Which metals are considered difficult to solder due to their oxides and high melting points?

Aluminum and magnesium are indeed challenging to solder due to their propensity to form oxides on their surfaces and their high melting points. Both metals easily develop a thin oxide layer that forms rapidly when exposed to air. This oxide not only acts as a barrier, preventing effective bonding during soldering but also has a melting point that is significantly higher than that of the base metals themselves. Consequently, this further complicates the soldering process because the solder needs to penetrate the oxide layer to create a strong joint.

Moreover, aluminum has a melting point of about 660 degrees Celsius (1220 degrees Fahrenheit), while magnesium has a melting point around 650 degrees Celsius (1202 degrees Fahrenheit). This means that achieving the necessary temperatures for soldering can be difficult, and the solder often does not adhere well unless special fluxes or techniques are employed. Therefore, these factors make aluminum and magnesium particularly difficult to solder effectively.