US20210292915A1

US20210292915A1 - Method for Coating a Motor Vehicle Bodyshell Part, and Motor Vehicle Bodyshell Part

Info

Publication number: US20210292915A1
Application number: US17/253,515
Authority: US
Inventors: Clemens Bergmann
Original assignee: Bayerische Motoren Werke AG
Current assignee: Bayerische Motoren Werke AG
Priority date: 2018-07-27
Filing date: 2019-07-03
Publication date: 2021-09-23
Also published as: CN112218968A; WO2020020588A1; DE102018212540A1

Abstract

A method for coating a motor vehicle part includes coating and subsequently painting. In the coating, a motor vehicle bodyshell part is coated with a metallic zinc coat as cathodic corrosion protection. In the subsequent painting, the coated motor vehicle bodyshell part is painted. The motor vehicle bodyshell part is chemically polished and/or electrochemically polished prior to the coating with the metallic zinc coat.

Description

BACKGROUND AND SUMMARY OF THE INVENTION

This disclosure relates to a method for coating a motor vehicle bodyshell part, and also to a motor vehicle bodyshell part.
A coated steel plate may have a base layer composed of steel, a coat composed for the most part of zinc being applied on the at least one upper side of said base layer by hot-dip coating. The coated steel plate can be painted after the zinc coat has been applied.
One object of this disclosure is to provide a method for coating a motor vehicle bodyshell part, and also a motor vehicle bodyshell part for a motor vehicle, which make a particularly high corrosion resistance of the motor vehicle bodyshell part possible.
This and other objects are achieved by a method for coating a motor vehicle bodyshell part, in which the motor vehicle bodyshell part is coated with a metallic zinc coat as cathodic corrosion protection and subsequently painted. In the case of metallic layers, a protective action against corrosion depends on a respective type of layer. A less noble layer protects the motor vehicle bodyshell part cathodically and functions in this case as sacrificial anode, which means that the base or less noble layer dissolves and a respective function of the motor vehicle bodyshell part is thereby obtained for as long as possible. Even relatively small imperfections or damage in the base or less noble layer initially have/has no serious impacts on account of a so-called electrochemical remote effect, in which the zinc, as a result of its less noble potential, attracts a corrosive attack and thus protects the motor vehicle bodyshell part against corrosion. Conventionally, steel is galvanized, wherein zinc functions as sacrificial anode. The metallic zinc coat of the motor vehicle bodyshell part thus serves as cathodic corrosion protection, since the metallic zinc coat functions as sacrificial anode.
In order to permit particularly advantageous protection of the motor vehicle bodyshell part against corrosion, provision is made according to this disclosure for the motor vehicle bodyshell part to be chemically polished and/or electrochemically polished prior to the coating with the metallic zinc coat. The chemical and/or electrochemical polishing leads to a particularly smooth surface of the motor vehicle bodyshell part, such that so-called zinc antennas can be avoided. In addition to the reduction in roughness of a surface of the motor vehicle bodyshell part, contaminants of the surface of the motor vehicle bodyshell part can be removed during the chemical and/or electrochemical polishing in order to avoid inclusion of the contaminants under the metallic zinc coat. The particularly smooth surface of the motor vehicle bodyshell part brought about by the chemical and/or electrochemical polishing makes it possible for the metallic zinc coat to likewise have a particularly flat and uniform surface with little roughness. This is advantageous in particular for a subsequent painting operation and thus a further layer application onto the metallic zinc coat, since all layers on or around the motor vehicle bodyshell part can be applied in a particularly uniform and smooth manner. A respective roughness of the surface of the motor vehicle bodyshell part leads to roughnesses, which increase from layer to layer on the surface of the motor vehicle bodyshell part, of the respective layers applied onto the motor vehicle bodyshell part, such as the zinc coat and possibly an additional paint coat. The reduction in the roughness of the surface of the motor vehicle bodyshell part thus leads to a particularly low roughness of a respective outermost layer applied onto the motor vehicle bodyshell part. For this purpose, the surface of the motor vehicle bodyshell part is chemically and/or electrochemically polished prior to the coating with the metallic zinc coat. The removal of contaminants from the surface of the motor vehicle bodyshell part by means of chemical and/or electrochemical polishing prior to the application of the zinc coat furthermore has the advantage that zinc coating-free locations can be avoided. These zinc coating-free locations could be produced in the case of contaminants, since zinc cannot be deposited on contaminants. The removal of contaminants from the surface of the motor vehicle bodyshell part by means of chemical and/or electrochemical polishing prior to the application of the zinc coat also has the advantage that it is possible for the inclusion of liquids or gases, from a galvanization process, in the contaminants to be prevented. During a drying process of a cathodic dip-painting operation, these included liquids or gases could generate a defect in a resultant not-cured paint layer, said defect possibly having a serious negative affect on the corrosion properties of the motor vehicle bodyshell part.
The electrochemical polishing, which can also be referred to as electropolishing, is a removal method, wherein metal of the motor vehicle bodyshell part is anodically removed using respective electrolytes which are matched specifically to a respective material of the motor vehicle bodyshell part. The electropolishing operation aims to reduce the surface roughness of the motor vehicle bodyshell part, that is to say to remove burrs, and also to provide a smoothness and possibly a gloss. Contaminants are also removed during the electropolishing operation. The electrolytes which are used as chemicals can differ depending on the material of the motor vehicle bodyshell part to be polished. During the chemical polishing operation, material on the surface of the motor vehicle bodyshell part is chemically removed, such that the motor vehicle bodyshell part can also be deburred and smoothed at locations which are difficult to access. In addition, it is possible to remove incipient cracks. In contrast with electropolishing, chemical polishing is a non-electrical method. As in the case of electropolishing, chemical polishing also involves the removal of the surface of the motor vehicle bodyshell part at locations which are inaccessible for mechanical methods. After the motor vehicle bodyshell part has been chemically and/or electrochemically polished, the motor vehicle bodyshell part is coated with the zinc coat and subsequently painted, wherein the motor vehicle bodyshell part can be cathodically dip-painted. The motor vehicle bodyshell part, which has been chemically and/or electrochemically polished prior to the coating, advantageously has a particularly smooth surface, such that a particularly uniform application of the zinc coat and further layers, such as, for example, paint layers, is possible. Since a smooth surface has fewer points of attack for damage in comparison with a particularly rough surface, and there is thus a particularly low risk of damage to the zinc coat, the coated motor vehicle bodyshell part has particularly good protection against corrosion.
In an advantageous configuration of this disclosure, provision is made for the motor vehicle bodyshell part to be generatively or additively manufactured or cast and to subsequently be chemically polished and/or electrochemically polished. The motor vehicle bodyshell part is thus produced in the context of an additive manufacturing operation or as a cast component, in particular by means of precision casting. Here, the motor vehicle bodyshell part is for example metallic and thus formed from a metal raw material. By way of example, the motor vehicle bodyshell part can be formed from steel or iron. A generatively manufactured or cast motor vehicle bodyshell part has a particularly high surface roughness on account of the respective production method. In order to reduce this surface roughness, the motor vehicle bodyshell part is chemically and/or electrochemically polished. By means of the method according to this disclosure, it is thus possible for motor vehicle bodyshell parts which have been generatively manufactured or cast to be provided with particularly good protection against corrosion by means of the zinc coat and the paint, with the result that the motor vehicle bodyshell part can be used for example in a wet region of a motor vehicle.
In a further advantageous configuration of this disclosure, it is provided that, after the chemical polishing and/or the electrochemical polishing and prior to the coating with the zinc coat, the motor vehicle bodyshell part is cleaned by means of ultrasound. In this way, a particularly high degree of cleanness of the surface of the motor vehicle bodyshell part can be achieved, since any contaminants not removed from the surface of the motor vehicle bodyshell part during the polishing thereof can be removed from the surface of the motor vehicle bodyshell part by means of ultrasound. The formation of cracks in the motor vehicle bodyshell part after the polishing and the ultrasonic cleaning and prior to the coating with the zinc coat can thus be kept particularly low. This makes it possible to coat the motor vehicle bodyshell part with the zinc coat in a particularly uniform manner, such that a particularly advantageous corrosion protection of the motor vehicle bodyshell part can be achieved.
A further advantageous configuration of this disclosure provides for the motor vehicle bodyshell part to be a body component. The motor vehicle bodyshell part is thus used for example in the wet region of the motor vehicle, there being, in principle, a particularly high risk of corrosion for components in said wet region. The method makes it possible for cast components or generatively manufactured motor vehicle bodyshell parts to be able to obtain a sufficient corrosion protection in order to be able to be used in the wet region of the motor vehicle with a predefined minimum service life. The method thus permits a particularly wide spectrum of use of generatively manufactured or cast motor vehicle bodyshell parts in motor vehicles, in particular in wet regions which are particularly susceptible to corrosion.
In a further configuration of this disclosure, it has been shown to be advantageous for the motor vehicle bodyshell part to be subjected to corundum blasting prior to the chemical polishing and/or the electrochemical polishing. In other words, the motor vehicle bodyshell part is subjected to abrasive surface treatment using corundum as blasting medium. In this way, particularly high roughnesses and also contaminants of the surface of the motor vehicle bodyshell part can be removed. In the context of the chemical polishing and/or electrochemical polishing, the roughness of the surface of the motor vehicle bodyshell part is then further reduced in order to obtain a particularly smooth surface of the motor vehicle bodyshell part. The smoother the surface of the motor vehicle bodyshell part after pretreatment thereof, in particular as a result of the polishing operation, the smoother the design possible for the metallic zinc coat of the motor vehicle bodyshell part. In this way, it is possible to achieve a particularly high corrosion resistance of the motor vehicle bodyshell part.
It has proven to be particularly advantageous for the motor vehicle bodyshell part to be hot-dip galvanized. This means that the motor vehicle bodyshell part is dip-coated with zinc after the motor vehicle bodyshell part has been chemically polished and/or electrochemically polished. Hot-dip galvanization is a particularly simple and rapid option for applying the metallic zinc coat to the motor vehicle bodyshell part. Furthermore, hot-dip galvanization makes it possible for any unevennesses in the surface of the vehicle bodyshell part to be compensated by means of the zinc coat, such that the metallic zinc coat has a lower roughness than that surface of the motor vehicle bodyshell part which is provided with the metallic zinc coat. The hot-dip galvanization of the motor vehicle bodyshell part thus makes it possible to produce a particularly corrosion-resistant motor vehicle bodyshell part.
This disclosure furthermore relates to a motor vehicle bodyshell part for a motor vehicle, having a base body which is coated with a zinc coat, and also having a paint layer which is arranged on the zinc coat. Here, the motor vehicle bodyshell part has for example been generatively manufactured or cast and can thus have a particularly high surface roughness. In order to achieve a particularly high corrosion resistance of the motor vehicle bodyshell part, provision is made according to this disclosure for the base body to be a chemically and/or electrochemically polished cast part and/or generatively manufactured component. In the context of the chemical and/or electrochemical polishing, it is possible for a surface roughness of the motor vehicle bodyshell part to be lowered at least to a defined maximum roughness. The cast and/or generatively manufactured motor vehicle bodyshell part thus has a surface with a particularly low surface roughness, wherein the surface is provided with the zinc coat. In particular, the motor vehicle bodyshell part is provided in its entirety with the zinc coat. A paint layer can be applied on the zinc coat, for example in the context of a cathodic dip-painting operation onto the zinc coat. The particularly smooth surface of the chemically and/or electrochemically polished motor vehicle bodyshell part makes it possible for the metallic zinc coat, in particular in the context of a hot-dip galvanizing operation, to be able to be applied onto the surface of the motor vehicle bodyshell part in a particularly uniform manner. Furthermore, on account of the particularly low surface roughness of the motor vehicle bodyshell part, the metallic zinc coat likewise has a particularly low surface roughness. In this way, the motor vehicle bodyshell part has particularly good protection against corrosion, since unevennesses in the zinc coat and in particular in a paint layer applied on the zinc coat are particularly susceptible to damage, and damage to the zinc coat or to the paint layer can lead to corrosion of the motor vehicle bodyshell part.
Further features of this disclosure emerge from the claims, the figures and the description of the figures. The features and combinations of features mentioned above in the description and also the features and combinations of features mentioned below in the description of the figures and/or shown in the figures alone can be used not only in the respectively specified combination but also in other combinations or on their own.
This disclosure is now explained in more detail on the basis of a preferred exemplary embodiments and with reference to the drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a method diagram for coating a motor vehicle bodyshell part, in which the motor vehicle bodyshell part is produced, chemically and/or electrochemically polished, coated with a metallic zinc coat and subsequently painted; and

FIG. 2 shows respective schematic views of a surface of the motor vehicle bodyshell part in different stages of a polishing operation or of a cleaning operation of the surface of the motor vehicle bodyshell part.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a method diagram for producing and coating a motor vehicle bodyshell part 5, wherein a particularly corrosion-resistant motor vehicle bodyshell part 5 is intended to be produced, which can be used in particular in a wet region of a motor vehicle.
In the method, in a first method step 1, the motor vehicle bodyshell part 5 is generatively manufactured or cast. This means that the motor vehicle bodyshell part 5 is either produced in a casting method or is constructed layerwise by additive manufacturing. In the present case, the motor vehicle bodyshell part 5 produced in the context of the first method step 1 is a body component. In a second method step 2, the motor vehicle bodyshell part 5 is pretreated. In the context of this pretreatment, the motor vehicle bodyshell part 5 is at least chemically and/or electrochemically polished. In addition, the motor vehicle bodyshell part 5 can be subjected to corundum blasting and/or ultrasonic cleaning. In a third method step 3, the motor vehicle bodyshell part 5 is then coated with a metallic zinc coat, in particular hot-dip galvanized. This metallic zinc coat serves as cathodic corrosion protection and thus functions as sacrificial anode for a particularly high corrosion resistance of the motor vehicle bodyshell part 5. In a fourth method step 4 subsequent to the galvanization, the motor vehicle bodyshell part 5 is painted, such that a paint is applied onto the motor vehicle bodyshell part 5, on the metallic zinc coat. In the present case, the motor vehicle bodyshell part 5 is cathodically dip-painted.
FIG. 2 illustrates a surface of the motor vehicle bodyshell part 5 in different states during the second method step 2 in the context of the pretreatment. The surface of the, in the present case, precision-cast motor vehicle bodyshell part 5 is shown in a plan view as a first state 6, after the casting operation and prior to a polishing operation. The surface of the motor vehicle bodyshell part 5 is shown both in a plan view and in a sectional view in a second state 7, after the surface of the motor vehicle bodyshell part 5 has been subjected to a corundum blasting operation. The surface of the motor vehicle bodyshell part 5 is furthermore illustrated in a third state 8 both in a plan view and in a sectional view, wherein the surface of the motor vehicle bodyshell part 5 in the third state 8 has been subjected to corundum blasting and then chemically polished. Moreover, the surface of the motor vehicle bodyshell part 5 is illustrated in a plan view and in a sectional view in a fourth state 9, in which the surface of the motor vehicle bodyshell part 5 has been subjected to corundum blasting, chemically polished, and also subjected to ultrasonic cleaning. Moreover, the surface of the motor vehicle bodyshell part 5 is illustrated in a plan view in the fifth state 10 thereof, wherein the surface of the motor vehicle bodyshell part 5 in the fifth state 10 has been subjected to corundum blasting twice, and thus chemically polished twice, and also subjected to ultrasonic cleaning.
In the second method step 2, the surface of the motor vehicle bodyshell part 5 passes first through the first state 6, then the second state 7, then the third state 8, then the fourth state 9 and subsequently the fifth state 10. In the present case, after reaching the fifth state 10, the surface of the motor vehicle bodyshell part 5 is provided with the zinc coat in the third method step 3. As can be seen in FIG. 2, contaminants and also surface roughnesses of the surface of the motor vehicle bodyshell part 5 decrease continuously from the first state 6 through to the fifth state 10. Consequently, by means of the pretreatment of the motor vehicle bodyshell part 5 and in particular the chemical and/or electrochemical polishing, it is possible to achieve a surface refinement of the surface of the motor vehicle bodyshell part 5, in particular a particularly clean and also particularly smooth surface of the motor vehicle bodyshell part 5.
Currently, no chemical surface finishing or surface cleaning of components composed of steel or iron, which have been produced by casting or additive manufacturing, has been established. A respective surface of these components is currently not provided with a closed paint layer and/or a closed zinc coat. Consequently, components produced in the context of additive manufacturing or cast manufacturing are currently not used in regions of a motor vehicle body which are subject to corrosion. Contaminants of the surface of these components can lead, after the respective surfaces have been galvanized and painted, to bubbles in the paint, which are undesired.
In order to overcome these disadvantages, the surface of the motor vehicle bodyshell part 5, which has been generatively manufactured or cast, is cleaned and/or leveled by a chemical aftertreatment, in particular chemical polishing and/or electrochemical polishing and/or electrochemical pulse polishing, in such a way that a sealed zinc layer and/or a sealed cathodic paint layer can be deposited on the surface thereof. The sealing of the paint layer and/or of the zinc coat can be additionally improved by subjecting the surface of the motor vehicle bodyshell part 5 to ultrasonic cleaning after the chemical and/or electrochemical polishing operation.
The described method makes it possible for the generatively manufactured or cast motor vehicle bodyshell part 5 to be able to be used in a motor vehicle body and in particular in a wet region of the motor vehicle without additional measures such as a coating with polyvinyl chloride. By means of the method, the generatively manufactured or cast motor vehicle bodyshell part 5 can be provided with the particularly advantageous corrosion properties described.

LIST OF REFERENCE DESIGNATIONS

1 First method step
2 Second method step
3 Third method step
4 Fourth method step
5 Motor vehicle bodyshell part
6 First state
7 Second state
8 Third state
9 Fourth state
10 Fifth state

Claims

1.-7. (canceled)

8. A method for coating a motor vehicle part, comprising:

coating a motor vehicle bodyshell part with a metallic zinc coat as cathodic corrosion protection; and

subsequently painting the coated motor vehicle bodyshell part, wherein

the motor vehicle bodyshell part is chemically polished and/or electrochemically polished prior to the coating with the metallic zinc coat.

9. The method according to claim 7, wherein the motor vehicle bodyshell part is generatively manufactured or cast and is subsequently chemically polished and/or electrochemically polished.

10. The method according to claim 8, wherein after the chemical polishing and/or the electrochemical polishing and prior to the coating with the zinc coat, the motor vehicle bodyshell part is cleaned by ultrasound.

11. The method according to claim 10, wherein the motor vehicle bodyshell part is a body component of the motor vehicle.

12. The method according to claim 11, further comprising:

subjecting the motor vehicle bodyshell part to corundum blasting prior to the chemical polishing and/or the electrochemical polishing.

13. The method according to claim 12, wherein the motor vehicle bodyshell part is hot-dip galvanized.

14. An apparatus comprising:

a motor vehicle bodyshell part having a base body which is coated with a zinc coat, and having a paint layer which is arranged on the zinc coat, wherein

the base body is a chemically and/or electrochemically polished cast part and/or generatively manufactured component.