As a professional in the field of materials science and engineering with a focus on metallurgy, I can provide you with a comprehensive answer to your query regarding the use of solder on aluminum.

Soldering aluminum is a unique process compared to soldering other metals due to its chemical and physical properties. Aluminum is a reactive metal with a high affinity for oxygen, forming a thin, protective oxide layer on its surface. This oxide layer can interfere with the soldering process, making it challenging to achieve a strong bond between the aluminum and the solder.

Soft Soldering Fluxes
The residues of some soft soldering fluxes may indeed be still active after soldering and must be removed. Fluxes are essential in the soldering process as they clean the surfaces to be joined, prevent oxidation, and promote the flow of solder. However, not all fluxes are suitable for aluminum. Aluminum soldering requires a flux that is specifically designed to handle the aluminum oxide layer. These fluxes are typically based on halide compounds, which are effective at penetrating and removing the oxide layer, allowing the solder to bond properly.

Solder Composition
Solders used for aluminum generally contain a mix of metals such as zinc, lead, cadmium, tin, copper, or aluminum. The composition of the solder is crucial as it affects the soldering temperature, flow characteristics, and the final joint strength. For aluminum, a solder alloy that is compatible with its melting point and does not react adversely with the aluminum is necessary.

Electrochemical Corrosion
One of the significant concerns when soldering aluminum is the potential for electrochemical corrosion. This can occur when dissimilar metals are in contact with each other, forming a galvanic cell that can lead to corrosion. Solders that contain tin may cause an electrochemical corrosion problem due to its galvanic potential with aluminum. Therefore, it is essential to select a solder alloy that minimizes this risk. Aluminum solder alloys often contain elements like zinc and sometimes a small amount of lead to improve wetting and reduce the risk of corrosion.

Soldering Process
The soldering process for aluminum involves several steps:


1. Cleaning: The aluminum surface must be thoroughly cleaned to remove any contaminants, including the natural oxide layer. This can be done using a chemical cleaner or an abrasive method.


2. Applying Flux: A suitable flux is applied to the cleaned aluminum surface to prevent re-oxidation and to facilitate the soldering process.


3. Heating: The aluminum is heated to a temperature that is within the solder's liquidus and solidus range. This temperature must be carefully controlled to avoid overheating the aluminum, which can lead to deformation or other issues.


4. Applying Solder: The solder is then applied to the heated aluminum. The solder should flow smoothly over the surface, indicating that it is bonding properly.


5. Cooling: After the solder has been applied, the joint is allowed to cool slowly to avoid thermal shock and to ensure a strong bond.


6. Cleaning Post-Soldering: Any residual flux or other contaminants are removed after the soldering process to prevent any long-term issues.

Safety Precautions
It is also important to note that safety precautions should be taken when soldering aluminum. Fumes from the flux and the solder itself can be hazardous, so proper ventilation and personal protective equipment are essential.

In conclusion, while it is possible to solder aluminum, it requires careful consideration of the solder alloy, flux type, and soldering process to ensure a successful and durable joint. The potential for electrochemical corrosion must be managed by selecting appropriate materials and following best practices in the soldering process.

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The residues of some soft soldering fluxes may be still active after soldering and must be removed. Solders used for aluminum generally contain zinc with some lead, cadmium, tin, copper, or aluminum. However, any solder that contains tin may cause an electrochemical corrosion problem due to its galvanic potential.read more >>