Final answer:
It is inadvisable to use soap to clean aluminum and magnesium engine parts because soap can cause corrosion, leave difficult-to-remove residue, and disrupt the protective passivation layer on these metals. The correct option is A.
Step-by-step explanation:
When cleaning aluminum and magnesium engine parts, it is inadvisable to soak them in solutions containing soap. The primary reason is that soap can react with these metals, causing corrosion. This is due to soaps forming insoluble calcium and magnesium compounds in hard water and because they are slightly alkaline in pH which can be problematic for metals sensitive to shifts in pH levels like aluminum and magnesium. Besides, the soaps and detergents might leave a residue that is hard to remove, which can be especially detrimental when aiming for a thorough clean of engine parts.
Furthermore, the presence of certain chemicals in soaps, such as salts, can exacerbate the corrosion process in metals. For example, salt water can speed up corrosion due to ions forming a salt bridge at the anodic and cathodic sites of metal surfaces. While magnesium has a natural passivation layer protecting it under normal circumstances, harsh soaps can disrupt this layer, resulting in increased susceptibility to corrosion.
Therefore, for cleaning purposes, it is preferred to use cleaners specifically designed for metals, which do not react negatively with aluminum and magnesium. These specialized cleaners often contain ingredients like lactic, citric, or phosphoric acid, which effectively clean without causing deleterious reactions. Moreover, nonferrous metal cleaners contain ammonia, ammonium soaps, and chelating agents, which can safely interact with magnesium and aluminum without the risk of corrosion or unwanted residue.