What is Acid Core Solder Used For? Here’s The Answer!

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If you think acid core solder sounds like an aggressive type of solder, you are absolutely right. The substance is used to tackle the toughest of soldering jobs and has many practical uses, but there are some instances where acid core solder should not be used. Follow along as we get to the core of it all.

Acid core solder is an aggressive flux that helps bond two pieces of metal. The acid core removes inhibiting oxidation from the seam of the two metals and lowers the melting point for optimal fusion.

Acid core solder has many uses beyond what most people would consider when they think of soldering. Read below to discover the projects small and large, personal and commercial, you can tackle with acid core solder. 

What is Acid Core Solder Made Of?

Acid core solder is a flux filled solder that usually comes in wire form. Other solders, like solid core solder require flux to be added. This makes acid core solder an effective choice when fusing metals a low melting temperature. The metals must have enough time to cool down to allow for the solder to bond properly.

Acid core solder is usually made up of zinc chloride, ammonium chloride, hydrochloric acid, and a gel or water based additive. The additive allows the metals to react and is just enough to get things started. If these metals and additives are not removed galvanic corrosion can occur between different types of metals.

Acid fluxes are either:

  • Water Based
  • Petroleum Based

Water based flux is less harsh on the environment and is being used more predominantly in the plumbing industry. 

Petroleum based fluxes are more difficult to clean and are better used in fusing sheet metal. Acid is not active outside the solution and cleaning water based flux is much easier.

Some acid core solder contains lead or lead alloys and should not be used to repair drinking water systems. The harsh chemicals in acid core solder can contaminate the area around and if not cleaned properly can end up in the water supply. 

Acid cores with silver, lead, or tin will not mix with water since they’re dissolved in salts, this ensures the bonding process is a controlled one.

How Does Acid Core Soldering Work?

Acid core solder neutralizes chemicals using halide activation by the use of an amine. This amine helps with the wetting process and enhances the amount of activation between metals. The oxidation is reduced and capillary action is maximized, this ensures that the metals do not degrade when they are being joined.

Acid core solder burns through aluminum and is only intended for use on specific metals and specific jobs. Acid core solder is not used in repairing electronics or electrical equipment since it’s corrosive. There is no effective way to clean off electronics or circuit boards that have been fused with an acid flux.

The metals or alloys used in each wire vary in function and effectiveness. Some metals burn at a lower temperature, but this is negligible with the use of acid core solder. Some of the commonly fused metals are:

  • Nickel
  • Steel
  • Copper
  • Other oxidized metals

Any foreign contaminants such as water and dirt can cause difficulties when soldering, these must be cleaned away to ensure the bond will hold over time. Any rust or debris can be sanded down to help with this process.

Flux residue is hygroscopic which means it absorbs moisture from the air, this will break down the materials integrity. To combat this, use a baking soda solution to neutralize the acid and ensure there’s no corrosion. 

The Different Types of Acid Core Solder

As we know, acid core soldering eliminates the need for added flux and allows for the joining of oxidized metals in a semi-permanent bond. However, the type of acid core solder used will differ from job to job. Here’s a look at the two types. 

Lead-Free Acid Core Solder

Lead is a common contaminant that is undesirable in some general repair applications. Most plumbing work requires the use of a lead-free solder. Make sure you check the labels carefully when choosing an acid core solder because most of them contain lead. Lead-free acid core solder is best used for:

  • General Plumbing
  • General Repair
  • Gutter systems
  • Jobs with environmental regulations

Common Types of Lead-Free Acid Core Solder

There are other alloys available if you are looking to use a certain kind of metal for a specific job.

Lead Containing Acid Core Solder

This type of acid core solder is normally composed of tin and lead and is used for general industrial purposes.  Keep in mind, the use of lead containing acid core solder is illegal in potable water systems.

Lead based acid core solder is best used for:

  • Fusing sheet metal
  • Metalworking
  • Automotive production/repair
  • Heat exchangers
  • Industrial manufacturing

The “core” of acid core solder accounts for about three percent of the wire. The acids are generally a chloride salt mixture. The benefit of this is that no flux needs to be added, as is the case with rosin core flux. The downside is, if the acids are left over after soldering the joints are subject to water damage and corrosion.

It is suggested to use a hot water rinse after soldering to remove any excess residue. To thoroughly remove flux residue you should use a neutralizing solution to ensure there is no corrosive material left over. It is important to note that the use of a neutralizer doesn’t guarantee removal of all corrosive material, especially with gel based flux.

Common Types of Lead Containing Acid Core Solder

Here’s two of the most common acid core solder that contain lead:

  • 50/50 50% Tin and 50% Lead from Weldcote
  • 40/60 40% Tin and 60% Lead from Oatey

If you use these, just make sure to follow all the safety instructions that come with them.

How is Acid Core Solder Used in Plumbing?

Acid core solder is most notably used in plumbing because of its high level of activation with oxidized metals. Metals commonly used in plumbing like copper are easily joined with the use of acid core. However, acid core solder is not to be used in drinking water systems and many plumbers are switching to water based acid core to eliminate excess residue.

  • Metal pipes
  • Sheet metal
  • Plumbing repair

Plumbing uses various metals that are generally heavily oxidized. Acid core soldering helps fuse metals that are tough to solder including nickel, steel, and other oxidized metals. The low melting point fusion of metals in the plumbing industry makes for quick and efficient work.   

How is Acid Core Solder Used in Metalworking?

Acid core solder is also used in metalworking and since it requires low heat it is an effective choice. Heating the metal causes oxidation which makes the material harder to fuse, the flux helps with this process by reducing oxidation. In metalworking specifically, providing a solid bond at a low temperature makes for faster and easier production.

  • General repair
  • Sheet metal
  • Industrial Manufacturing

Acid core soldering is a metalworkers dream providing efficient ways to tackle jobs as big ships and as intricate as small car parts.

Acid Core Solder: For Unique Applications

Acid core is an aggressive type of flux used to reduce the melting point of metals that are fused together. This type of soldering is best used for plumbing and industrial purposes and is not intended for use on electronics.

If you plan on using acid core solder for any of your projects, make sure you’re aware of the safety implications–especially if you’re using leaded acid core solder. 



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