CA1117894A

CA1117894A - Production of multiple zinc-containing coatings

Info

Publication number: CA1117894A
Application number: CA000294313A
Authority: CA
Inventors: Richard J. Clauss
Original assignee: Oxy Metal Industries Corp
Current assignee: Oxy Metal Industries Corp
Priority date: 1977-01-13
Filing date: 1978-01-04
Publication date: 1982-02-09
Also published as: JPS5389835A; DE2800258C2; FR2377460B1; JPS5756557B2; FR2377460A1; DE2800258A1; GB1567101A

Abstract

ABSTRACT OF THE DISCLOSURE

Markedly superior corrosion protective properties are imparted to metal-bearing surfaces at substantially reduced costs by depositing upon said surfaces one or more superimposed zinc containing layers located intermediate the metal bearing surfaces and a sacrificial layer of generally pure zinc. The initial layer which is in direct contact with the metal bearing surface includes therein from about 3 to about 15% of an alloy-ing element selected from the group consisting of nickel, iron or cobalt and a second layer deposited upon the initial layer having a relatively lower content of the alloying element as compared with the initial layer. In this manner penetration of rust inducing substances into the metal bearing surfaces is effectively precluded under substantially all environmental conditions normally encountered.

Description

~34 BACKGROUND OF TH~ INVENTION
It is known in the art to which this invention per-tains to provide upon steel, iron or related metal-bearing surfaces an electrodeposited essentially pure zinc coating for corrosion protection purposes. It is conventional in the elec-trochemical deposition of a zinc coating upon a steel substrate, subsequent to cleaning, rinsing and acid pickling, to plate from a cyanide zinc bath, and after rinsing, to provide upon the zinc plated surface a passive film in the form of a chrom-ate salt. This may of course take various forms, such as aclear coating which resembles nickel, or various textures and ; colors which assume the appearance of olive drab or colored zinc. However, experience has well indicated that under ex-posure to known salt spray and humidity environments, or to a locale in which there is a constant salt spray of high salt concentrations coupled with high humidity, the zinc layer under such conditions penetrates into the steel substrate, and rust or other undesirable conditions are manifest. It has been pro-posed in order to overcome these difficulties that the zinc ~O coating be increased in thickness, however, while a modest improvement has been noted in the area of corrosion protection, the final product obtained after relatively long exposure to an industrial or marine environment is generally not completely acceptable after lengthy service periods.
SUMMARY OF T~E INVENTION
It has now been discovered by applicant that substan-tial cost economies may be effected not only through material savings, and in accordance with one aspect of the invention additionally by a reduction in the processing steps, while at the same time markedly improving the resistance of the metal-bearing surface to corrosion-inducing substances by depositing upon the metal-bearing surface a plurality of layers of zinc B 1 ~

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and/or alloys thereof in which the alloying elements may be nickel, cobalt, or iron, the initial layer deposited on the metal-bearing surface having an alloy content not substantially greater than about 15% nickel, iron or cobalt, and the layer immediately thereto having an alloy content of from bétween about zero to markedly less than the maximum alloy content of said initial layer.
In accordance with the invention, there is provided an article of manufacture for utilization in an environment hav-ing present therein corrosion-inducing substances, which com-prises a substrate presenting a metal-bearing surface of steel or iron-based alloys, and a plurality of superimposed zinc-containing layers deposited upon said metal-bearing surface, the initial layer in direct contact with said metal-bearing surface including therein from about 3 to about 15% of an alloy-ing element selected from the group consisting of nickel, iron or cobalt and a second layer deposited upon said initial layer having a relatively lower content of said alloying element as compared with said initial layer.
These irnportant improvements over the prior art ap-proaches may be achieved by proceeding in the following manner.
In one embodiment thereof, the invention may be practiced by providing upon a steel, iron or related substrate presenting a metal-bearing surface a "duplex" type coating comprising on the substrate an initial layer of nickel-zinc, cobalt-zinc or iron-zinc which is followed by a conventional zinc deposit. As will be apparent from the more detailed description of the invention now to follow, the alloy layer may have a total thickness as low as about 0.05 mils and the conventional zinc deposit approxi-mately the same thickness. This is quite in contrast to the prior art in which a zinc deposit is solely utilized, having generally a greater thickness, and even then corrosion problems

- 2 -present themselves under normal environments to which the coat-ed substrate is exposed. As one explanation for the novel results achieved when the initial layer is a zinc alloy deposit, the alloy deposit may be less electrochemically corrod-ible than zinc alone, and accordingly, the zinc coating dis-solves preferentially to the zinc alloy layer, and thereby delays penetration of corrosion-inducing substances to the steel or iron substrate. To be more specific, a deposit which is essentially pure zinc or has a relatively high zinc content is anodic to both the zinc alloy which has a relatively lower zinc content and also to the substrate, such as steel or iron.
This appears to explain why the outermost layer or layers rela-tively high in zinc content corrode preferentially and thereby delay the penetration action to the substrate.
More specifically, in accordance with the invention, there is provided a method for the production of one or more zinc-containing coatings upon a substrate presenting a metal-bearing surface of steel or iron-based alloys, which comprises .
contacting said metal-bearing surface with an electroplating solution under conditions of relatively low or essentially zero agitation effective to deposit upon the metal-bearing surface an initial zinc-containing coating having therein about 3 to about 15% of an alloying element selected from the group con-sisting of nickel, iron or cobalt, and then contacting said coated metal-bearing surface with an electroplating solution under conditions of relatively high agitation effective to deposit upon said initial coating a second coating having a relatively lower content of said alloying element as compared with said initial coating.
The plating procedure described immediately herein-above does, however, require the use of two separate electro-plating solutions. This possible disadvantage in certain types l~q~

of installations may be overcome by the now to be described additional embodiment of ~his invention. This is accomplished by the deposition of alternate coatings having varying alloy compositions from a single solution by the novel technique of changing the agitation of the electrolyte solution. Stated briefly, a coating of relatively high nickel, cobalt or iron content alloyed with zinc is deposited followed by a coatin~ of relatively lower content of the above metals in the alloy, each of these being deposited from a sep~rate solution whereby the entire electrodeposition process is greatly simplified. In effect, the novel results of this invention are achieved by employing only one solution in contrast to two separate plating baths as earlier described. As will now be appreciated, many layers of different alloy compositions may be deposited in relatively close time sequence from a single solution by variable agitation. If desired, the uppermost layer may have its alloy content so precisely controlled that for practical purposes the final electrodeposit is essentially zinc. In this manner, it is possible for certain applications to entirely eliminate the overcoat of what is regarded in the art as an essentially pure layer~ In any event, regardless of the particular manner in which the novel concepts of this invention are practiced, it has been found that the initial layer electro-deposited on the metal~bearing surface should have an alloy content not substantially greater than about 15% of nickel, iron or cobalt, and that the layer immediate thereto should have an alloy content which varies from about zero to markedly less than the maximum alloy content of the initial layer. In this manner, the mentioned material and production ecomonies are obtained, and the possibility of corrosion penetration rendered substantially remote.

~1~7~4 It is also within the contemplation o~ this invention that improvements may be achieved in the resistance of a metal-bearing surface to corrosion-inducing substances by applying to the surface through spraying techniques one or more superimposed coatings containing zinc alloying elements located intermediate the metal-bearing surface and a specified layer of generally pure zinc, the latter of which may be deposited by spraying, paintlng or electroplating methods.
Preferred spraying techniques relating to the novel concepts of this invention will be disclosed in detail hereinafter.
DESCRIPTION OF THE PREFERRED EMBODIMENT
In order to more fully appreciate the named advances made by ap,olicant over the discussed prior art structures and procedures, there are set forth hereinbelow a number of zinc-_ 5 _ ~, ,, .f ~i,, 1117~4 U lO,G03 containing formulations, deposition methods, and test data Il results obtained when coated specimens were exposed to known salt spray and humidity environments and also two conditions ¦ in which there was a constant salt spray of high salt i concentrations coupled with high humidity. First, there was ¦ formulated a zinc sulfate solutlon which functioned both as a control in the production of prior art substantially pure Izinc coatings, and also as a superimposed electroplate upon ia zinc alloy deposit in which the alloying element W-fS either ¦nickel, iron or cobalt. This formulation is disclosed in the ¦example now to follow. I
¦ EXAMPLE I
¦ 1800 mils of a zinc sulfate solution was prepared jfrom a composition which contained 216 grams ZnSO4 H2O, 69 grams IH3BO3, 36 grams (NH4)2SO4, and 18 grams sodium gluconate. This solution was filtered after preparation using a small amount of filter aid and the pH was adjusted to 5.4. Air agitation was employed.
, EXAMPLE II
I A zinc-nickel solution was prepared having the following ¦composition j Znso4~H2o 130 g/l , NiC12~6H2O 153 g/l l ~13B03 15 g/l I (Nl~4)2S4 20 g/l ¦ The above solution was maintained at a temperature between ¦7S and 80~. and had a pH of about 4.5. Utilizing t]liS solution, ¦Ithe nickel content in the alloy obtai,ned was approximately 9,2%.
~INO agitation was employed.

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U l0, 03 ¦ EXAMPLE III
l ZnC12 65 g/l ¦ NiC12-6H2 120 g/l ¦ NH4C1 160 g/l S ¦ NaC2l~3O2 6 g/l ¦ This formulation was maintained during preparation at between 65 and 85F., and had a pH of between 5 and 5.5. The alloy deposit ob~ained therefrom had a nickel content of about 13.3%. Agitation was not employed.
'10 EXAMPLE IV
ZnSO4 H2O 30 g/l NiC12 6H2o 210 g/l (NH4)2S04 20 g/l I H3BO3 45 g/l This particular solution was maintained at a temperature of between 75 and 80F. and had a pH of about 5 to 5.5. The alloy deposit obtained therefrom had a nickel content of about 'i 19.8%. No agitation was used.
; To illustrate the invention further, 1800 mils of a I -zinc-iron solution was prepared with the following composition: ;
EXAMPLE V
FeC12 4H2O . ~ 126 R/l ZnC12 (774 ~/1) 21 ml H3BO3 69 gtl KCl 36 gjl sodium gluconate 18 g/l This solution was filtered using a small amount of ilter aid and the pH was adjusted to about 3.7 using lO~ NaO~
The solution temperature Wcls about 75 to 80~., and agitation ¦ was e~nploye~.
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U 1~,603 It has been stated earlier that the objectives of this . I invention can be achieved when the element alloyed with zinc ¦ is cobalt. An exemplary formulation was prepared having the . composition as follows:
EXA~IPLE VI
.. ZnS0~ H20 60 g/l CoC12 90 g/l H3B03 45 g/l . NH~Cl 20 g/l The solution temperature was maintained at 75 to 80P., the pH was about 2.5 to 3.0, and no agitation was used. I

The majority of the formulations set forth above, I
subsequent to electroplating upon steel panels having linear dimensions of approximately 4 inches by 6 inches, were subjected to neutral salt spray tests and also prolonged exposure to a corrosive environment such as is present at Kure Beach, North Carollna. In rating the results of these tésts, the procedure of ASTM Standard B537 was followed.
In preparation for the first series of salt spray tests, control panels were plated with the solution of Example I above ' -.for 10 minutes at. 3.5 amperes (about 25 ASF) with air agitation.
A second.sét of steel panels were first plated with the formulation of Example V, previously referred to, for 5 minutes at 3.5 amperes with air agitation, with rinsing, and thereafter plated for about 5 minutes at 3.5 amperes witIl air agitation from a solution of the type shown in Example I. Also in prep-. aration for neutral salt spray testing, a third set of steel panels l~sing the formu.lation of Example V was plated for approxi-mately 2 and 1/2 minutcs with no air agitation, 2 and 1/2 minutes`
with air agitation, 2 ancl 1/2 minutes with no air agitation, and 2 and 1/2 minutes Wit]l air agitation. This provided a multi-1~7t3~
U 10,G03 layer deposi~ with varying iron contents, and when air agita~
was used, the iron con~ent in the alloy was approximately 5 to 7~.
The results obtained with the specimens described above were electroplated and exposed to a 5% neutral salt spray 1, environment in accordance with ASTM Standard B117 are set forth below in Table A. It may be noted therefrom that the specimens were observed after periods o~ 24, 44 and 6~ hours, and the legend "WCP" refers to a white corrosion deposit, as is known 0 to the art. The specimens designated '~zinc plated control" had a thickness of about 0.2 mils, and the other specimens a tota]
I plated thickness of approximately 0.2 mils. ,, ¦ Other specimens were prepared substantially as above described, and the salt spray test results appear in Table B
below. In this table, the systems designated as zinc deposit employ the general formulation of ~xample I, and the multi-layer specimens used variations of compositions above set forth in Examples II and III. Blank spaces in the chart indicate that no observations were made at that particular time.
0 ¦ I~ may be seen from a review of the data presented in I Tables A and B that panels of steel or an iron-based alloy when ¦ electroplated in accordance with the novel concepts of this invention significantly out-performed panels which carried solely , a zinc coating of the same thickness. It is ~uite clear from ,5 I this that material economies are effected,and a superior product is obtained.
O~ even greater signi~icance is the comparative corrosion exposure data JIOW to be presented. All tests were performed at I a marine test site at Kure Beacll, Nortll Carolina, which is an ¦ environment charactcrized by constant salt spray o~ hi~h salt ¦ concentrations couplcd l~ith hi~h humidity.

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The formulations employed in preparation of panels used in corrosion exposure testing were basically the same as above discussed in connection with the neutral salt spray tests.
Referring first to Table C, it will be noted that the 0.1 mil zinc panels showed significant failure after about . 2 months exposure, and 0.3 mil zinc panels underwent slight : 10 ¦ deterioration after 2 months to the end of this particular test at 12 months. Had this period been extended therebeyond, complete failure could be anticipated. Quite by contrast, the duplex panels of this invention, the only formulation difference I therebetween being the nickel content, had a top rating up to I the end of the 8 month period, and then showed a failure reading.
Table D below sets forth marine corrosion expos~re test results over differing periods with alloy formulations varying somewhat over those of the last discussed Table. Again, it is ~uite clear from a study of this test data that panels prepared by applicant t S herein disclosed procedures and carrying thereon zinc alloy electroplated coatings out-performed the prior art simple zinc electroplates of the same or even greater thickness-es.
I Table ~ appearing hereinafter likewise shows the superior-ity of the present invention in an eleven month exposure test.
The 0.1 mil ZillC specimens were complete failures~ and even tripling the zinc coating gave a reading of only 3. On the I other hand, a multiple zinc-nickel coating using varying degrees I of ~gitation scored the highest possible rating o~ 10.
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(1.-) U 10,603 It has been pointed out hereinabove that by proceeding in accordance with the novel concepts of this invention there may be provided an ultimate article which includes as a sub-strate a steel or iron-based alloy having electroplated there-S upon one or more superimposed coatings containing zinc alloying elements located intermediate the base or substrate and a ¦ sacrificial layer of generally pure zinc. Of course, upon the latter layer there may be provided a passive film in the form of a chromate salt. In this manner markedly superior corrosion L0 protective properties are imparted to the base or substrate and penetration of rust inducing substances into the metal bearing surfaces is effectively precluded under substantially all environmental conditions normally encountered, and simul-l taneously substantial production economies are effective. It !5 is an important aspect of this invention that in the ultimate article produced there be provided a multi-layer deposit which contains two or more layers of zinc or zinc alloys in which the alloying elements are nickel, iron or cobalt, and also ln which in any two consecutive layers the initial layer has a higher alloy content than any subsequent coatings eLectrodeposit-¦ ed thereupon, and further, by this same invention the alloy ~I content of the initial layer doe.s not exceed more than about 15% of iron, cobalt or nickel. Experience to date has indicated that when the initial layer of the zinc alloy deposit, whether ,5 the alloying element be nickel, cobalt or iron, is substantially less than about 3% of the alloying element or is greater than ¦ approximately 15~ of the alloy;ng element, the important ¦ advantages of superior corrosion protection at reduced produc-¦ tion economies do not prevail. This is believed quite well O ¦ indicated when reference is made to Table D hereillabove, whici I portrays ~ha~ wllcn under corroslon exposure exceedillg abou~ 9 I .`
, 1¦ t-LG) U 10,603 j~
il l months and a nickel alloying content of greater than about 15.0% there was substantial deterioration of the specimens.
It is accordingly highly desirable for reasons of cost economies I and high corrosion protective resistance that the amount of ¦ the alloying element, whether it be nickel, iron or cobalt, be ~ithin the range of approximately 3 to 15%. .
. The novel results achieved above have been particularly directed to the utilization of electroplating methods. However, ¦ and as was earlier noted, it is also within the contemplation . ~ of this invention that improvements may be achieved in the I resistance of a metal-bearing surface to corrosion-inducing : ¦ substances by applying to the surface through spraying techniques .¦ one or more superimposed coatings containing zinc alloying elements located intermediate the metal-bearing surface and .5 a specified layer of generally pure zinc, the latter of which may be deposited by spraying, painting or electroplating.
methods. An effective technique which may be utilized is ¦ described in a publication entitled "Protective Coatings for ¦ Metal" authored by Burns and Bradley, published in 1955 by 0 ¦ Reinhold Publishing Co., and particularly in Chapter Four 51 bearing a title o "Sprayed Metal Coatings". Quite obviously, ii the process described in this publication has application with I the present invention, and by proceeding in accordance with - . ¦ the teachings therein, ready control may be effected as to the amount o the zinc alloying element to be used in the practice of this invention.
Various modifications have been discussed herein as I to th~ compositions and procedures of th~s invention, and ¦I changes and modi~i.cations therein may of course be eected ~ il without departin~ :fro]n thc spi.rit o t]lis invelltiotl or the : 1¦ scopé of the subjo:i.necl clainls.

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. An article of manufacture for utilization in an en-vironment having present therein corrosion-inducing substances, which comprises a substrate presenting a metal-bearing surface of steel or iron-based alloys, and a plurality of superimposed zinc-containing layers deposited upon said metal-bearing sur-face, the initial layer in direct contact with said metal-bearing surface including therein from about 3 to about 15%
of an alloying element selected from the group consisting of nickel, iron or cobalt and a second layer deposited upon said initial layer having a relatively lower content of said alloy-ing element as compared with said initial layer.

2. An article of manufacture as defined in Claim 1, in which there is provided upon the plurality of superimposed zinc-containing layers an uppermost coating of essentially pure zinc.

3. An article of manufacture as defined in Claim 1, in which there is provided upon the second layer of superimposed zinc-containing layers a passive film.

4. An article of manufacture for utilization in an en-vironment having present therein corrosion-inducing substances, which comprises a substrate presenting a metal-bearing surface of steel or iron-based alloys, and a plurality of layers of zinc and/or alloys thereof in which the alloying element is nickel, the initial layer deposited on the metal-bearing sur-face having a nickel content between about 4.0 to 14.0 percent, and the layer immediately adjacent thereto having a nickel content of from about zero to about 4.0 percent.

5. A multi-layered electroplated article of commerce having superior resistance to corrosion, comprising a steel or iron alloy base, a first electrodeposited coating of a zinc alloy on said base in which the alloying element is nickel, iron or cobalt present in an amount of about 3 to about 15%
and a second electrodeposited zinc-containing coating on said first coating and having present therein from about zero to less than the maximum alloy content of said first coating.

6. An article of manufacture as defined in Claim 1, in which the alloy content of the initial layer is from about 3.0 to approximately 15.0% nickel, iron or cobalt, and the alloy content of the second layer is essentially zero to pro-vide a substantially pure sacrificial zinc coating on said initial layer, thereby delaying penetration of corrosion-inducing substances to the substrate.

7. A method for the production of one or more zinc-containing layers upon a substrate presenting a metal-bearing surface of steel or iron-based alloys, which comprises deposit-ing on the metal-bearing surface in direct contact therewith, by electroplating techniques, an initial zinc-containing layer having therein about 3 to about 15% of an alloying element selected from the group consisting of nickel, iron or cobalt, and then depositing upon said initial layer a second layer having a relatively lower content of said alloying element as compared with said initial layer.

8. A method for the production of one or more zinc-con-taining coatings upon a substrate presenting a metal-bearing surface of steel or iron-based alloys, which comprises contact-ing said metal-bearing surface with an electroplating solution under conditions of relatively low or essentially zero agita-tion effective to deposit upon the metal-bearing surface an initial zinc-containing coating having therein about 3 to about 15% of an alloying element selected from the group consisting of nickel, iron or cobalt, and then contacting said coated metal-bearing surface with an electroplating solution under conditions of relatively high agitation effective to deposit upon said initial coating a second coating having a relatively lower content of said alloying element as compared with said initial coating.