Typically, RO systems control TDS levels within the 50-250ppm range. Although not as expensive as DI water, RO treatment does require chemicals to regenerate membranes. RO water can improve coating performance and reduce pretreatment chemical usage when used as make-up water for chemical baths. However, DI water is preferable as a final rinse, because the typical purity of RO water falls short of levels required for best coating performance. If source water purity is adequate to produce the desired coating-corrosion resistance, overflowing or counterflowing rinse stages can be effective and less costly.
Water is conserved by overflowing or backflowing final rinse water into earlier rinse stages.
Similarly, first rinse TDS should not exceed four times source water TDS, and second rinse no more than three times source. Variability in source water TDS levels and rinse contamination from part mix and production rate chemicals makes TDS monitoring absolutely critical in this approach. Furthermore, when production final rinse levels exceed 100ppm, coating performance testing is required to confirm desired resistance properties. Rinse water quality's key impact on coating performance necessitates vigilant monitoring, treatment and control to ensure corrosion performance. DI water final rinse is a necessity for high-performance applications.