____ To achieve a quality paint finish on the various steel substrates on a continuous coil coating paint line, a water-break-free, clean surface must be achieved so that the paint film will adhere to the metal and it will stay that way for years without loss of adhesion or the existence of under film corrosion. The importance of this condition cannot be overemphasized. There has been some debate about the most effective method to clean and prepare galvanized steel and galvalume that has been passivated at the mill and shipped to the coater for painting. I will discuss what I believe is the best approach in cleaning and pretreating these two passivated substrates prior to painting at a coil coater.
Inorganic Chemical Passivations – Chromium Based
____ Metallic coated steel sheet and coil will be susceptible to storage stain if not treated to minimize this possibility. For many years, the traditional practice has been to treat galvanized steel and galvalume coil with an aqueous solution of chromic acid, chromium salts, and mineral salts that will produce a thin coating on the surface of the steel. This is an inorganic chemical process or "passivation" and it is applied near the end of the coating line in the steel mill. The mechanism of how this passivation occurs is not fully understood except that we know it is a dissolution and precipitation reaction, similar to what occurs during phosphating. The passivation solution dissolves a very small amount of the metal at the surface and it forms a protective film containing complex chromium and metal compounds. The chromium contained in passivation solutions comes in two valences: hexavalent, which is Cr+6 or trivalent, which is Cr+3. Hexavalent chrome is the type that has been regulated and limited by RoHS directives and it is the one that is considered a "chromate" passivation and will be the focus of most of my discussion here.
____ Recommendations of 1-2 mg/ft2 of chrome resulting from the correct chromate concentration, pH and exposure time. Less than half of the total chromium content on the surface of the steel is hexavalent chromium in a complex mixture of metal salts and oxides.
____ Galvanized steel coil produced on hot-dip galvanizing lines has an aluminum oxide layer deposited on the surface of the metal. This is a result of the aluminum that is used to inhibit the zinc-iron alloying reaction and it forms very rapidly when the strip exits the zinc pot. Chromate passivation cannot be effective unless this oxide layer is completely removed during the passivation process/reaction. Fluoride is, therefore, used in the passivation solution to dissolve the surface oxide layer prior to the deposition of the chromate compounds.
____ The zinc surface is protected through a barrier and passivation reaction. The barrier is the chromium oxide, and the hexavalent chromium contained in the film acts to re-passivate exposed metal. The mechanism here occurs when water comes in contact with the thin film and dissolves the hexavalent chromium to form a chromate solution, and this is then a fresh passivation solution for the metal surface. This is the basis of the "self-healing" property of chromate passivation films. However, the benefits of this "self-healing" property are limited under wet conditions where water is present between metal surfaces in the coil or in sheets. In that case, storage stains can occur. In any case, wet strip or dry, Cr+6 eventually oxidizes to Cr+3 .
Paintability of Chromate-Passivated Zinc-Coated Steel
____ Specifically in the case of zinc-coated hot-dip galvanized steel, chromate passivations are generally not considered paintable without very extreme removal methods and procedures. They are also not considered as substrates that can be effectively phosphate coated. Experience with chromate passivates indicates that even with aggressive brushing, cleaning and high cleaner bath concentrations and temperatures, chrome-based inorganic passivates usually cannot be completely removed. There have been instances under specific line conditions where a very aggressive acid cleaner bath has removed most of the chromate on a hot-dip galvanized surface with the assistance of brushes. However, it has also been proven that any chromate residue that remains on the steel surface can significantly and negatively impact the uniform deposition of zinc phosphate pretreatments. Passivation films also decrease the adhesion of most paints to the zinc surface. Passivated hot-dip galvanized surfaces have exhibited paint voids caused by lack of paint adhesion when those surfaces are previously passivated with a chromium-based passivation solution prior to painting.
____ Another consideration with chromate passivations on hot-dip galvanized steel is the fact that the chrome that is removed may contaminate the cleaning, rinsing and pretreatment solutions on continuous paint lines. There are then additional waste treatment implications and procedures to be considered specifically with the chrome in the spent cleaner and pretreatment baths and the water rinses. Also, chromium passivation interferes negatively with the spot weldability of galvanized steel sheet. Chrome poisons the copper electrodes by softening them and severely reducing tip life. That is the reason why most galvanized sheet that is to be fabricated and spot welded is ordered as unpassivated.
Paintability of Chromate-Passivated Galvalume (Al-Zn)
____ Chrome passivated Galvalume (Al-Zn) has proven to be paintable as it can be reliably cleaned and pretreated. In fact, passivated Galvalume is routinely pretreated on coil coating paint lines where some of the chrome passivation is removed and then a fresh chrome dried-in-place pretreatment is applied on top of the remaining chrome passivate to fill in any coating voids on the substrate surface. A Galvalume coil processed in this manner exhibits excellent corrosion resistance and paint adhesion.
____ It is very difficult to obtain all of the benefits of hexavalent chromium passivation with a more environmentally friendly trivalent chrome product. In spite of the fact that this has become more popular recently with the RoHS directives in place, trivalent Cr must be applied at much heavier coating weights in order to approach achieving the corrosion protection of a hexavalent Cr product. Many of these trivalent products are paintable.
____ Although chromium-free passivations exist in both organic and inorganic forms, the application parameters have been found to be much more critical than those of Cr+6 systems. Coating weights need to be as high as 30mg/ft.2 with the non-chrome systems in order to achieve comparable performance to hexavalent Cr systems. Paintability with chrome-free passivations, however, has been shown to be similar to that seen with chromate passivations.
____ Paintability of Chromate-Passivated GalvanealPassivated Galvaneal (zinc-iron) can be successfully painted in the continuous coil coating process because of its microscopically rough surface and acceptance of chrome pretreatments. However, chrome contamination of cleaning and phosphate baths is once again a concern.
____ In summary, great care must be taken to assess the condition of metal being painted on a continuous coil coating paint line, especially if it is passivated with a chromium-based passivation. Hot-dip galvanized and Galvalume present different dynamics and substrate issues. It is extremely important to properly clean and pretreat the steel surface to remove as much of the passivation as possible. Hot-dip galvanized steel is very difficult if not impossible to properly clean for subsequent painting, while passivated Galvalume has been shown to be an acceptable substrate for subsequent painting and is painted on coil paint lines successfully. Passivated Galvaneal can also be successfully cleaned, pretreated and painted. All chromium-based passivations present waste treatment and disposal problems in the cleaner, water rinse and pretreatment stages of coil lines where attempts are made to remove the passivation prior to painting. There are chrome-free and trivalent Cr alternatives available, but the coating weights needed to achieve acceptable results can be very high and, therefore, operationally and cost prohibitive.
Information provided by Bill Kegel, President/Founder, WEK Coating Consultants.