CA1046912A - Treatment of metal surfaces with trivalent chromium solutions - Google Patents
Treatment of metal surfaces with trivalent chromium solutionsInfo
- Publication number
- CA1046912A CA1046912A CA228,380A CA228380A CA1046912A CA 1046912 A CA1046912 A CA 1046912A CA 228380 A CA228380 A CA 228380A CA 1046912 A CA1046912 A CA 1046912A
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- CA
- Canada
- Prior art keywords
- metal article
- treatment
- trivalent chromium
- chromium
- article
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
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Classifications
-
- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C22/00—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
- C23C22/05—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions
- C23C22/06—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6
- C23C22/48—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6 not containing phosphates, hexavalent chromium compounds, fluorides or complex fluorides, molybdates, tungstates, vanadates or oxalates
- C23C22/53—Treatment of zinc or alloys based thereon
-
- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C22/00—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
- C23C22/05—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions
- C23C22/06—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6
- C23C22/48—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6 not containing phosphates, hexavalent chromium compounds, fluorides or complex fluorides, molybdates, tungstates, vanadates or oxalates
-
- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C22/00—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
- C23C22/82—After-treatment
- C23C22/83—Chemical after-treatment
-
- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2222/00—Aspects relating to chemical surface treatment of metallic material by reaction of the surface with a reactive medium
- C23C2222/10—Use of solutions containing trivalent chromium but free of hexavalent chromium
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- Chemical & Material Sciences (AREA)
- General Chemical & Material Sciences (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Engineering & Computer Science (AREA)
- Materials Engineering (AREA)
- Mechanical Engineering (AREA)
- Metallurgy (AREA)
- Organic Chemistry (AREA)
- Chemical Treatment Of Metals (AREA)
- Application Of Or Painting With Fluid Materials (AREA)
Abstract
Title: TREATMENT OF METAL SURFACES WITH TRIVALENT
CHROMIUM SOLUTIONS
Inventor: Donald Edward Wells Abstract of the Disclosure:
Conversion coatings are produced on ferrous metal or galvanized surfaces by treatment with an aqueous solu-tion of a trivalent chromium compound. The solution is preferably formed by the reduction of a hexavalent chromium solution with an alkali metal sulfite or bisulfite. The temperature of the metal treatment is substantially lower than is required to produce an iron phosphate coating, and the absence of hexavalent chromium results in a substantial decrease in pollutants in the effluent. The conversion coating obtained is usually deep blue in color and improves paint adhesion of the treated surface and corrosion resis-tance, especially under paint and other, siccative organic coatings.
CHROMIUM SOLUTIONS
Inventor: Donald Edward Wells Abstract of the Disclosure:
Conversion coatings are produced on ferrous metal or galvanized surfaces by treatment with an aqueous solu-tion of a trivalent chromium compound. The solution is preferably formed by the reduction of a hexavalent chromium solution with an alkali metal sulfite or bisulfite. The temperature of the metal treatment is substantially lower than is required to produce an iron phosphate coating, and the absence of hexavalent chromium results in a substantial decrease in pollutants in the effluent. The conversion coating obtained is usually deep blue in color and improves paint adhesion of the treated surface and corrosion resis-tance, especially under paint and other, siccative organic coatings.
Description
~469~Z
This invention relates to a method for treating ferrous metal and galvanized articles, and to articles thus treated~ More particularly, it relates to a method for producing a:conversion coating on a ferrous metal or galvanized article which comprises contacting said article with an .
aqueous solution of a trivalent chromium compound, It has been the practice for many years to form conversion coatings on meta~ sur~aces by treatment with solutions of various chemicals whieh react with the surface to form a coating which protects the metal against corrosion and also serves as a paint base. Among the commonly used conversion coating compositions are aqueous phosphate and chromate solutions, Many of these are known in the art;
I among the simplest of the phosphate compositions are the - 15 so-called "iron phosphates" which comprise solutions of alka-li metal phosphates or the like, and which react with the iron of the ferrous metal surface to form an iron phosphate coa~ing~ Most iron phosphate solutions must be contacted with the metal at elevated temperatures, typically 50~C, or above. In view of the current need to conserve energy, it is of interest to develop conversion coating solutions which may be used at lower temperatures, especially ~`
room temperature, Other conversion coating compositions in general use are the chromium-containing solutions known in the art.
.
.. .
These generally contain hexavalent chromium in the form of chromate or dichromate, frequently in combination with trivalent chromium or in forms such that partial reduction to trivalent chromium takes place during treatment of the ferrous metal surface, Recent emphasis on water pollution ...., : - `
~ ~ .
problems has drawn attention to the fact that chromates are serious pollutants To meet water quality standards, it is frequently necessary to go through a multi-sta~e purification sequence in order to remove chromates from metal treatment process effluents, ~ypical steps in this sequence include reduction ~f the hexavalent chromium to trivalent chromium and precipitation with lime or some similar chemlcal. The result is that the chromate content of the effluent water is decreased, frequently to zero or near zero, but the expense ~ of the treatment process to the user is quite high A principal object of the present invention, therefore, is to develop new methods and compositions for the treatment of ferrous metal surfaces to improve adhesion of siccative coatings and to increase corrosion resistance, especially when the surfaces are further coated with such ; siccative coatings ~ A further object is to provide a treatment system ;~
.
for ferrous and galvanized metals which provides results similar to those of iron phosphates and other phosphate conversion coatings, but which can be applied at low tem-peratures, ` Another object is to provide a treatment system which can be used without a resulting discharge of hexavalent , chromium or other pollutants as park of the effluent Other objects will in part be obvious and will in ~part appear hereinafter.
The present invention is based on the discovery i ~ j that aqueous solutions containing chromium entirely in the e ~ , trivalent stage are advantageously used to produce conver-,, 30 ~ sion coatings on ferrous and galvanized metals, especially steel and galvanized steel.
~111469~Z
Any aqueous solution of a trivalent chromium com-pound may be used in the method of this invention. Thus, solutions of chromium sulfate, chromium nitrate and the like are useful. The preferred trivalent chromium solutions, however, are those prepared by reduction of an aqueous hexavalent chromium-containing solution. Many suitable reducing agents, both organic and inorganic, are knownO The organic ones in~lude such materials as methanol, ethanol, ethylene glycol, formaldehyde, hydroquinone and the like.
The methods of reducing hexavalent chromium with -~
organic reducing agents are ~nerally known in the art.
For example, U.S. patents 3,063,877 and 3,404,045 describe methods or reducing chromium trioxlde with formaldehyde and methanol, respectively. ~owever, the amounts of the reducing agents used according to those patents are insuf-ficient fGr complete reduction of hexavalent to trivalent chromium. According to the present invention, the amount of reducing agent is increased to at least the amount required for complete reduction.
AmOng the suitable inorganic redu~ing agents are alkali metal iodides, ferrous salts, sulfur dioxide, alkali `i metal`sulfites or bisulfites and the like. The alkali metal sulfites and bisulfite~, notably the bisulfites and especially ~,~ sodium and potassium bisulfite, are preferred. As with the . - i ~ ~5 organic reducing agents previously described the inorganic .
~` reducing agents are employed in amounts sufficient to com-pletely reduce hexavalent to trivalent chromium. HOwever, a substantial excess of sulfite or bisulfite over this amount should generally not be present since the presence of sulfite or bisulfite in the final treatment solution sometimes _ r--\ .
1~916~.z results in the formation of "blush rust" on the metal surace being treated. In general, the amount of sulfite or bisulfite employed should be less than about a 1~ excess (by weight) over the stoichiometric amount required for complete reduc-tion of hexavalent to trivalent chromium. If, because of incom~lete reaction, further reducing agent is necessary, methanol or a similar organic reducing agent, or an inorganic -reducing agent other than suifite or bisulfite, should be used to complete the reduction.
The preparation of trivalent chromium solutions Nseful (usually after dilution with water as described ~ ~ ;
hereinafter) in the method of this invention is illustrated by the following examples. All parts are b~ weight. Equiva- ;
lents of acid are calculated on the basis of acidic hydrogens.
An equivalent of a material to be oxidized or reduced is its molecular weight divided by the number of units of valence change in the oxidation~reduction reaction.
Example 1 ;~
: . -- . .
A solution of 51 parts (9.55 e~uivalents) of ` methanol in 144 parts of water is added dropwise, with external cooling, to a solution of 300 ~arts ~8.96 equiva-lents) of 99.5% assay chromium trioxide in 204 ~arts of `
water. After methanol addition is com~lete, 900 parts (9.25 . .
equivalents) of concentrated hydrochloric acid is added ., . ~ , .
over several hours, with stirring, at a temperature of a4_~8C The product is the desired trivalent chromium solution. ~ -!
The procedure of Example 1 is repeated, except that 850 parts (9.04 equivalents) of concentrated nitric / ' ' ~ .
~~~
;9~L2 acid i9 used, A similar product is obtained, Example 3 The procedure of Exam~le 1 is repeated, except that 550 parts (9.17 equivalents) of glacial acetic acid is used, A similar product is obtained, Example 4 The procedure of Example 1 is repeated, except that 465 parts (9.12 equivalents) bf concentrated sul~uric acid is used, A similar product is obtained, -Example 5 Solid sodium bisulfite, 12,2 parts (0,2~5 equiva-lent), is added gradually, with stirring~ to a solution of 7~8 parts (0,2~3 equivalent) of 99.5~ assay chromium trioxide in 80 parts of water. The addition rate of the sodium bisulfite is such that the temperature does not ' exceed 65C, The product is the desired trivalent chromium solution, In the metal treatment operation in which the ~ " method of this invention is used, the metal surface is usually k first cleaned by chemical and/or physical means to remove any grease, dirt and oxides, When the trivalent chrornium solution contains a surfactant as described hereLnafter, the initial chernical cleaning stage may be unnecessary, The surface (or any portion thereof for which 25 ~ treatment is desired) is then rinsed-with water and treated ` ~ w1th the tri~alent chromium solution, Treatment may be by any of the commonly used techniques such as spraying, brushing, dipping, roller-coating, reverse roller-coat-Lng, and~flow-coating, ~he solutions of the present invention 1.', ~ ~ ' , ~
469~2 are particularly useful in a spray system. The concentra- -tion of chromium in the metal treatment solution is ~enerall~
about 0.01-0.~% b~ weight and may be somewhat lower for spray applications (e.g., 0.05-0.11%) than for immersion applications (e.g., about 0.11-0.2%).
The pH of the trivalent chromium solution during application is about 3.5-6.0, usually a . 0-5Ø Since a con-centrate prepared from an already formed trivalent chromium salt (e.g., chromium chloride or nitrate~, or by reduction of hexavalent chromium with certain reducing agents such as sulfur dioxide, may be too acidic to produce a metal treatment solution having a pH within this range, it may be necessary to adjust the pH of the concentxate or treatment solution by adding an alkaline reagent thereto. Such reagent (usually ammonium hydroxide, sodium hydroxide or potassium hydroxide~ and preferably one of the latter two) r, is most preferably added to the concentrate.
On the other hand, when bisulfîte or the like is the reducing agent it may sometimes be necessar~ to acidify the water used for diluting the concentrate to form the treatment solution, so as to avoid the formation of a pre-cipitate or a colloid during dilution. When this is done, the ~ater should have a pH after adjustment below 7.0, pre-; ferably about 5.0-5.5. The preferred acid for adjusting pH
is sulfuric acid.
` ( The temperature of application of the trivalent chromium solution to the metal surface is usuallv about ` 0-50C., although it may be higher (up to about 75C.) if , . .. . .. .
l desired, and is preferably about 10-30C~ Thus, it will be .~ j .
appreciated that the method of this invention requires the use of temperatures much lower than are generally required 10~6912 for the application of an iron phosphate coating, and con-sequently that a lower energy input is necessary. This low energy input is another advantage of the method of this invention.
Following the trivalent chromium treatment, the metal surface is usually rinsed with water, usually also at a temperature below about 50C. and advantageously at room temperature, and is then dried. Drying may be by simply air-blowing at room temperature or may be at higher temp-eratures, usually up to about 65C.
~ The conversion coating produced on a ferrous metal surface by the method of this invention is usually deep blue in color~ In addition to furnishing an attrac-tive appearance to the metal, it improves corrosion resistance and paint adhesion.
After a metal article has been treated in accor-dance with the method of this invention, it is preferred to apply an organic coating composition which may be a siccative coating such as paint, lacquer, varnish, synthetic resin, enamel or the like (which are preferred), an electrostat-ically deposited powder coating, or any other suitable type.
s Examples of siccative coatings which may be used are the acrylic, alkyd, epoxy, phenolic, melamine and polyvinyl alcohol resins and paints.
` 25 Application of a siccative coating composition can be effected by any of the ordinary techni~ues such as brushing, spraying, dipping, roller-coating, flow-coating, electrostatic or electrophoretic attraction, etc. I The coated article is dried in the manner best suited for the siccative coating composition employed, e.g., by air-drylng at ambient or elevated temperature, baking in an oven, ~ ` ` ' ~ ' ,:,, - .
~ 7 ~,:,: ; : , ~0~6~2 or baking under infra-red lamps. In most instances, the thickness of the dried ~ilm of the siccative organic coating composition will be about 0.1 10.0 mils, more often about 0.3-5.0 mils.
From the above description, it will be apparent that the advantages of this invention include the avoidance of hexavalent chromium as a pollutant in the process effluent, lower energy requirement for formation of a con-version coating, improved paint adhesion of the treated metal, and improved corrosion resistance, especially of the painted metal surface.
It is within the scope of this invention to include in the trivalent chromium solution a minor amount of a sur~actant, usually a non-ionic surfactant. When the sur-factant is present, the chromium solution mà~ be used as a cleaner as well as a conversion coating~solution, thus ' eliminating the initial chemical cleaning step described hereinabove.
The effectiveness of the method of this invention for forming conversion coatings on ferrous metal surfaces ., .
is demonstrated by a procedure in which panels of cold-rolled steel are cleaned by spraying with a sodium phosphate-based cleaner at 66-72C. and rinsed with water at room temperature. They are then sprayed at 18-22C. with a solution of Example 5. The treated panels are rinsed again with water, dried under infrared lamps and painted with a while alkyd-melamine baking enamel. Six sets of test panels are processed at the end of consecutive half-days of working of the spray line.
A set of control panels is similarly cleaned and ~ rinsed, treated at 50-60C. with a commercial chlorate- ;
`?i ~
x , : :
:~; 8 i9~2 accelerated iron phosphate conversion coating solution having a p~l of 5-~, and rinsed with water and with a commercial hexavalent-trivalent chromium post-rinse solu-tion. These panels are also painted as described above.
The paint film on each panel is ruptured down to -~ the bare metal by scoring a six-inch line on the surface of the panel. The scored panel is placed in a cabinet containing a 5% aqueous sodium chloride solution at 95F.
Air is bubbled through the solution to produce a corrosive salt atmosphere which acts on the surface of the test panel suspended above the level of the salt solution. The panels remain in this atmosphere for 120 hours after which they are removed, washed with water and dried with a cloth. A
pressure-sensitive tape is then applied to each panel and removed suddenly. This ~rocedure is repeated until no more paint can be removed in this manner. The loss o~ adhesion . , .
; caused by corrosion from the scribed line is measured in thirty-seconds of an inch. -~
' When measured in this manner, the average loss of adhesion for the panels treated by the method of this inven-tion is 0.7S thirty-seconds of an inch. The average for the`
control panels is 0.~4 thirty-seconds of an inch. Thus, the method of this invention gives results when the chromium olution is applied at room temperature or below, which are ~' ~5 comparable to those provided by a commercial iron phosphatesolution applied at elevate~ temperatures and ~ollowed by a commexcial hexavalent chromium-containing rinse.
The trivalent chromium solutions used in the method of this invention may also be used as post-rinses for phosphated metals, especially ferrous metals. The phosphating treatment used prior to this post-rinse may he ~'', :~ ~
~ 9 ~ ' ', 1~46912 iron phosphate, zinc phosphate, calcium- or magnesium-modified zinc phosphate, or the like.
... ... .
: , ., . . .:: .
~ ' ... .
:: :.
: . :
,~: :~: '- :, ~- ~ . . . - .
~, :
.~ : :, . , : , ~: : :. -:,~ I :
~. :, -: . . .. ~
This invention relates to a method for treating ferrous metal and galvanized articles, and to articles thus treated~ More particularly, it relates to a method for producing a:conversion coating on a ferrous metal or galvanized article which comprises contacting said article with an .
aqueous solution of a trivalent chromium compound, It has been the practice for many years to form conversion coatings on meta~ sur~aces by treatment with solutions of various chemicals whieh react with the surface to form a coating which protects the metal against corrosion and also serves as a paint base. Among the commonly used conversion coating compositions are aqueous phosphate and chromate solutions, Many of these are known in the art;
I among the simplest of the phosphate compositions are the - 15 so-called "iron phosphates" which comprise solutions of alka-li metal phosphates or the like, and which react with the iron of the ferrous metal surface to form an iron phosphate coa~ing~ Most iron phosphate solutions must be contacted with the metal at elevated temperatures, typically 50~C, or above. In view of the current need to conserve energy, it is of interest to develop conversion coating solutions which may be used at lower temperatures, especially ~`
room temperature, Other conversion coating compositions in general use are the chromium-containing solutions known in the art.
.
.. .
These generally contain hexavalent chromium in the form of chromate or dichromate, frequently in combination with trivalent chromium or in forms such that partial reduction to trivalent chromium takes place during treatment of the ferrous metal surface, Recent emphasis on water pollution ...., : - `
~ ~ .
problems has drawn attention to the fact that chromates are serious pollutants To meet water quality standards, it is frequently necessary to go through a multi-sta~e purification sequence in order to remove chromates from metal treatment process effluents, ~ypical steps in this sequence include reduction ~f the hexavalent chromium to trivalent chromium and precipitation with lime or some similar chemlcal. The result is that the chromate content of the effluent water is decreased, frequently to zero or near zero, but the expense ~ of the treatment process to the user is quite high A principal object of the present invention, therefore, is to develop new methods and compositions for the treatment of ferrous metal surfaces to improve adhesion of siccative coatings and to increase corrosion resistance, especially when the surfaces are further coated with such ; siccative coatings ~ A further object is to provide a treatment system ;~
.
for ferrous and galvanized metals which provides results similar to those of iron phosphates and other phosphate conversion coatings, but which can be applied at low tem-peratures, ` Another object is to provide a treatment system which can be used without a resulting discharge of hexavalent , chromium or other pollutants as park of the effluent Other objects will in part be obvious and will in ~part appear hereinafter.
The present invention is based on the discovery i ~ j that aqueous solutions containing chromium entirely in the e ~ , trivalent stage are advantageously used to produce conver-,, 30 ~ sion coatings on ferrous and galvanized metals, especially steel and galvanized steel.
~111469~Z
Any aqueous solution of a trivalent chromium com-pound may be used in the method of this invention. Thus, solutions of chromium sulfate, chromium nitrate and the like are useful. The preferred trivalent chromium solutions, however, are those prepared by reduction of an aqueous hexavalent chromium-containing solution. Many suitable reducing agents, both organic and inorganic, are knownO The organic ones in~lude such materials as methanol, ethanol, ethylene glycol, formaldehyde, hydroquinone and the like.
The methods of reducing hexavalent chromium with -~
organic reducing agents are ~nerally known in the art.
For example, U.S. patents 3,063,877 and 3,404,045 describe methods or reducing chromium trioxlde with formaldehyde and methanol, respectively. ~owever, the amounts of the reducing agents used according to those patents are insuf-ficient fGr complete reduction of hexavalent to trivalent chromium. According to the present invention, the amount of reducing agent is increased to at least the amount required for complete reduction.
AmOng the suitable inorganic redu~ing agents are alkali metal iodides, ferrous salts, sulfur dioxide, alkali `i metal`sulfites or bisulfites and the like. The alkali metal sulfites and bisulfite~, notably the bisulfites and especially ~,~ sodium and potassium bisulfite, are preferred. As with the . - i ~ ~5 organic reducing agents previously described the inorganic .
~` reducing agents are employed in amounts sufficient to com-pletely reduce hexavalent to trivalent chromium. HOwever, a substantial excess of sulfite or bisulfite over this amount should generally not be present since the presence of sulfite or bisulfite in the final treatment solution sometimes _ r--\ .
1~916~.z results in the formation of "blush rust" on the metal surace being treated. In general, the amount of sulfite or bisulfite employed should be less than about a 1~ excess (by weight) over the stoichiometric amount required for complete reduc-tion of hexavalent to trivalent chromium. If, because of incom~lete reaction, further reducing agent is necessary, methanol or a similar organic reducing agent, or an inorganic -reducing agent other than suifite or bisulfite, should be used to complete the reduction.
The preparation of trivalent chromium solutions Nseful (usually after dilution with water as described ~ ~ ;
hereinafter) in the method of this invention is illustrated by the following examples. All parts are b~ weight. Equiva- ;
lents of acid are calculated on the basis of acidic hydrogens.
An equivalent of a material to be oxidized or reduced is its molecular weight divided by the number of units of valence change in the oxidation~reduction reaction.
Example 1 ;~
: . -- . .
A solution of 51 parts (9.55 e~uivalents) of ` methanol in 144 parts of water is added dropwise, with external cooling, to a solution of 300 ~arts ~8.96 equiva-lents) of 99.5% assay chromium trioxide in 204 ~arts of `
water. After methanol addition is com~lete, 900 parts (9.25 . .
equivalents) of concentrated hydrochloric acid is added ., . ~ , .
over several hours, with stirring, at a temperature of a4_~8C The product is the desired trivalent chromium solution. ~ -!
The procedure of Example 1 is repeated, except that 850 parts (9.04 equivalents) of concentrated nitric / ' ' ~ .
~~~
;9~L2 acid i9 used, A similar product is obtained, Example 3 The procedure of Exam~le 1 is repeated, except that 550 parts (9.17 equivalents) of glacial acetic acid is used, A similar product is obtained, Example 4 The procedure of Example 1 is repeated, except that 465 parts (9.12 equivalents) bf concentrated sul~uric acid is used, A similar product is obtained, -Example 5 Solid sodium bisulfite, 12,2 parts (0,2~5 equiva-lent), is added gradually, with stirring~ to a solution of 7~8 parts (0,2~3 equivalent) of 99.5~ assay chromium trioxide in 80 parts of water. The addition rate of the sodium bisulfite is such that the temperature does not ' exceed 65C, The product is the desired trivalent chromium solution, In the metal treatment operation in which the ~ " method of this invention is used, the metal surface is usually k first cleaned by chemical and/or physical means to remove any grease, dirt and oxides, When the trivalent chrornium solution contains a surfactant as described hereLnafter, the initial chernical cleaning stage may be unnecessary, The surface (or any portion thereof for which 25 ~ treatment is desired) is then rinsed-with water and treated ` ~ w1th the tri~alent chromium solution, Treatment may be by any of the commonly used techniques such as spraying, brushing, dipping, roller-coating, reverse roller-coat-Lng, and~flow-coating, ~he solutions of the present invention 1.', ~ ~ ' , ~
469~2 are particularly useful in a spray system. The concentra- -tion of chromium in the metal treatment solution is ~enerall~
about 0.01-0.~% b~ weight and may be somewhat lower for spray applications (e.g., 0.05-0.11%) than for immersion applications (e.g., about 0.11-0.2%).
The pH of the trivalent chromium solution during application is about 3.5-6.0, usually a . 0-5Ø Since a con-centrate prepared from an already formed trivalent chromium salt (e.g., chromium chloride or nitrate~, or by reduction of hexavalent chromium with certain reducing agents such as sulfur dioxide, may be too acidic to produce a metal treatment solution having a pH within this range, it may be necessary to adjust the pH of the concentxate or treatment solution by adding an alkaline reagent thereto. Such reagent (usually ammonium hydroxide, sodium hydroxide or potassium hydroxide~ and preferably one of the latter two) r, is most preferably added to the concentrate.
On the other hand, when bisulfîte or the like is the reducing agent it may sometimes be necessar~ to acidify the water used for diluting the concentrate to form the treatment solution, so as to avoid the formation of a pre-cipitate or a colloid during dilution. When this is done, the ~ater should have a pH after adjustment below 7.0, pre-; ferably about 5.0-5.5. The preferred acid for adjusting pH
is sulfuric acid.
` ( The temperature of application of the trivalent chromium solution to the metal surface is usuallv about ` 0-50C., although it may be higher (up to about 75C.) if , . .. . .. .
l desired, and is preferably about 10-30C~ Thus, it will be .~ j .
appreciated that the method of this invention requires the use of temperatures much lower than are generally required 10~6912 for the application of an iron phosphate coating, and con-sequently that a lower energy input is necessary. This low energy input is another advantage of the method of this invention.
Following the trivalent chromium treatment, the metal surface is usually rinsed with water, usually also at a temperature below about 50C. and advantageously at room temperature, and is then dried. Drying may be by simply air-blowing at room temperature or may be at higher temp-eratures, usually up to about 65C.
~ The conversion coating produced on a ferrous metal surface by the method of this invention is usually deep blue in color~ In addition to furnishing an attrac-tive appearance to the metal, it improves corrosion resistance and paint adhesion.
After a metal article has been treated in accor-dance with the method of this invention, it is preferred to apply an organic coating composition which may be a siccative coating such as paint, lacquer, varnish, synthetic resin, enamel or the like (which are preferred), an electrostat-ically deposited powder coating, or any other suitable type.
s Examples of siccative coatings which may be used are the acrylic, alkyd, epoxy, phenolic, melamine and polyvinyl alcohol resins and paints.
` 25 Application of a siccative coating composition can be effected by any of the ordinary techni~ues such as brushing, spraying, dipping, roller-coating, flow-coating, electrostatic or electrophoretic attraction, etc. I The coated article is dried in the manner best suited for the siccative coating composition employed, e.g., by air-drylng at ambient or elevated temperature, baking in an oven, ~ ` ` ' ~ ' ,:,, - .
~ 7 ~,:,: ; : , ~0~6~2 or baking under infra-red lamps. In most instances, the thickness of the dried ~ilm of the siccative organic coating composition will be about 0.1 10.0 mils, more often about 0.3-5.0 mils.
From the above description, it will be apparent that the advantages of this invention include the avoidance of hexavalent chromium as a pollutant in the process effluent, lower energy requirement for formation of a con-version coating, improved paint adhesion of the treated metal, and improved corrosion resistance, especially of the painted metal surface.
It is within the scope of this invention to include in the trivalent chromium solution a minor amount of a sur~actant, usually a non-ionic surfactant. When the sur-factant is present, the chromium solution mà~ be used as a cleaner as well as a conversion coating~solution, thus ' eliminating the initial chemical cleaning step described hereinabove.
The effectiveness of the method of this invention for forming conversion coatings on ferrous metal surfaces ., .
is demonstrated by a procedure in which panels of cold-rolled steel are cleaned by spraying with a sodium phosphate-based cleaner at 66-72C. and rinsed with water at room temperature. They are then sprayed at 18-22C. with a solution of Example 5. The treated panels are rinsed again with water, dried under infrared lamps and painted with a while alkyd-melamine baking enamel. Six sets of test panels are processed at the end of consecutive half-days of working of the spray line.
A set of control panels is similarly cleaned and ~ rinsed, treated at 50-60C. with a commercial chlorate- ;
`?i ~
x , : :
:~; 8 i9~2 accelerated iron phosphate conversion coating solution having a p~l of 5-~, and rinsed with water and with a commercial hexavalent-trivalent chromium post-rinse solu-tion. These panels are also painted as described above.
The paint film on each panel is ruptured down to -~ the bare metal by scoring a six-inch line on the surface of the panel. The scored panel is placed in a cabinet containing a 5% aqueous sodium chloride solution at 95F.
Air is bubbled through the solution to produce a corrosive salt atmosphere which acts on the surface of the test panel suspended above the level of the salt solution. The panels remain in this atmosphere for 120 hours after which they are removed, washed with water and dried with a cloth. A
pressure-sensitive tape is then applied to each panel and removed suddenly. This ~rocedure is repeated until no more paint can be removed in this manner. The loss o~ adhesion . , .
; caused by corrosion from the scribed line is measured in thirty-seconds of an inch. -~
' When measured in this manner, the average loss of adhesion for the panels treated by the method of this inven-tion is 0.7S thirty-seconds of an inch. The average for the`
control panels is 0.~4 thirty-seconds of an inch. Thus, the method of this invention gives results when the chromium olution is applied at room temperature or below, which are ~' ~5 comparable to those provided by a commercial iron phosphatesolution applied at elevate~ temperatures and ~ollowed by a commexcial hexavalent chromium-containing rinse.
The trivalent chromium solutions used in the method of this invention may also be used as post-rinses for phosphated metals, especially ferrous metals. The phosphating treatment used prior to this post-rinse may he ~'', :~ ~
~ 9 ~ ' ', 1~46912 iron phosphate, zinc phosphate, calcium- or magnesium-modified zinc phosphate, or the like.
... ... .
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~. :, -: . . .. ~
Claims (24)
1. A method for producing a conversion coating on a ferrous metal or galvanized article which comprises contacting said article with an aqueous solution of a trivalent chromium compound.
2. A method according to claim 1 wherein the aqueous solution is obtained by reduction of an aqueous hexavalent chromium-containing solution.
3. A method according to claim 2 wherein the reducing agent for the hexavalent chromium is an alkali metal sulfite or bisulfite.
4. A method according to claim 3 wherein the amount of sulfite or bisulfite used for reduction is less than about a 1% excess, by weight, over the stoichiometric amount required for reduction of hexavalent to trivalent chromium.
5. A method according to claim 4 wherein the metal article is rinsed with water after being contacted with said aqueous solution.
6. A method according to claim 5 wherein the temperature of application of said aqueous solution to said article is about 10-30°C.
7. A method according to claim 4 wherein the aqueous solution contains about 0.01-0.2% chromium by weight.
8. A method according to claim 7 wherein the pH
of the aqueous solution is about 3.5-6Ø
of the aqueous solution is about 3.5-6Ø
9. A method according to claim 8 wherein the reducing agent is sodium or potassium bisulfite.
10. A method according to claim 9 wherein the metal article is rinsed with water after being contacted with said aqueous solution.
11. A method according to claim 10 wherein the temperature of application of said aqueous solution to said article is about 10-30°C.
12. A metal article having on its surface a conversion coating produced by contacting said article with an aqueous solution of a trivalent chromium compound.
13. A metal article having on its surface a conversion coating produced by the method of claim 2.
14. A ferrous metal article having on its surface a conversion coating produced by the method of claim 2.
15. A ferrous metal article having on its surface a conversion coating produced by the method of claim 5.
16. A ferrous metal article having on its surface a conversion coating produced by the method of claim 9.
17. A ferrous metal article having on its surface a conversion coating produced by the method of claim 10.
18. A metal article according to claim 12 which has been further coated with an organic coating.
19. A metal article according to claim 12 which has been further coated with a siccative organic coating.
20. A metal article according to claim 13 which has been further coated with a siccative organic coating.
21. A ferrous metal article according to claim 14 which has been further coated with a siccative organic coating.
22. A ferrous metal article according to claim 15 which has been further coated with a siccative organic coating.
23. A ferrous metal article according to claim 16 which has been further coated with a siccative organic coating.
24. A ferrous metal article according to claim 17 which has been further coated with a siccative organic coating.
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US47969774A | 1974-06-17 | 1974-06-17 | |
| US50389174A | 1974-09-06 | 1974-09-06 | |
| US55489675A | 1975-03-03 | 1975-03-03 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1046912A true CA1046912A (en) | 1979-01-23 |
Family
ID=27413496
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA228,380A Expired CA1046912A (en) | 1974-06-17 | 1975-06-03 | Treatment of metal surfaces with trivalent chromium solutions |
Country Status (5)
| Country | Link |
|---|---|
| JP (1) | JPS5123445A (en) |
| CA (1) | CA1046912A (en) |
| DE (1) | DE2526832A1 (en) |
| FR (1) | FR2274706A1 (en) |
| GB (1) | GB1461244A (en) |
Families Citing this family (19)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP3392008B2 (en) | 1996-10-30 | 2003-03-31 | 日本表面化学株式会社 | Metal protective film forming treatment agent and treatment method |
| US4171231A (en) * | 1978-04-27 | 1979-10-16 | R. O. Hull & Company, Inc. | Coating solutions of trivalent chromium for coating zinc surfaces |
| US4263059A (en) * | 1979-12-21 | 1981-04-21 | Rohco, Inc. | Coating solutions of trivalent chromium for coating zinc and cadmium surfaces |
| CA1228000A (en) * | 1981-04-16 | 1987-10-13 | David E. Crotty | Chromium appearance passivate solution and process |
| FR2520758A1 (en) * | 1982-01-29 | 1983-08-05 | Produits Ind Cie Fse | COMPOSITION AND METHOD FOR THE TREATMENT OF PHOSPHATE METAL SURFACES |
| DE3429532A1 (en) * | 1984-08-10 | 1986-02-20 | Gerhard Collardin GmbH, 5000 Köln | METHOD FOR PASSIVATING LEAD AND LEAD-BASED SURFACES |
| US5415702A (en) * | 1993-09-02 | 1995-05-16 | Mcgean-Rohco, Inc. | Black chromium-containing conversion coatings on zinc-nickel and zinc-iron alloys |
| US5393353A (en) * | 1993-09-16 | 1995-02-28 | Mcgean-Rohco, Inc. | Chromium-free black zinc-nickel alloy surfaces |
| US5393354A (en) * | 1993-10-07 | 1995-02-28 | Mcgean-Rohco, Inc. | Iridescent chromium coatings and method |
| US7314671B1 (en) | 1996-04-19 | 2008-01-01 | Surtec International Gmbh | Chromium(VI)-free conversion layer and method for producing it |
| DE19615664A1 (en) * | 1996-04-19 | 1997-10-23 | Surtec Produkte Und Systeme Fu | Chromium (VI) free chromate layer and process for its production |
| US6190464B1 (en) | 1998-09-24 | 2001-02-20 | Nisshin Steel Co., Ltd. | Chromating solution and chromated metal sheet |
| US7101469B2 (en) | 2004-11-10 | 2006-09-05 | Atotech Deutschland Gmbh | Metal pieces and articles having improved corrosion resistance |
| EP3569734A1 (en) | 2018-05-18 | 2019-11-20 | Henkel AG & Co. KGaA | Passivation composition based on trivalent chromium |
| EP3663435B1 (en) * | 2018-12-05 | 2024-03-13 | Henkel AG & Co. KGaA | Passivation composition based on mixtures of phosphoric and phosphonic acids |
| WO2021139955A1 (en) | 2020-01-06 | 2021-07-15 | Henkel Ag & Co. Kgaa | Passivation composition suitable for inner surfaces of zinc coated steel tanks storing hydrocarbons |
| CN112663037B (en) * | 2020-12-04 | 2023-02-03 | 佛山市顺德区美硕金属表面技术有限公司 | Vitrification agent, preparation method and application thereof |
| EP4274865A1 (en) | 2021-01-06 | 2023-11-15 | Henkel AG & Co. KGaA | Improved cr(iii)-based passivation for zinc-aluminum coated steel |
| GB2603194A (en) | 2021-02-01 | 2022-08-03 | Henkel Ag & Co Kgaa | Improved cr(iii) based dry-in-place coating composition for zinc coated steel |
-
1975
- 1975-05-12 GB GB1989075A patent/GB1461244A/en not_active Expired
- 1975-06-03 CA CA228,380A patent/CA1046912A/en not_active Expired
- 1975-06-13 FR FR7518549A patent/FR2274706A1/en active Granted
- 1975-06-16 DE DE19752526832 patent/DE2526832A1/en active Pending
- 1975-06-16 JP JP7210775A patent/JPS5123445A/en active Pending
Also Published As
| Publication number | Publication date |
|---|---|
| JPS5123445A (en) | 1976-02-25 |
| DE2526832A1 (en) | 1976-01-02 |
| GB1461244A (en) | 1977-01-13 |
| FR2274706A1 (en) | 1976-01-09 |
| FR2274706B1 (en) | 1979-06-15 |
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