## 3 Comparison of Rinse and No-Rinse Processes  
Substantial differences exist between the rinse and no-rinse conversion-coating systems, regardless of the presence of heavy metals in the process fluids. Both conversion treatments are carried out using multiple-process steps (Figure 2). In the case of the rinse process, the metal sheet/foil is immersed in a degreasing bath (often an alkaline aqueous solution) to remove oil and grease due to rolling, followed by subsequent washing with demineralised water. After this, an acid etching is required to \"desmut\" the metal surface; then, after another rinsing, it is possible to treat the metal by applying the conversion coating by dipping the sheet/foil in an aqueous solution containing chromic acid (CrVI) or chromate-phosphate acidic aqueous solution (containing CrIII), followed by the last rinsing-bath. Usually, each washing step is performed by using  
Figure 2: Comparison of steps for rinse and no-rinse processes. two washing tanks, separated from each other, to ensure the cleaning of the rolled parts. The active ingredients react with the metal surface, forming the protective saline deposit. Then, the laminate has to be dried in an oven to form a solid layer on the surface. In this process, reaction products are also formed in the bulk of the conversion bath. They do not deposit on the laminate and pollute the solution, which is therefore subject to ageing and must be frequently renewed. Therefore, the rinse process leads to the formation of sludge to be disposed of as special waste, as well as larger water consumption. The first four steps (alkaline degreasing, rinse, acid etching and rinse) of the no-rinse treatments are carried out following the same steps as in the rinse process. The deposition/formation of the conversion layer differs greatly, as the last phase of the material washing is absent. Furthermore, the no-rinse treatments differ from the conventional ones also for the following reasons: (a) the solutions used are much more concentrated, so they are usually diluted before use; (b) they can be spread, and this implies a different mechanism of film formation, if compared to the rinse process; (c) the no-rinse product contains an organic film-forming part (typically consisting of a polyvinyl-phenolic resin), allowing a uniform film-thickness of the deposited layer; (d) in addition, a better dosage of the active ingredients allows for reducing the amount of product used in the process. For those reasons, no-rinse products are called \"for total consumption\", meaning that the product utilised in the treatment plant is fully deposited on the substrate. In this way, it is possible to avoid the ageing of the treatment bath and the production of waste. Therefore, the no-rinse products, unlike the rinse ones, do not require a washing downstream to the pre-treatment. Their use allows the elimination of wastewater polluted by the active ingredients and then the subsequent sludge-disposal, thus reducing costs and carrying out the process with a lower environmental-impact. Figure 3 shows a picture of the no-rinse conversion-coating plant. In conclusion, the no-rinse process shows several benefits and few drawbacks, compared to the rinse one, as reported in Table 1.  
The rinse process takes place by immersing the aluminium-alloy sheets in the treatment solution, ensuring that both sides of the foils are treated in the same manner, showing the same amount of deposited product. When using the no-rinse process, it could occur that, due to non-perfect spray nozzles or roller-coater alignment, the amount of material deposited could differ between the two faces of the sheet. Both processes are continuous, showing treatment speeds of up to 200 m/min. The no-rinse process allows the use of smaller-sized treatment plants, due to the lack of at least two washing-tanks. It is estimated that the no-rinse plants take up approximately 30% less surface than the rinse ones.