Soldering Copper to Steel Using Salt Water and Floor Cleaner – Why It Works

I wanted to de-solder components(inrush current limiting resistor, PC817C optocoupler and, S8050 Transistor etc) of a CFL bulb and mobile adapters to learn about AC to DC power supply, voltage regulator and SMPS(switched mode power supplu. But all my 3 soldering iron are not working(not heating up) and so I wanted to learn how to repair soldering iron. In doing so instead I saw a video which demonstrated how to solder copper to steel using basic tools and household chemicals. The method caught my attention because this knowledge can be very helpful in diy projects where you have to make joints between copper and aluminum materials. The technique involved dipping a soldering iron tip and a piece of solder wire into salt water, applying power, and later using floor cleaner as flux to complete the solder joint.But after digging into the science, it turns out this is a brilliant electrochemical pre-treatment method—a DIY form of electroplating or flux activation. Here I am writing about this method of joining steel to copper, if you want to join copper to aluminum read this lemon for soldering copper to aluminum guide.

⚙️ Materials

• Soldering iron

• Solder wire (Sn-Pb or Sn-Cu alloy)

• Saltwater solution (NaCl + H₂O)

• Floor cleaner (contains acids or ammonia)

• Copper and steel parts to join

• 12V DC power source or similar

🔬 What Really Happens in Salt Water

The soldering iron tip (probably connected to positive) and solder wire (to negative) form a basic electrolytic cell in salt water:

🔋 Electrolytic Setup:

• Anode (Soldering Iron Tip, +):

  • May oxidize slightly, but mostly just acts as current source.

• Cathode (Solder wire, –):

  • Undergoes electrochemical cleaning or activation.

  • May deposit elemental tin (Sn) or remove surface oxidation, exposing fresh solder metal.

    

⚛️ Chemical Reactions:

At the cathode (solder wire side):

$$2H_2O + 2e^- \rightarrow H_2(g) + 2OH^-$$

This creates bubbles of hydrogen gas, which also help mechanically loosen any oxide or contaminant on the solder.

The water becomes slightly basic near the solder, which may cause SnO or PbO layers to dissolve or convert.

At the anode (iron tip side):

$$2Cl^- \rightarrow Cl_2(g) + 2e^-$$

This forms chlorine gas—an aggressive oxidizer that can also help remove oxides in small amounts. This is similar to electrolysis cleaning of soldering iron tip with saltwater solution.

🧪 What's the Effect?

The solder wire, after this treatment, has:

• A cleaner surface, free from oxides.

• Possibly a thin layer of freshly exposed tin, more reactive.

• A texture and chemistry that makes it wet and bond to steel better when used with heat.

The floor cleaner, added during actual soldering, serves as a flux, removing final oxide traces and allowing the molten solder to flow and stick:

Typical Flux Reaction (with HCl from floor cleaner):

$$Fe_2O_3 + 6HCl \rightarrow 2FeCl_3 + 3H_2O$$

This cleans the steel, which normally resists soldering due to surface oxidation.

 

🛠️ Final Soldering Step

Once pre-treated:

• Heat the soldering iron.

• Apply a drop of floor cleaner mixer to the joint area.

• Touch the solder wire to the steel and copper—solder wets both surfaces.

• The joint forms.

The key: the electrochemically activated solder wire + chemical flux + localized heating = success.

🎯 Why This Works