Washing removes contamination using water, and water is not a neutral substance – it carries dissolved minerals, and when it evaporates it leaves them behind. Where the rinse water is hard, or where a site uses recovered or brackish water, a wash process deposits a film on every container it processes. The containers come out looking clean and carry a residue that matters for some applications.
What hard water leaves
Calcium and magnesium salts, deposited as a thin film and visible as water spotting once dry. On most industrial containers that is cosmetic. On a container that will carry a painted or plated part, or anything where surface cleanliness matters, it is a contaminant – and it is being applied by the cleaning process, which is a difficult thing to explain to a customer.
Saline or recovered water is worse
Chlorides are the concern, because chloride residues promote corrosion and because a rinse with saline water deposits salt on the container surface. A container washed with brackish water and then loaded with steel parts has had salt introduced into a closed environment – which is close to the worst thing that can be done to that load. Where water quality is poor, the rinse stage is the one to fix rather than the wash stage.
The rinse is the stage that matters
A wash with a poor-quality water supply followed by a rinse with good water leaves far less residue than the reverse. Where water quality is an issue and only one stage can use treated water, it should be the final rinse. That is a cheap change – a deionised or reverse-osmosis rinse on the final stage only – and it solves most residue problems.
Detergent residue is the same problem
An incomplete rinse leaves detergent film, which is hygroscopic in some formulations and attracts moisture, and which can affect a subsequent coating or adhesion process. Where containers are washed and then used for parts that get painted or bonded, rinse completeness matters. The usual cause of incomplete rinsing is a rinse stage that is too short or a water supply that has been turned down to save cost.
Testing for residue
A simple conductivity test on the final rinse water, compared against the incoming supply, shows whether dissolved solids are being removed or added. A wipe test on a dried container, with a white cloth and a little solvent, shows whether anything is being deposited. Both are cheap and both can be run weekly. Where a residue problem is suspected, these two tests locate it immediately.
Drying, and why it makes residue visible
Residue is invisible while the container is wet and obvious once it is dry, which is why water spotting appears at the drying stage rather than the wash stage. Where a drying stage uses heat, the spotting is worse, because rapid evaporation concentrates the deposit. A slower, air-assisted dry produces less visible spotting for the same water quality. See drying containers after washing.
What to specify
The incoming water quality, whether the final rinse uses treated water, the rinse duration, and a periodic residue check. Four items, and for a wash serving a sensitive application all four belong in the process documentation. Where a customer audits the wash, these are the questions that get asked, and having the answers is worth more than having a clean-looking container.
The cost of fixing it
A treated final rinse is a modest capital cost and a small running cost, and it is usually far cheaper than dealing with the consequences of residue – rejected loads, adhesion failures, or a corrosion problem that appears to come from nowhere. Where a site has hard or poor water, this is one of the highest-value changes available in a wash process.