US3177134A

US3177134A - Method of forming corrosion resistant coatings

Info

Publication number: US3177134A
Application number: US82397A
Authority: US
Inventors: Gartner Gustav; Ruhle Rudolf; Mutschler Gerda
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1961-01-13
Filing date: 1961-01-13
Publication date: 1965-04-06
Anticipated expiration: 1982-04-06

Description

BSI'HOD OF FORAIING CORROSION RESISTANT Gustav Grtner. Gerlingen, Stuttgart, Rudolf Rhle,- Snittgart-Zufcnhausen, and Gerda Mutschler, Stuttgart,

gort, Germany Y Filed Ian. 13, 1961, Ser. N 82,397 l2 Claims. (Cl. 204--192) The present invention relates to a method of forming a present invention is concerned with'a method for producing on a lacquer-covered metallic body a firmly adhering,

corrosion resistant, reecting metal coating. I Broadly, the method ot' the present invention is carried out by vapor deposition in a partial vacuum of an Y intermediate layer consisting of a metal oxide, deposition of intermediate layer being carried out in the specific manner described further below, and thereafter vapor de positing an aluminum coating on the intermediate metal oxide Ayer.

The novel features whic. are considered as character istie for the invention are set forth in particular in th:

appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection vvith the accompanying dieu-ing, in which the drawing is allow diagram of the present method.

lt is primarily contemplated to use the method of the present invention for the forming of coatings on surfaces which `will be manually contacted and whieh'up to nov.' usually had to be provided with galvanic coatings.

lt is an object ofthe presen invention to provide suc rclectng aluminum coatings in a particu arly simple and economicalmnnner and so that the thus-formed aluminum ccznmg will he corrosion resistant and firmly adhering to its support.

Decrraive layers v-:hich up to now `were formed for instan.: on vapor deposited 'neutralizing intermediate or scoaratng layers of zinc sulo'e, tin oxide, beryllium oxide, barium sutndc, chromium, or silicon oxide possess the desired pernzanency only when not exposed to severe conditions of wear. Thus, the adherence of the metallic layer formed on such conventional sepcrating layers is not sufficiently lirm, or even when using a chromium intermediate layer, the decorative, reliecting metallic surface layer will not withstand the severe stresses in n corrosion promoting environment to which for instance portions or automobile trim, such as the metal rings surround- This togaconsiderable vinethe headlights are exposed. fr exlent is due to the fact-that suchprir art metallic surfece layers cannot be completely insulated or el-: icallyV neutralizedlt is thus a further object of the present invention to provide c method for producing a metallic reflecting coating, particularly an aluminum coating which is not subject to the above discussed disadvantages of prior art coatings of this lcind.

scripton and 'ofthe appended claims. .With the above .metallic body a iirmly adhering, corrosion resistant, re-

`the lacquer-covered body with an oxygen atmosphere v Other objectsl and alvantages-of the present invention f vsill become apparent from a further readmgof the de-fv 7' and other objects in view, the prisent invention 'is con- 'y cerned with a method for forming on a lacquer-covered Germany, assignors to Robert Bosch G.rn.b.H., Stuttl corrosion resistant coating and, more particularly, the

`maintained :it subatmcspheric pressure, vaporizing chromium in the atmosphere so as to deposit the same on lacquercovered metallic body,'the chromium sx1- body, and forming by vapor deposition an aluminum coab,

'tained at subatrnospheric pressure cf about lCt-3 mm. Hg, 'j

'are produced by forming a chr important according to the present invention that in such burned oli, as will be described more detailturtl'xerv insulated from the metallic body.

llo-.weven even by taking the greatest possible care in applyingthe diced by the oxygen o the atmosphere while being de-g posited so as lto form a layer of chromium oxide on the ing on the layer of chromium oxide, the lacquer covering and the chromium oxide layer electrically insulating the aluminum coating from the metallic body.A

A preferred manner of carrying out the method of the present invention comprises the steps of contacting the lacquer-covered body with an oxygen atmosphere main-1 vaporizing chromium in the atmospheres so as to deposit the same on the lacquer-covered metallic body, the chro- 'i mium being oxidized by the oxygen nf the atmosphere while being deposited so as to form a layer of chromium"" oxide on the body, forming by vapor deposition in an inert i argon atmosphere maintained at subatmospherie pressure of about l0n3 mm. Hg, the atmosphere being ionized by glovl discharge, an aluminum coating on the layer of cl1ro- A mium oxide, the lacquer covering and the chromium i' oxide layer substantially but not completely electrically insulating the aluminum coat' .g from thc metallic 'ot'idy` s without, however, completely' eliminating the formation of small aluminum bridges extending transversely through the chromium oxide layer and the lacquer covering from the aluminum coating to the metallic body, electrically burning off the aluminum bridges so as to completely insulate the aluminum coating rorn the metallic body, and Y fon-ning by vapor deposition of silicon in an oxygen atmosphere maintained at a subatmosphctic pressure of ,Y abou 104 nim. Hg a prot :tive layer of silicon oxides en the aluminum coating.

'According to the present invention, aluminum coatings omium oxide covering or ayer on the lacquer-covered metallic body by vapor deposition of chromium in a diluted 'oxygen atmosphere. Thereby it is accomplished that the vaporied chromium, while being deposited, will be transformed into chromium oxide. Thereafter, an aluminum coating is vapor deposited on the thus formed chromium oxide layer. Jhen only portions of the metallic body are to be thus treated i and, consequently only portions of the metallic body are-* lacquer covered, then it is essential to prevent the formation of aluminum bridges atthe edges of the lacquer covering, since such aluminum edges would l'orm an electrical t contact between the aluminum coating and the only partially lacquer-covered metallic body. For this reason, in cases where not the entire metallic body is covered with lacquer, it is contemplated according to the present invention to masl: the edge portions ofthe lacquer covering prior to vapor deposition of the aluminum coating so that, l 1,- after removal of the masliin", the edge portion of the lacquer covering will extend outwardly beyond the aluminurn coating :incl consequently lateral contact between the aluminum coating and the metallic body will not' occur.

lt is frequently unavoidable thnt cracks or fissures occur 'I or are formed in the lacquer covering and in the chro' 4mum oxide layer. Upon vapor deposition of the alulni",A

num coating, aluminum will penetrate such cracks or f I fissures and small bridges will be formed, electrically con-4 Y necxing the aluminum coating with the metallic body. lt is cases, these small bridges are de:troyed, i.e'., electrically below, in order to maintain the alu'minum coating safely l Basically, corrosion is prevented byavoiding'A any poss.,- bility for the localized formation of galvanic elements..

intermediate layers, it is still possible asf;

` f as much as possible, the present invention proposes first of metallic base body. 'In order to reduce this possibility "all tolrcplace a current conducting chromium layer-although the .same would possess the desired high degree of adherence to the underlying lacquer layer as well as to discussed aboverthat .conductive bridgesiwill be" 'formed between the outer decorative metallic coating and the @time j' to use natural resin lacquers and it is furthermore also the superposed vapor deposited aluminum coatingwith an electrically insulating chromium oxide layer. The

chromium oxide layer is formed by vapor deposition of chromium in a diluted oxygen atmosphere so that oxidation of the chromium will take place after the same has been vaporized. Thus, the electrically conductive nf tcrmecliate chromium layer which requires careful insulation, is .replaced with a nor`tconductive chromium oxide `\layer-so-that-neither between the aluminum coating and the intermediate layer, nor between the intermediate layer and the metallic base body a corrosion causing galvanic element will be formed.

The aluminum coating is preferably applied by vapor deposition in an argon atmosphere which is ionized by glow discharge.

ln order to assure complete electrical insulation of the aluminum coating from the metallic base body, any small metallic, electrically conductive bridges which may connect the aluminum coating with the metallic base body are then burned ott electrically. By thus burning off any conductive bridges which may have been formed due to the inadvertent presence vof conductive particles in the intermediate and lacquer layers, or due to penetration of cracks and pin holes by alumnum, complete electric insulation of the aluminum coating from the metallic base body is assured. The electrical burning off of such small bridges can be carried out in various ways, for instance by discharge of a condenser, or by application of either direct or alternating current. Preferably, the thus treated bodies are then subjected to appropriate testing in order to assure that complete insulation of the aluminum layer has been achieved.

-ln some cases it is difficult to prevent corrosion at the edges of the body which is to be aluminum coated. This may be due to the fact that the lacquer layer on such edge -portions usttallyrisvery-thin; of poor adherence, or may have torn during drying.` Sometimes, particularly when the metallic base body is formed with sharp edges, the lacquer layer has rcceded and the metal` 'c sharp cdgcs are exposed. For all these rez-sons, the instalaxon of the aluminum coating in the vicinity of such edge portions required particular attention.. In order to assure electric insulation between thc alutttinum coating and the metallic support in the region of these edge portions, it is preferred to mask the edge portions during the vapor deposition Stepsl particularly during the vapor deposition of the aluminum coating.

As a further safeguard against corrosion and for additional protection of the reflecting or mirror-like aluminum coating against mechanical attacks, it is also contemplated according to the present invention to form on the aluminum coating a protective layer of silicon oxides formed by vapor deposition in an oxygen atmosphere at reduced pressure of between about x10-5 and 5x10*3 mm. Hg under simultaneous ionization by glow discharge. The degree of partial vacuum which is applied depends in each case primarily on the conditions which are required in order to maintain the glow discharge and on the distance between the bod;` which is to be coated and the source of the vapors whichare to be deposited. r U

The chromiufr., aluminum and silicon which are used for vapor deposition' should have a purity of at least 99%, and in the case of aluminum preferably of 99.9%. In-

stead of aluminum, also aluminum alloys containing small l quantities of chromium and/or magnesium may be lvaperized. I v 'I l Vaporization offchromium and silicon in the oxygen atmosphere, as described above, will result in oxidation a manner similar to the formation of the intermediate lacquer known in the art. Such lacquers may be based vthe present invention, without, however, limiting the iup vention to the specic details 'of the example. A The lacquer-covered metallic body which is to be in capacity from 4 to 40 nf. and from 2O to 150 volts..

so that vthe respective The metallic base body may be covered by any suitablel on synthetic resins, for instance alkyd-rnelarnine resins, urea resins or epoxy resins, however, it is also possible within the scope of the present invention to cover the aluminum coating or the protective layer of silicon oxides l thereon with a suitable transparent lacquer.

The following example is given as illustrative only of coated is placed into the vapor deposition chamber which also contains the chromium which is to be vaporized. f The chamber is then evacuated at room temperature to y a pressure not exceeding 5 10"l mm. Hg. Thereafter,

oxygen gas is introduced into the chamber until the pressure has risen to about 10-3 mm. Hg. The chromium is then vaporized by electric heating aud the t'nus formed chromium vapors, after reacting with the oxygen atmosphe. will precipitate on the relatively cold lacquercovered metallic body. Thereafter, aluminum is introduced into the chamber `and pressure therein is again 'i reduced to below 5x10'-4 mm. Hg. At this very low residual pressure, the aluminum is vaporized by being heated electrically to about lOOO C., which may be carried out with a' current of a few volts and about amperes. j The vaporization and deposition of aluminum can also be carried out in a diterent manner, namely by intro ducing, after evacuation of the chamber to S l0f4 mm. l-lg, argon until the pressure rises to about 104. During subsequent vaporization of the aluminum, a potential of several thousand volts is applied to an electrode located in the chamber, for instance about 10,000 volts until glow discharge starts and about 1G00 volts thereafter. Thus i produced glow discharge will increase the kinetic energy of the aluminum vapor so that the same will become hotter and this will improve the adherence of the aluminum coating formed by the deposition of the aluminum vapors. For instance, as described in '(75. Patent No. 1,710,747, one terminal may be attached to an electrode located within the vaporzation chamber or vessel, and the terminal of opposite polarity may be attached to themetal wall of the vaporization vessel. Upon application of the above described potential to the two tcrmi nais, glow discharge will then take place between the electrode and the wall of the vaporization vessel.

Tire thus formed aluminum coating may still include 1' arranged in the circuit, which condenser is first charged I" and will then be discharged by connecting Ithe circuit including the metallic body. This discharge will pass N through the above discussed electrically conductive bridges l Y and will destroy 4the same. Preferably, burning eff ofA the bridges is started with relatively low voltage and a" small condenser and voltage and condenser capacity are "5* then increased until Vall bridges are destroyed. Good resuits were obtained with a series of condensers ranging 'I However, these values are not necessarily limiting.

After the aluminum coatinghas been applied as del-rf;

. scribed above and the conductive bridges have been burned oli, the thus coated body is lacquered, or a protective layer of silicon oxides is applied b v vapor deposition in.

chromium oxide layer described above, except that, of

.- and 'to form a layer of chromium oxide on said lacquerby applying current course, the chromium standpoint of prior art. fairly constitute essential characteristics of the generic or specific aspects of this invention i and, therefore,` such to be comprehended within the meaning and range ot`v adaptations should and are intended equivalence of the following claims. '-f- What is claimed as new and desired to be secured by Letters Patent is:

1. A method for forming on a lacquer-covered metallic body a iirmly adhering, corrosion resistant, reecting metal coating, comprising the steps of contacting said Ylacquer-covered body with an oxygen atmosphere mainn said atmosphere s tained at subatmospheric pressure; vaporizing chromium o as to oxidize vaporized chromium covered metallic body; and forming hy vapor deposition an aluminum coating on said layer of chromium oxide,

the lacquer covering and said chromium oxide layer electrically insulating said aluminum coating rom said metal- 2. A method for forming on a lacquer-covered metallic body a firmly adherin g, corrosion resistant, reecting metal coating, comprising the steps of contacting said lacquercovered `vody with an oxygen atmosphere maintained at subatmospheric press ure of about -3 mm. Hg; Vaporizing chromium in said atmosphere so as to oxidize vaporized chromium and to form a layer of crornium oxide on said lacquer-covered metallic body; and forming by vapor deposition an aluminum coating on said layer of chromium oxide, the lacquer covering and said chromium oxide layer electrically insulating said aluminum coating from said metallic body.

3. A method for forming on a lacquer-co 'ered metallic body a firmly adhering, corrosion resistant, reecting metal coating, comprising the 'steps of contacting said lacquer-covered bodyvriih-enoxy cn atmosphere maintained at subatmospheric pressure; vapor-lling chromium A"'in's'aid atmosphere so`as to oxidrze vaporizedhromium and to form a layer covered metallic bod of chromium oxide on said lacquery; and forming by vapor deposition in an inert atmosphere maintained at subatmospheric pressure an aluminum coating on said loyer of chromium oxide, the lacquer covering and said chromium oxide layer electrically insulating said aluminum coating rom said metallic body.

and to form a layer oi chromium oxide on saidlacquercovered metallic body; and forming by vapor deposition in an inert atmosphere maintained at subatmospheric pressure, said atmosp an aluminum coating here being ionizedby glow discharge, on said layer of chromium oxide, the

y lacquer covering and said chromium oxide layer electrically insulating said aluminum coating from said metellic body.

" 5. A methof'for forming on a lacquer-covered metallic -'body a firmly adhering, corrosion resistant, reflecting metal coating, comprising the-steps of contacting said in said atmosphere s and tfr form a layer lacquer-covered body with an oxygen atmosphere maintained at subatmospheric pressure; vapori-ing chromium c as to oxidize vaporized chromium of chromium oxide on said lacquer- :ovcred metallic body; and forming by vapor deposition in an inert argon atmosphere maintained at subatmosphcric pressure of about l0" mm. Hg an aluminum coating on said layer of chromiumcxide, the lacquer as starting material is replaced by Without further analysis, the foregoing willso fully reveal the gist of the present invention that others cau knowledge readily adapt it for various applications without omitting features that, from the covering and said chromium oxide layer electrically nsulating said aluminum coating from said metallic body.

metal coating, com rising tbe/steps of contacting Said 1 6. A method for forming on a lacquer-covered meta1lic. body a rmly adhering, corrosion resistant, rcecting lacquer-covered body with an oxygen atmosphere maintained at subatmospheric pressure; vaporizing chromium in said atmosphere so as to oxidize vaporized chromium and to form a layer of chromium oxide on said lacquer- \f covered'metallic body; forming by vapor deposition an alurnnumcoating on said layer of chromium oxide, the i lacquer coveripgand said chromium oxide layer substanl tially but not completely electrically insulating said aluminum coating from said metallic body; and electrically 2 burning oli aluminum bridges extending transversely through said chromium oxide layer and said lacquer covering from said aluminum coatingto said metallic body so as to completely insulate said aluminum coating from said metallic body.

7. A method for forming on a lacquer-covered metallic body a rmly adhering, corrosion resistant, 'redecting metal coating, comprising the steps of contactingsaid lacquer-covered body with an oxygen atmosphere main tained at subatmospheric pressure of about 10*3 mm. Hg; vaporizing chromium in atmosphere so as to oxidzc vaporized chromium and to form a layer of chromium oxide on said lacquer-covered metallic body; forming by vapor deposition in an inert argon atmosphere maintained at subatmospheric pressure of about 103 mm. Hg, said atmosphere being ionized by glow discharge, an aluminunr coating; on said layer of chromium oxide, the lacquer covering and said chromium oxide layer substantially but not completely electrically insulating said aluminum coating trom said metallic body; and electrically burning off aluminum bridges extending transversely through said chromium oxide layer and said lacquer covering from said aluminum coating to said metallic body so as to compleely insulate said aluminum coating from said metallic body.

8. A method for forming on a lacquer-covered portion of a metallic body a rmly adhering, corrosion resistant, rcccting metal coating, comprising the steps of masking the peripheral portion of said lacquer covering; contacting said lacquer-covered portion of said body with an oxygen atmosphere maintained at subatmospheric pressure', vaporizing chromium in said atmosphere so as to oxidize vaporized chromium and to form a layer of chromium oxidr.` on said lacquer-covered metallic bodv; and forming by vapor deposition on aluminum coating on said layer of chromium oxide, the lacquer covering, the masking of the peripheral portion thereof and said chromium oxide layer electrically insulating said aluminum coating from said metallic body.

9. A method for forming on a lacquer-covered portion of a metallic body a firmly adhering, corrosion resistant, rctiecting metal coating, comprising the steps of contacting said lacquer-covered body with an -oxygen atmoslphere maintained atV subatmospheric pressure; Vaporizing chromium in said atmosphere so as to oxidize vaporized chromium and to form a layer of chromium oxide on said lacquer-covered metallic body, masking the peripheral portion of said lacquer covering; and forming by vapor deposition an aluminum coating on said layer of chromium oxide, the lacquer covering, the masking of the peripheral portion thereof and said chromium oxide layer electrically insulating said aluminumcoating from said metallic body.

l0. A method for forming on a lacquer-covered metallic body a firmly adhering, corrosion resistant, r:lecting j metal coating, comprising the steps of contacting said lacquer-covered body with an oxygen atmosphere rnantained at subatmospheric pressure; vapon'zing chromium in said atmosphere so as to oxidixe vaporized chromium and to form a layer ot chromium oxide on said lacquercovered metallic body', forming by vapor deposition an aluminum coating on said lacquer covering and said chromium oxide layer electrically insulating said aluminum coating from said metallic body; and forming by vapor deposition of silicon in an oxygen atmosphere maintained `at subatmospheric pressure a protective layer of silicon oxides o n said aluminum coating.

l1. A method for forming on a lacquer-covered metallic body a rmlyadhering, corrosion resistant, re'ccting metal coating, comprising thesteps of contacting said lacquer-covered body with an oxygen atmosphere maintained at subatmcspheric pressure; vaporizing chromium in said atmosphere so as to oxidi yaporized chromium and to form a layer of chromium oxide on said lacquercovered metallic body; forming by vapor deposition an aluminum coating on said layer of chromium oxide, the lacquer covering and said chromium oxide. layer electrically insulating said aluminum coating from said metallic body; and forming by vapor deposition o silicon layer of chromium 'oJ-tide,` thein an oxygen atmosphere maintained at subatmospberic 20 pressure of about 10"4 mm. Hg a protective l:1}.-r of silicon oxides on said aluminum coating.

l2. A method for forming on a lacquer-covered metallic body a firmly adhering. corrosion resistant, rcecting metal coating, comprising the steps of contacting said lacquercovered body with an oxygen atmosphere maintained at subatmospheric pressure of about l0"s mm. Hg; vaporizing chromium in said atmosphere so as to oxidize vaporized chromium and to form a layer of chromium oxide or. said lacquer-covered rnetztlic body;

forming-by vapor deposition in an inert argon atmos- Y phere maintained :rt'subatrnospheric pressure of aboutA r l0-"mm. Hg, said atmosphere being ionized by g,lo`.v.' 4 discharge, an aluminum coating on said layer of chromium oxide, the lacquer coyering and said chromium oxide' layer substantially but not completely electrically insularing said aluminum coating from said metallic body; electrically burning of aluminum bridges extending' trans- Yersely through said chromium oxide layer and saidv lacquer covering from said aluminum coating to said metallic body so as to completely insulate said aluminum coating from said metallic body; and. forming by vapor deposition oi silicon in an oxygen atmosphere maintained at a subatrnospheric pressure of about 10'* mm. Hg a protective layer-of silicon oxides on said aluminum coating.

References Cited by the Exam if;

UNITEDv STATES Parnu-rs1 2,628,921 2/53 Weinneh 117-62 2,917,318 12/59 Thomson 117-71 2. 9 s.g.= s,1o/eo Brown 117--113 2,957,782 11i/en .acuer 2'9-197 x OTHER REFERENCES Holland: Vacuum Deposition of Thin Films (1956),

John Vtiley, New York, pp. 80-'S3, 169-176, 1145-3144 and A50-455 relied on. A RICHARD D. Nevins, Primary Examiner.' i

Claims

1. A METHOD FOR FORMING ON A LACQUER-COVERED METALLIC BODY A FIRMLY ADHERING, CORROSION RESISTANT, REFLECTING METAL COATING, COMPRISING THE STEPS OF CONTACTING SAID LACQUER-COVERED BODY WITH AN OXYGEN ATMOSPHERE MAINTAINED AT SUBATMOSPHERIC PRESSURE; VAPORIZING CHROMIUM IN SAID ATMOSPHERE SO AS TO OXIDIZE VAPORIZED CHROMIUM AND TO FORM A LAYER OF CHROMIUM OXIDE ON SAID LACQUERCOVERED METALLIC BODY; AND FORMING BY VAPOR DEPOSITION AN ALUMINUM COATING ON SAID LAYER OF CHROMIUM OXIDE, THE LACQUER COVERING AND SAID CHROMIUM OXIDE LAYER ELECTRICALLY INSULATING SAID ALUMINUM COATING FROM SAID METALLIC BODY.