intensity factor can reach and exceed the material KISCC. As soon as this happens, the stress corrosion cracking phenomenon starts finally, justifying the shorter working life of Circuit n.2.
Conflicts of Interest
|BM1||Base metal—Pipe side|
|HAZ1||Heat-affected zone—Pipe side|
|MZ1||Melted zone—Pipe side|
|BM2||Base metal—Flange side|
|HAZ2||Heat-affected zone—Flange side|
|MZ2||Melted zone—At the crevice tip|
|HV0.3||Micro-hardness value—Load equal to 300 gf|
|SCC||Stress corrosion cracking|
|KI||Stress intensity factor in Loading mode I|
|KIC||Fracture toughness in Loading mode I|
|KISCC||Critical stress intensity factor under SCC conditions|
|da/dt||Crack growth rate|
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Casaroli, A.; Boniardi, M.V.; Rivolta, B.; Gerosa, R.; Iacoviello, F. Metallurgical Failure Analysis of Closed Water Circuit Containing Molybdate-Based Inhibitor. Metals 2023, 13, 723. https://doi.org/10.3390/met13040723
Casaroli A, Boniardi MV, Rivolta B, Gerosa R, Iacoviello F. Metallurgical Failure Analysis of Closed Water Circuit Containing Molybdate-Based Inhibitor. Metals. 2023; 13(4):723. https://doi.org/10.3390/met13040723Chicago/Turabian Style
Casaroli, Andrea, Marco Virginio Boniardi, Barbara Rivolta, Riccardo Gerosa, and Francesco Iacoviello. 2023. "Metallurgical Failure Analysis of Closed Water Circuit Containing Molybdate-Based Inhibitor" Metals 13, no. 4: 723. https://doi.org/10.3390/met13040723