In metal cleaning processes, water is not only used as a cleaning solution by dissolving acid or alkali agent but also used for "water washing" processes before and after the metal cleaning. Therefore, the water quality is an extremely important factor in determining the cleanliness of the metal surfaces.

(1) Impurities in city water

In general, city water (surface/ground water) will be used as cleaning water without special pretreatment. However, the city water usually contains numerous impurities as shown in [Table 1].

[Table 1] Impurities in city water

(2) Hardness components

In metal cleaning processes, impurities as Ca, Mg, and Fe cause more damages to metal surfaces. Salt content dissolved in the water deactivates the detergency of soap and synthetic detergent. In addition, hardly soluble or insoluble salt products generated here will then attach to the metal surface you are about to clean. This cross-contamination degrades the cleaning performance.
In addition, Fe exists in solution as divalent salt. However, it is easily oxidized by oxygen to form insoluble trivalent iron salt, which also contaminates the products you are about to clean. The Fe content is extremely low for tap water because of its oxidation treatment.
In most cases, the dissolved amount of Ca and Mg is expressed by an index called "hardness". Water hardness is expressed as mg per liter of water (in units of ppm) by converting all of the mineral (Ca, Mg) salts dissolved in the water into the equivalent of CaCO₃ (calcium carbonate). [Table 2] shows the grade of water hardness.

[Table 2] Grade of hardness

Based on the content of city water, using this water for washing in the finishing stage of metal cleaning will leave stains after drying. To prevent this, remove minerals from city water by the ion-exchange resin and use demineralized water instead. For washing processes requiring higher cleanliness, use water treated with advanced processing, such as microfiltration, demineralization, degassing, and disinfection.