Effect of purified water in pre-treatment
Water purity plays an important role
Purified water refers to water that has been purified from particulate matter, typically by means of return osmosis (R.O) or deionisation (D.I). Purity is usually measured by the conductivity of the water (µS/cm), and the more conductive particles in the water, the more impure it is. Typical dirt particles include various salts, surfactants, biocides, metal particles, etc. In addition, raw water quality varies with the seasons, and the local water supply network also has an impact on the amount of particulate matter in tap water.
Particles can form electrochemical pairs on the metal surface, causing an immediate corrosion reaction (flash rust) and/or reducing paint adhesion. In addition, any dirt particles will weaken the corrosion protection and paint adhesion, which is why severe corrosion may later occur underneath the paint surface if moisture has penetrated the porous paint surface.
The use of purified water has increased with the use of thin film technologies for chemical solutions and rinsing. If the water purity level is not sufficient, the effectiveness of the thin film technology will be reduced and, in the worst case, may become inoperable. For pre-treatment processes using purified water, a level of 30–50 µS/cm is usually sufficient, while tap water conductivity in Finland is in the range of 200–300 µS/cm and is not sufficient to replace purified water without quality-related consequences.
Where do the dirt particles on the metal surface come from?
One might think that purified water is better than tap water at rinsing the surface of the metal to be treated from residues of previous treatment or chemicals, for example. However, that is not what is happening here; instead, what causes this is that Finnish tap water contains a surprising amount of dirt particles that are harmful to the pre-treatment process.
In addition to water, there are other sources of dirt particles as well, such as:
– Using the wrong chemicals or equipment to remove the dirt in question
– Too few rinsing steps
– Technical faults or defects in the equipment (e.g. mixing of wash and rinse waters)
– Using too high a line speed for the washing power of the process
– Neglected maintenance of equipment (lime, sludge, sediment)
– The interval between washes or rinses is too long (water conductivity, pH levels)
– Humidity and dirt in the production area
Washing chemicals and rinse waters may look clear and of good quality, but in reality, the cleanliness of the surface after treatments is not sufficient to achieve the desired corrosion protection and paint adhesion.
Tap water or purified water?
The amount of dirt particles in purified water is, therefore, significantly lower than in tap water, and this has a major impact on the quality of the final result. There are solutions on the market:
(i) which use tap water and do not call for the use of purified water
(ii) whose quality is clearly improved if tap water is replaced by purified water
(iii) which need purified water to function stably and to a high standard of quality.
Iron phosphating has been replaced by three-to-five-step thin film technologies, mostly using tap water. If purified water is recommended for the process, it is very likely that replacing it with tap water will lead to quality problems sooner or later. The waterless one-step Toran is a high quality, cost-effective solution when you need an oil-removing pre-treatment to improve paint adhesion and corrosion protection. Learn more here.
When aiming for high-end quality levels in water-based metal pre-treatment solutions, the use of purified water is essential. For example, the Pronortec thin film technology achieves a higher level of quality than comparable technologies (zirconium/silane/titanium). Learn more here.
Find out more about the high quality and easy-to-use purified water solutions we offer; learn more here.