Acids are substances with a pH below 7, used in car washing primarily to dissolve mineral contamination that alkaline products cannot remove. While alkalis handle fats and organic dirt, acids target inorganic deposits like limescale, road salt residue, and brake dust.
Common acids in car wash chemistry:
- Hydrofluoric acid (HF) — extremely effective on mineral deposits but highly toxic; increasingly restricted and being phased out.
- Phosphoric acid — good all-round performer for limescale and light rust.
- Citric acid — mild, biodegradable option for maintenance washes and as a chelating agent.
- Oxalic acid — effective against rust and iron deposits from brake dust.
Safety considerations:
Acidic products require careful handling. Always consult the Safety Data Sheet and use appropriate PPE. On vehicle surfaces, acids can etch glass and damage chrome if left too long — contact time control is essential.
In a professional two-step washing process, the acid step follows the alkaline active foam stage, ensuring both organic and mineral contaminants are removed. For a detailed comparison of when to use each, read alkaline vs acidic foams.