Substrate-Specific Surface Pretreatment

Substrate-Specific Surface Pretreatment Aluminum and Aluminum Alloys Surface pretreatment of aluminum and aluminum alloys comprises cleaning, degreasing, etching and possibly formation of conversion coatings. Aluminum castings are not pickled but are grit-blasted. Degreasing and etching systems and processes are in principle similar to the pretreatment of ferrous materials. The largest proportion of aluminum products to date are those to be anodized. The pretreatment of these, especially semi-finished aluminum products, is specified in DIN 17611 and is summarized in Table 1.   Aluminum surfaces can be cleaned of oils using organic solvents, aqueous, emulsified or weakly alkaline degreasing solutions and acid degreasers with low phosphating effect (%1 to 10 orthophosphoric acid, technically %85). When degreasing with alkali, thorough rinsing with water must follow.

Etching

Etching is generally alkaline, sometimes acidic. For alkaline etching processes, sodium hydroxide or potassium hydroxide is used as the etching agent in concentrations of 10 to 30%. Etching time is 1 to 3 minutes. After etching, thorough rinsing with water is performed and neutralization with 20% nitric acid is carried out. For acid etching, sulfuric acid and/or hydrofluoric acid is used. Special protective measures are required when etching with hydrofluoric acid.

Formation of Conversion Coatings

Conversion coatings are applied to improve the adhesion of organic coatings to the naturally smooth and dense aluminum surface. Oxide, anodize and phosphate coatings, generally integrated into the organic coating process, are particularly important. The process is identical to the production of conversion coatings.

Copper

The top layer is removed from oxidized or tarnished copper surfaces by processing with abrasive felt rollers, grit-blasting with pumice/water mixture or pickling in diluted H2SO4. Additional etching in Na2S2O7 is also possible. Coatings do not adhere sufficiently to smooth copper surfaces. Pretreatment comprises degreasing with organic solvents and mechanical pretreatment of the surface. In special cases such as printed circuit board technology, the adhesion strength of photoresists can be significantly increased, for example by spraying with pumice powder.

Zinc

Particularly for subsequent organic coatings, thermal sprayed zinc and zinc castings require no special surface pretreatment; zinc sheet, hot-dip galvanized components and electroplated zinc require pretreatment consisting of degreasing and phosphating of the surface. After a process time of maximum 30 minutes, rinsing is performed with cold and warm water and subsequently dried. Old surfaces covered with zinc carbonate coating are cleaned of dust and dirt by mechanical processes.

Plastics

Plastic pretreatment generally has two purposes. First, degreasing to achieve suitable wettability using the processes described above, and second, roughening of the surface to create mechanical adhesion points. A typical example is pretreatment of ABS plastics and polypropylene for metallization without external current (see Figure 1), where chemical treatment takes place. For plastics that are difficult to damage by chemical means, plasma cleaning and activation are again suitable. An extreme case is pretreatment of Teflon films (PTFE) with molten sodium. Films treated in this way can be bonded and metallized without external current through activation for special components.

Rinsing

In almost all active process steps of surface technology carried out in an aqueous environment, rinsing follows. This fact is generally not particularly emphasized in process descriptions. The purpose of rinsing is to: • Stop the defined active process, • Minimize the carryover of electrolytes and thus achieve savings in bath content, • Prevent possible subsequent chemical reactions on the surface, • Enable recovery of contents and water. If deionized water (defined purity, water hardness) is used for rinsing and appropriate equipment and technologies are used, compliance with the rinsing criterion R results in the required coating quality. At the same time, the demand for water and wastewater savings from an environmental protection and occupational health perspective can be met.   Here, Co is the concentration of salts in the electrolyte or treatment solution and C is the equilibrium concentration in the rinsing water. Equilibrium is established between the ions transferred from the substrate surface to the rinsing water and the ions returning to the surface. This applies to non-flowing rinsing water. This corresponds to the situation of a pedestal sink. The inverse of the rinsing criterion R shows the degree of dilution X to be achieved in rinsing. Compliance with the rinsing criterion R and as a result the limit value of the permissible salt concentration in rinsing water residues on goods determines the quantity of fresh water required. With the amount of entrained electrolyte D in l/min and the amount of required fresh water W in l/min, the following applies: W = D . R In other words: the amount of fresh water required to carry out a rinsing step is influenced by the rinsing criterion R and the amount of electrolyte or solution D carried into the rinsing water. Water demand increases with the concentration of entrained electrolyte and the area rinsed per unit time. Residues from application solutions adhere to all surfaces. Numerous research results exist. In general, the following can be stated: • High-concentration electrolytes (e.g. nickel) result in high carryover losses of approximately 40 to 70 ml/m² for flat, vertical surfaces. • With minimum dripping times of 6 to 10 seconds, carryover losses approach a minimum. • By adding wetting agents, carryover losses can be minimized by approximately 20 to 30%. In practice, empirical values for R are used to determine fresh water demand during rinsing. Table 2 contains values for the rinsing criterion. To obtain sophisticated final surfaces in optics, electronics, vacuum technology and silicon technology, rinsing criteria of 100,000 and higher are required. Depending on requirements and conditions, rinsing can be performed with completely desalted water, drinking water or circulation or process water. From a technical standpoint, immersion rinsing is the easiest to implement, whereby products are immersed in a fixed volume of rinsing water. The concentration of electrolyte introduced increases with the number of "dips". This rinsing technique is known as standing rinsing. With continuous fresh water flow, the disadvantages of standing rinsing can be eliminated at the expense of higher water consumption through flow rinsing. Single-stage rinsing with uncontrolled fresh water supply and insufficient movement of goods increases water demand and worsens the rinsing result. It is advisable to combine rinsing techniques with each other and thus utilize advantages while keeping disadvantages as small as possible. This is achieved, for example, with a multiple rinsing cascade having a downstream rinsing bath. In countercurrent rinsing, water in a series of rinsing tanks is directed from one to the next. The rinsing water and workpieces to be rinsed move in countercurrent, with workpieces rinsed first in the bath with the highest electrolyte concentration and finally in the tank where fresh water is supplied. The results in Table 3 reflect these findings for rinsing application. Spray rinsing is particularly suitable for flat and smooth workpieces such as printed circuit boards. Rinsing water is sprayed onto the workpiece surface from nozzles under pressure, which means that high electrolyte concentration can be achieved in the rinsing water. In principle, a system requires only a single spray rinsing station, but this does not allow separate control of the rinsing water. In addition, simple measures are possible to a limited extent to reduce carryover of bath solution into the rinsing water. These are: • Removal of electrolyte film by draining, squeezing, wiping, blowing, suction, • Wiping and scrubbing, • Drain-friendly design (small smooth surfaces; no scooping or capillary movement), • Low-concentration baths, • Increased rinsing temperature (meaning lower viscosity), • Low exit speeds. After the final coating step, rinsing is followed by drying to remove the water layer from the surface very rapidly and evenly, for example to prevent tarnishing or staining. A wetting agent is usually added to the final rinse. References • Vorbehandlung als Herausforderung, Metalloberflaeche, RITUPER • Reinigung in die Fertigung integrieren, Metalloberflaeche, LANDAU • Prozesssicherheit in der Galvanotechnik, Metalloberflaeche, PENZ • Wirtschaftliche und umweltgerechte Teilereinigung, Galvanotechnik, FALLOT İzzet Aydın General Manager Hillebrand Chemicals Kimyasal Pazarlama Ltd. Şti.