Salt Spray Corrosion Testing For Aero-Engine Metal Materials

Due to the high temperature, high humidity and high salt Marine environment, the corrosion problem of sea-based aircraft engine is particularly prominent. Although some Salt spray corrosion testing have been carried out for engine materials in the early stage, the test duration is mostly 96h recommended by GJB 150.11A-2009 "Salt spray Test Method for Military Equipment Environmental Test Method". With the increasing requirements for material corrosion resistance of sea-based aircraft engines, The early test time can not meet the requirements of the engine for long-term environmental adaptability and high reliability of the material. Therefore, in this study, three typical metal materials, namely 0Crl8Ni9 stainless steel, ZL114A aluminum alloy and TC4 (special grade) titanium alloy, were selected as test materials. By increasing the number of test cycles, the salt spray corrosion test lasting 240h was selected to analyze its corrosion resistance. The corrosion behavior and corrosion mechanism of three kinds of materials under salt spray conditions are summarized in order to provide design basis for the selection of aero engine materials and protective measures in Marine environment.

Test material

The test materials were selected as 0Crl8Ni9 (surface passivation) stainless steel, ZL114A (surface chromic acid anodizing) aluminum alloy and TC4 (super grade) titanium alloy. Among them:
a) 0Crl8Ni9 stainless steel is commonly used in the market of commercial stainless steel, its silicon (Si) content is less than 0.9% (mass fraction, the same as below), manganese (Mn) content is about 1.95%, but also after solution treatment, annealing and stabilization treatment.
b) The titanium (Ti) content of ZL 1 14 A aluminum alloy is between 0.10 and 0.20%, without adding beryllium (Be) element, low impurity and excellent performance.
C) of TC4 titanium alloy (super) chemical composition: iron (Fe) < 0.30%, carbon (c) < 0.10%, nitrogen (N) < 0.05%, hydrogen (H) 0.015%, or less oxygen (0) of 0.20% or less, 5.5 ~ 6.8% aluminum (AI), vanadium (V) 3.5 ~ 4.5%.
The size of the test sample is about 100mm x 100mm x 5 mm. The three materials were cleaned with alcohol and ultra-pure water before salt spray corrosion test. After cleaning, they were dried with cold water and placed in a clean test container.

Salt spray test method

According to GJB 150.11A-2009 "Military equipment environmental test method salt spray test", salt spray corrosion machine was used to carry out salt spray test. The temperature of the salt spray machine was adjusted to 35°C±2°C, the pH was 6.5~7.2, and 24h spray wetting +24h drying was a cycle. The Salt spray corrosion testing was carried out for 2 cycles (96h), 4 cycles (192h) and 5 cycles (240h), respectively. In order to ensure that the settlement rate of the salt solution is (1-3) mL/ (80cm2*h), the salt spray settlement rate and the pH value of the settlement solution are measured at least every 24h during the entire spray period. After repeated for 5 times, 3 tablets were taken out for observation after 96h, 192h and 240h respectively, and the salt spray corrosion performance was evaluated. After the test, clean the surface of the test sample with alcohol and dry it with cold air. The salt spray corrosion samples were observed and analyzed by visual inspection, weighing, X-ray diffraction (XRD) and scanning electron microscopy (SEM).

Conclusion

• With the longer duration of Salt spray corrosion testing, 0Crl8Ni9 stainless steel test sample local corrosion, corrosion area is very small. The diffraction peak changes, the position is slightly shifted, and the peak width of Crl.07Fel8.93 becomes larger. The reason is that the inclusion of a small amount of corrosion products leads to the deterioration of crystallinity, fewer corrosion products, and no corresponding oxides are detected. 0Crl8Ni9 stainless steel can meet the requirements of material corrosion properties in the general salt spray environment, if you need to use in a harsher salt spray environment, you need to consider adding protective measures.
• ZL114A aluminum alloy is seriously corroded under salt spray conditions, and the corrosion rate is the highest. As the salt spray time becomes longer, the corrosion area increases, and no corrosion oxides are detected, and the corrosion is mainly intergranular corrosion. Surface protection treatment should be increased when used in salt spray environment.
• TC4 (extra grade) titanium alloy has the lowest corrosion rate, except for slight corrosion at the defect caused by the machining part, no corrosion was seen in the rest of the parts, and no corrosion products were detected by phase analysis. TC4 (special grade) titanium alloy has excellent corrosion resistance, and can not be used in the salt spray environment to improve the surface protection treatment of corrosion resistance.
• Corrosion resistance of 3 kinds of metal materials TC4 (special grade) titanium alloy > 0 Crl8Ni9 stainless steel > ZL114A aluminum alloy.
• Through 240h salt spray test, the corrosion resistance of the three metal materials TC4 (super grade) titanium alloy is better than 0Crl8Ni9 stainless steel is better than ZL114A aluminum alloy. TC4 (special grade) titanium alloy has excellent corrosion resistance, and surface protection measures can not be used in the Marine salt spray environment; 0Crl8Ni9 stainless steel has good corrosion resistance, and passivation measures can be generally used in the Marine salt spray environment. If the use requirements are longer, surface protection measures such as painting need to be increased; ZL114A aluminum alloy has general corrosion resistance. In addition to surface anodizing treatment, surface protection measures such as painting should be added when used in Marine salt spray environment.