I think phosphoric acid is what you want.

That or you could try a Lye (Oven Cleaner) solution with water.

http://carcarebuzz.com/what-to-watch-for-in-wheel-tire-cleaning-chemicals/

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Cleaning Chemicals Evolve

Just as wheel and tire technologies and designs have changed over the years, so too have the chemicals used to clean them. This evolution is most notable in the move toward less aggressive, less caustic chemicals, and products made with special additives to boost cleaning power and performance.

To understand how wheel and tire cleaning chemicals work, it’s helpful to understand where chemicals fall on the pH scale. The pH of a chemical is measured according to a logarithmic scale of 0 to14. Chemicals with a pH of 0 to 5 are considered acidic. Stomach acid, lemon juice and coffee are common items with an acidic pH. Inorganic soils and soils held in place by magnetic forces are most easily removed with low pH products.

Chemicals with a pH of 6 to 8 are considered neutral. Natural stream water and milk would be considered “neutral.”

Chemicals with a pH of 9 to 14 are considered alkaline. Bleach and drain opener are two examples of items with an alkaline pH. Soils held in place by electrical charge are best removed with high-alkaline products. Keep in mind that while all caustics are alkaline, not all alkaline solutions are caustic.

It is important to note that pH is not an indication of the concentration of a solution. The proper method used to determine solution concentration is titration.

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Alkaline wheel and tire cleaning chemicals include active ingredients such as sodium metasilicate (metso), sodium carbonate (soda ash), sodium hydroxide (caustic soda, NaOH), and potassium hydroxide (caustic potash, KOH). Years ago, wash operators tended toward chemistries with significant amounts of caustic to “burn” dirt and grime off of wheels. Eventually, the damage caused by use of aggressive caustics led operators toward acidic chemicals. Operators should steer away from wheel/tire cleaners with a pH above 12.5 if they do not want a product containing caustic soda, which can damage wheels.

Acidic wheel and tire cleaning chemicals may include the following sources of acidity: citric acid, sulfuric acid, hydrofluoric acid (HF), hydrochloric acid (muratic acid), phosphoric acid, hydroxyacetic acid, and oxalic acid.

In general, acidic cleaners most always provide a clean wheel with relative ease, although the type of acid used will clearly affect the cleaning result. The decision on whether or not to use hydrofluoric acid, which may pose a worker safety issue if not properly managed—is an individual decision that may vary from operator to operator. But with new types of acidic cleaners on the market, hydrofluoric acid is used less and less these days.

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Detergents and solvents are used as alternatives to caustic and acidic materials to clean wheels and tires more safely. In fact, there are widely available, non-acidic, non-caustic alkaline cleaning chemicals that do at least 95 percent of what acids and caustics can do, especially when coupled with high-pressure water and friction. The ideal wheel/tire cleaner would have a detergency and solvency component with no caustic soda and a pH of less than 12.5.

Solvents—butyl and butyl substitutes, d-limonene, terpenes, and petroleum distillates—are often added to cleaning chemicals to increase their degreasing power. Buffering agents such as complex phosphates, sodium carbonates, and sodium silicates also may be added to cleaning products to stabilize their pH during the washing process. Some additives are seasonally specific—for example, to boost cleaning performance during dry seasons or when there is a lot of salt on the road.

Surfactants can either increase or decrease surface tension and improve wetting of the soil/surface, allowing better penetration of water to the soil and helping to prevent soils from being re-deposited on the surface. It is important to be aware of the difference in the types of surfactants. Mixing an anionic (negatively charged) surfactant such as a foamer with a cationic (positively charged) surfactant such as a drying agent or spray wax will cause them to neutralize each other and form a gooey solid that will stop up the application system.

Detergency and friction-based chemicals are best for salt deposits, while solvents are best used for absorption of clays, dirt and tar.

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