Conveying rack cleaning in an electrodeposition process

Abstract

A method for anti-corrosion coating of metal components, in series, comprising a plurality of wet-chemical treatment steps concluding in cathodic electrodeposition, in which method each component is received by a conveying rack; then the component and conveying rack proceed through all treatment steps; the finished coated component is separated from the conveying rack; and an uncoated component is then received by the same conveying rack for coating; wherein build-up of solid coating deposits on the conveying rack is prevented by using an additional treatment step before cleaning/degreasing, passivation and electrodeposition, thereby avoiding extraction of individual conveying elements for removing coating deposits; wherein removal of cathodic electrodeposition coating constituents from conveying racks is achieved by contacting the conveying racks carrying components to be coated with an aqueous acidic agent, containing phosphoric acid, before the wet-chemical treatment steps for cleaning/degreasing, passivation and cathodic electrodeposition.

1 . A method for anti-corrosion coating of metal components in series, the method comprising at least one cleaning/degreasing step, a passivation step and a cathodic electrodeposition step, as wet-chemical treatment steps in each case, in a process sequence for the anti-corrosion coating of the metal components, the cleaning/degreasing preceding the passivation and taking place by contact with an aqueous cleaning solution having a pH value of above 6, and the treatment step of the cathodic electrodeposition following that of the passivation and taking place by contact with a dip coating;
 wherein each component in the series to be pretreated is received by a conveying rack, then a transport pair comprising the component and the conveying rack is moved through the wet-chemical treatment steps according to the process sequence, the transport pair is separated only after a last treatment step in the process sequence for anti-corrosion coating and a component provided with anti-corrosion coating is extracted thereby releasing the conveying rack which then receives a next component in the series in order to repeat the process sequence for the anti-corrosion coating of said next component, the conveying rack going through the process sequence as often as is necessary to carry out anti-corrosion coating of the series of components;   wherein at least a part of each conveying rack, that is brought into contact with the dip coating in the cathodic electrodeposition, is contacted with an aqueous pre-cleaning agent in a pre-cleaning step taking place after said last treatment step and after the next component to be pretreated has been received, but before the treatment step of cleaning/degreasing, wherein the aqueous pre-cleaning agent has a total acid content of at least 3 points and contains phosphoric acid.   
     
     
         2 . The method according to  claim 1 , wherein the dip coating in the cathodic electrodeposition is based on an epoxy resin as a binder, and the epoxy resin is selected from amine-modified polyepoxides. 
     
     
         3 . The method according to  claim 2 , wherein the aqueous pre-cleaning agent has a pH of less than 5.5 and does not fall below 2.5. 
     
     
         4 . The method according to  claim 3 , wherein the aqueous pre-cleaning agent has a pH of less than 5 and the total acid content thereof is at least 10 points; and the dip coating comprises blocked and/or unblocked organic compounds containing isocyanate groups as hardeners and further comprises at least one water-soluble compound of the element bismuth and/or of the element yttrium. 
     
     
         5 . The method according to  claim 1 , wherein the aqueous pre-cleaning agent has a total phosphate content of at least 3 g/kg and less than 50 g/kg of the aqueous pre-cleaning agent, the phosphate in each case calculated as PO 4 . 
     
     
         6 . The method according to  claim 5 , wherein the total phosphate content is at least 10 g/kg of the aqueous pre-cleaning agent; and said pre-cleaning agent further comprises one or more surfactants. 
     
     
         7 . The method according to  claim 6 , wherein the aqueous pre-cleaning agent further comprises one or more non-ionic surfactants. 
     
     
         8 . The method according to  claim 7 , wherein the one or more non-ionic surfactants are selected from ethoxylated and/or propoxylated C8-C12 fatty alcohols having a degree of ethoxylation of 5-11 and degree of propoxylation of 1-7. 
     
     
         9 . The method according to  claim 1 , wherein at least the part of each conveying rack which has been brought into contact with the cathodic electrodeposition, and optionally also the component to be provided with anti-corrosion coating which is received by the conveying rack, is brought into contact with the aqueous pre-cleaning agent in the pre-cleaning step by immersion, flushing and/or spraying. 
     
     
         10 . The method according to  claim 1 , wherein bringing into contact with the aqueous pre-cleaning agent in the pre-cleaning step takes place at a temperature in a range of from 40 to 60° C. 
     
     
         11 . The method according to  claim 1 , further comprising contacting the part of the conveying rack, which is brought into contact with the dip coating in the cathodic electrodeposition, with an aqueous wetting agent after the cathodic electrodeposition step, but before the pre-cleaning step, and optionally also before the conveying rack receives the component to be provided with anti-corrosion coating, wherein the aqueous wetting agent comprises at least one water-soluble alcohol and/or at least one water-soluble glycol ether. 
     
     
         12 . The method according to  claim 11 , wherein the at least one water-soluble alcohol is selected from 1,2-propanediol; ethylene glycol; glycerol; 1,4-butanediol and combinations thereof; and contact with the wetting agent takes place for at least 5 seconds and no longer than 60 seconds. 
     
     
         13 . The method according to  claim 1 , wherein the treatment step following the pre-cleaning and the passivation in each case follows these steps immediately, with or without an intermediate rinsing step, but without a drying step. 
     
     
         14 . The method according to  claim 1 , wherein the treatment step of cleaning/degreasing immediately follows that of pre-cleaning without an intermediate rinsing step and without a drying step. 
     
     
         15 . The method according to  claim 1 , wherein the passivation step is carried out by contact an acidic aqueous zinc phosphating treatment solution. 
     
     
         16 . The method according to  claim 1 , wherein the treatment step of cleaning/degreasing is carried out on the basis of an alkaline aqueous cleaning solution having a pH above 8. 
     
     
         17 . The method according to  claim 1 , wherein baking of the dip coating applied in the cathodic electrodeposition step, in order to form a hardened paint coating, follows the process sequence for the anti-corrosion coating of components in series using conveying racks, which are not conveying racks associated with the process sequence for anti-corrosion coating.