Surface Treatments to Protect Conventional and Rheo-High Pressure Die Cast Al-Si Alloys from Corrosion

Rheocasting process integrated with the high pressure die casting method (Rheo-HPDC) has established itself as a new promising technology to produce high-quality components. However, different types of microstructural segregation induced by the semi-solid process influence the properties of the final component. The semi-solid microstructural features and the new compositions require detailed corrosion studies and verification. The first part of this thesis deals with microstructural and corrosion studies of the conventional and Rheo-HPDC Al-Si alloys. In this part, corrosion properties of two Al–Si alloys containing 2.5 and 4.5 wt % silicon cast by Rheo-HPDC method were examined in the diluted Harrison solution using polarization and electrochemical impedance spectroscopy (EIS) techniques on as-cast and ground surfaces. The microstructural studies revealed that samples taken from different positions (with respect to the feeding gate) contain different fractions of solid and liquid parts of the initial slurry. It was shown that the Rheo-HPDC Al-Si alloys are prone to the localized form of corrosion inside the eutectic region at the interface of aluminum with silicon phase and intermetallic particles. Electrochemical behavior of as-cast, ground surface, and bulk material was shown to be different due to the presence of a segregated skin layer and the surface quality. Corrosion properties of the two Al-Si alloys cast by the conventional and Rheo-HPDC process were also evaluated and compared in 0.01, 0.05, 0.1 and 0.6 M NaCl solutions. The conventional HPDC and semi-solid alloys presented similar EIS responses. However, the semi-solid samples with a lower fraction of the eutectic phase showed slightly higher impedance values in the more diluted sodium chloride solutions. Corrosion morphological features, including localized corrosion, trenching and co-operative corrosion rings were comparable for both types of alloys. However, in the anodic polarization test, the semi-solid alloys presented a higher resistance to pitting corrosion. To protect aluminum alloys from corrosion, chromium-based conversion coating has been successfully used for decades, due to its extensive protection. However, rising concerns and new restrictions on the environmental hazards of Cr (VI) compounds have led to intensive efforts to develop alternative coatings. The second and third parts of this thesis address the effort to investigate two alternatives in this field. Cerium-based conversion coatings were deposited on the conventional and Rheo-HPDC Al-Si alloys by immersion in cerium nitrate aqueous solutions. Different parameters were studied to optimize the conversion coating, and NaCl or H2O2 were also added to the solution to modify or accelerate the deposition process. The results revealed that applying cerium-based conversion coating on Al-Si alloys, is possible and a selective deposition is obtained due to the presence of iron-rich intermetallic particles inside the eutectic region. Under the accelerated conditions, the deposition mechanism includes dissolution of the aluminum matrix, selective dissolution of aluminum from the noble intermetallic particles, oxidation of iron from these particles, and the deposition of cerium hydroxide/oxide layer. The results revealed that the improvement in corrosion resistance in the presence of selectively deposited cerium-based conversion coating is more significant compared to the homogenous coating obtained from the conversion solution containing H2O2. The aluminum alloy with a higher amount of silicon showed more active surface during the conversion process which reduces the required concentration of Ce(NO3)3 but also makes it difficult to work with more aggressive solutions. In the third part of this thesis, the possible protective effect of polypyrrole coating on pure aluminum and Rheo-HPDC Al-Si alloys was investigated. Different electropolymerization solutions containing the Py monomer, SDS, DHBDS (Tiron), C6H8O7 and NaNO3 were used. The presence of nitrate anions led to the passivation of the aluminum electrode (both pure and alloy) during the electropolymerization and to the deposition of a thicker/more conductive coating. These facts resulted in longer and more efficient corrosion protection in NaCl solutions. This polypyrrole coating was able to keep the alloys’ surface potential noble for at least 168 hours. Which can be attributed to the anodic protection provided by the reduction of the polymer. It was shown that the presence of silicon phase or intermetallic particles has a positive effect on the electropolymerization of polypyrrole film. Therefore, the coatings deposited on the alloys possess higher thicknesses compared to those deposited on the pure aluminum. In the presence of chloride ions, all coatings suffered from the formation of blisters as a result of severe (localized) galvanic interaction of polypyrrole with aluminum. This may question the application of polypyrrole coating in concentrated NaCl solutions. However, it is shown that the protection efficiency can be improved by altering the solution chemistry which affects the polymer/metal interface and the conductivity and the barrier properties of the coating. Therefore, the application of polypyrrole in corrosion protection is not totally ruled out but needs specific considerations.