Pretreatment of superalloys and stainless steels for electroplating

Process for activating surfaces of superalloys and stainless steels prior to electroplating a metal such as chromium thereon, whereby a metal deposit having excellent adhesion to the substrate is obtained. The article is activated by anodic etching in a novel activation solution containing about 55% to 80% by volume of H.sub.2 SO.sub.4 and about 1% to 10% by volume of commercial 52% hydrofluoric acid, preferably an aqueous bath containing 65% by volume of 95-98% H.sub.2 SO.sub.4 and 5% by volume of 52% hydrofluoric acid at 30 A/dm.sup.2 and 20.degree. C. for one minute.

BACKGROUND OF THE INVENTION 
The present invention relates to a process for pretreating readily 
passivated metals, notably stainless steels and superalloys, for 
electroplating metals, such as chromium, nickel and cobalt, to produce 
deposits of greatly improved adhesion to the substrate. 
The utility of superalloys in some defense oriented applications can be 
greatly improved by electroplating a coating of a metal, such as chromium, 
to change the surface properties of the alloy. However, superalloys, like 
stainless steels, are readily passivated by the formation of surface oxide 
films. Such films present special problems for electroplating, since they 
can reduce deposit adhesion, and special cleaning and processing is 
required to remove the films prior to plating, as is well known in the 
art. 
Various methods are known for activating stainless steels for 
electroplating. These methods are generally discussed in ASTM Designation 
B254-53, "Preparation of and Plating Stainless Steels", September (1964), 
and include (a) cathodic or anodic treatments in alkaline or acidic 
solutions. (b) immersion treatments and (c) simultaneous activation using 
a strike intermediate layer. A strike layer of intermediate metal such as 
nickel, cobalt, zinc and cadmium is commonly employed to protect the 
stainless steel after activation; and since the strike layer itself is 
applied by electroplating, at least two electroplating steps are required 
in the process. Further, the effectiveness of the activation and strike 
layer decreases as the chromium content of the stainless steel increases. 
Superalloys are alloys developed for very high temperature service where 
relatively high stresses are encountered and where oxidation resistance is 
frequently required (Metals Handbook 8th Edition Vol. 1, page 37). They 
are generally nickel, cobalt or iron-base alloys wherein the high 
temperature properties are derived through solid solution strengthening 
and hardening by precipitated metal carbide and gamma prime phases. 
Although superalloys are similar to stainless steels, it is not known 
whether the same activation methods are suitable for both. The few 
references on superalloys suggests the need for an intermediate strike 
layer in a viable activating process. However, the presence of the 
intermediate strike layer can exclude the electroplated alloy from service 
in high temperature environments. 
It is an object of the present invention to provide a method for 
pretreating superalloys and stainless steels which overcomes the 
aforementioned problems and eliminates the need for an intermediate strike 
layer.

SUMMARY AND DETAILED DESCRIPTION OF THE INVENTION 
In accordance with the process of the present invention the surface of a 
stainless steel or superalloy article prior to electroplating is activated 
by anodic etching in a novel activation solution containing about from 55% 
to 80% by volume of 95-98% sulfuric acid and about 1 to 10% by volume of 
52% hydrofluoric acid. When the activated surface is electroplated in 
conventional manner with a metal such as chromium, the metal deposit 
obtained thereby without the use of an intermediate metal strike possesses 
greatly improved adhesion to the substrate. In similar manner, by 
utilizing the novel activation treatment of the present invention, other 
metals such as nickel, cobalt, copper and iron can be electroplated in 
conventional manner without the use of an intermediate metal strike to 
produce metal deposits characterized by excellent adhesion to the 
substrate. 
The present invention is particularly suitable for activating nickel, 
cobalt and iron-base superalloys and stainless steels. 
In carrying out the process of the present invention the stainless steel or 
superalloy article surface is submitted to anodic etching in the novel 
activation bath, i.e. by employing the article as the anode and using an 
inert cathode, such as lead or platinum. The article is preferably 
electroplated directly following the activation treatment, since it has 
been found that an intermediate rinse with water before electroplating the 
substrate with a metal, such as chromium, tends to decrease the adhesion 
of the metal deposit to the substrate. Also, it has been found that 
little, if any, improvement of adhesion of the electroplated metal to the 
substrate results when the article is submitted to cathodic etching in the 
novel activation solution under otherwise similar conditions. 
The aqueous activation solutions employed in the process of the present 
invention contains about from 55% to 80% by volume of 95-98% H.sub.2 
SO.sub.4 (d. 1.84) and about 1% to 10% by volume of 52% by hydrofluoric 
acid of commerce, and preferably about 65% by volume of 95-98% H.sub.2 
SO.sub.4 and about 3 to 7% by volume of 52% hydrofluoric acid. Activation 
baths containing substantially more than 80 volume % of 95-98% H.sub.2 
SO.sub.4 promote excellent adhesion of electroplated metal deposits; but 
they are less preferred, since they are more costly and possess lower 
conductivity and higher viscosity. When the amount of 52% hydrofluoro acid 
in the activation solutions is increased substantially above 10% by volume 
of reduced substantially below 1% by volume, the electroplated metal 
deposits obtained possess considerably inferior adhesion to the substrate. 
Also, as shown in examples 10 and 11 below, aqueous solutions containing 
substantially lower concentrations of sulfuric acid, e.g. 30% by volume of 
95-98% sulfuric acid, in mixture with 5% by volume of commercial 52% 
hydrofluoric acid, are essentially ineffective for activation and 
improving the adhesion of metal deposits electroplated on superalloy 
substrates. 
The activation treatment can be carried out at a current density of at 
least about 20 A/dm.sup.2, preferably between about 30 and 120 A/dm.sup.2, 
and at a temperature preferably between room temperature and about 
50.degree. C. The duration of the activation treatment can vary widely. 
Excellent results can be obtained by treating the surface of the article 
in the activation solution for a period of 30 to 120 seconds. 
The following examples illustrate specific embodiments of the method of 
carrying out the process of the present invention as well as other 
methods. 
ACTIVATION PROCEDURE 
Cylindrical samples, each one-inch long and three-quarters inch in 
diameter, were machined from GC-27, HS-25 and U-700 bar stock. (See Table 
1 for compositions of these alloys) Each sample was secured in a parallel 
bar holder for grinding an end face of the cylindrical sample, the use of 
a parallel bar holder insuring that the sample can be removed and 
repositioned without changing surface alignment. After grinding the end 
face, the circumferential area of each sample was masked with heat 
shrinkable polypropylene tubing to expose only the flat end face for 
plating. No precleaning procedure was used prior to the surface activation 
treatment as the sample was used immediately following surface grinding. 
The samples thus prepared were activated using the acid etching solutions 
and methods described in Table 2. 
PLATING PROCEDURE 
The plating solution employed was prepared by dissolving 250 grams/liter of 
CrO.sub.3 (chromic acid) and 2.5 grams/liter of 95-98% sulfuric acid is 
distilled water. The samples obtained by the activation procedure 
described above were plated in the solution in a two liter beaker, using a 
platinum anode and rotation of the cathode (sample) with no other 
mechanical agitation. High contraction (HC) chromium was plated at a 
current density of 31 amperes per square decimeter (A/dm.sup.2) and a 
solution temperature of 55.degree..+-.1.degree. C., while low contraction 
(LC) chromium was plated at a current density of 150 A/dm.sup.2 and at a 
temperature of 85.degree..+-.1.degree. C. 
ADHESION TESTS 
The following procedure was employed to evaluate the HC and LC chromium 
deposits on the superalloy substrates obtained in the foregoing examples. 
The sample with chromium deposited to a thickness of 75.mu. was returned to 
the parallel bar holder and secured, after which parallel grooves were cut 
across the plated surface to a depth just below the interface using a 
small metal shaper equipped with a carbide tool. The degree of adhesion 
was evaluated with the aid of a low power optical microscope. The optimum 
conditions for the adhesion test were determined by studying chromium 
deposits of various thicknesses (25 to 150 m), grooves of various depths 
(0.125 to 1 mm from the interface), distance of groove separation (0.75 to 
2.5 mm) and different speeds of the cutting tool. The optimum results were 
obtained by employing a 75.mu. thick chromium deposit with the parallel 
grooves cut to a depth of 175.mu. from the interface at a separation of 1 
mm and using a tool speed of about 7.5 cm/sec. 
The foregoing test is sufficiently sensitive to distinguish the following 
four classes of adhesion: 
Excellent: fracture occurs only within the chromium layer or the coating on 
the land remains undamaged. 
Good: fracture occurs predominantly in the chromium layer. 
Poor: fracture occurs predominantly at the interface. 
No adhesion: coating detaches completely at the interface or spalls during 
plating. 
Table 2 sets forth an evaluation of the adhesion of chromium coatings 
electroplated on the superalloys after various activation treatments. The 
results show that the activation process of the present invention is the 
only method which produced a significant improvement in the adhesion of 
the chromium deposit. 
Example 1 shows the adhesion obtained on a freshly ground superalloy 
surface without an activation pretreatment. 
Example 2 shows a process described in page 203, Electroplating Engineering 
Handbook, Third Edition, A. K. Graham Ed., Van Nostrand Reinhold Co. 
(1971). This is the only process found in a literature search which does 
not employ strike plating in the activation step. This treatment consists 
in the following order: a one minute immersion in a 20% hydrochloric acid 
solution, a water rinse, a one-half minute immersion in a 67% nitric acid 
solution, a water rinse, a 1/2 minute immersion in the chromic acid 
plating solution followed by plating in the same solution. 
Example 3 shows cathodic pretreatment in a conventional chromic acid 
plating solution, i.e., the alloy sample is the cathode. 
Example 4 shows anodic pretreatment in the chromic acid plating solution, 
wherein the alloy sample is the anode. 
Example 5-9 show anodic and cathodic pretreatments in 30%, 65% and 
concentrated sulfuric acid solutions. 
Examples 10 and 11 show anodic and cathodic pretreatments in an aqueous 
solution containing 30% by volume of 95-98% H.sub.2 SO.sub.4 and 5% by 
volume of commercial 52% hydrofluoric acid. This solution is recommended 
for use on stainless steels (Metal Finishing Handbook, 77 (13), January 
1979) and is said to be excellent for the simultaneous activation of 
landing gear steel and nickel plated surfaces (G. T. Sink, Plating and 
Surface Finishing, 63 (8), 20 (1976)). The adhesion of the chromium plate 
obtained on all three superalloys using both anodic and cathodic 
pretreatments in this H.sub.2 SO.sub.4 -HF solution with one exception was 
"poor" or "no adhesion." 
Example 12 illustrates the activation process of the present invention. It 
shows anodic pretreatment in an aqueous solution containing 65% by volume 
of concentrated sulfuric acid and 5% by volume of commercial hydrofluoric 
acid (650 ml. 95-98% H.sub.2 SO.sub.4 d. 1.84 and 50 ml commercial 52% 
hydrofluoric acid diluted to one liter with distilled water). Excellent 
adhesion of metal deposits were thereby obtained on all three superalloys. 
Table 3 shows the effects of hydrofluoric acid concentration and etching 
time on the adhesion of chromium deposits on U-700 alloy. In these 
examples the amount of 52% hydrofluoric acid was varied from 1.25 to 7 
percent by volume and the etching time was varied from 15 to 120 seconds 
while the amount of concentrated (95-98%) H.sub.2 SO.sub.4 was maintained 
at 65% by volume. Etching was conducted anodically at a current density of 
31 A/dm.sup.2. Except for the inconsistent results obtained with the 15 
second etch, excellent adhesion of the chromium deposit was obtained when 
longer etching times were employed. 
The process of the present invention can also be utilized for stainless 
steels. For example the activation solution and procedure described in 
Example 12 was employed for plating chromium by the aforementioned 
procedure on 304, 316 and 410 stainless steels (see Table 1 for the 
compositions of these stainless steels). The adhesion of the chromium 
deposits thus produced on each of these substrates was found to be 
excellent, as measured by the aforementioned adhesion test. 
The foregoing disclosure is merely illustrative of the principles of this 
invention and is not to be interpreted in a limiting sense. We wish it to 
be understood that we do not desire to be limited to the exact details 
described because obvious modifications will occur to a person skilled in 
the art. 
TABLE 1 
__________________________________________________________________________ 
PRIMARY COMPOSITION OF SELECTED 
SUPERALLOYS AND STAINLESS STEELS 
Ni Co Fe Cr Mo Mn Ti 
Al 
W C Si 
P S 
__________________________________________________________________________ 
U-700 
Bal. 18.5 
0.5 
15 5.0 
-- 3.5 
4.4 
-- 
0.07 
-- 
-- -- 
HS-25 
10 Bal. 
3.0 
20 -- 1.5 
-- 
-- 
15 
0.10 
0.5 
-- -- 
CG-27 
38 -- Bal. 
13 5.5 
0.1 
2.5 
1.5 
-- 
0.05 
0.1 
-- -- 
304 8-10.5 
-- Bal. 
18-20 
-- 2.0 
-- 
-- 
-- 
0.08 
1.0 
0.045 
0.030 
316 10-14 
-- Bal. 
16-18 
2-3 
2.0 
-- 
-- 
-- 
0.08 
1.0 
0.045 
0.030 
410 -- -- Bal. 
12 -- -- -- 
-- 
-- 
0.02 
0.8 
0.006 
0.025 
__________________________________________________________________________ 
TABLE 2 
__________________________________________________________________________ 
ADHESION OF HC CHROMIUM ON SUPERALLOYS 
Adhesion 
Example 
Activation (Etching) Solution 
Pretreatment Condition 
CG-27 U-700 HS-25 
__________________________________________________________________________ 
1 None Freshly Ground Surface 
Poor Poor Poor 
2 20% HCl, 67% HNO.sub.3, 250/ 
Immersion, water rinse 
Excellent 
Poor Poor 
2.5 g/l; CrO.sub.3 /H.sub.2 SO.sub.4 
1 min., 1/2 min., 1/2 min. 
3 250/2.5 g/l; CrO.sub.3 /H.sub.2 SO.sub.4 
Cathodic, 2.4 v, 15 min. 
Good No adhesion 
No adhesion 
4 250/2.5 g/P; CrO.sub.3 /H.sub.2 SO.sub.4 
Anodic, 31 A/dm.sup.2, 2 min. 
Good No adhesion 
No adhesion 
5 30% H.sub.2 SO.sub.4 
Anodic, 31 A/dm.sup.2, 1 min. 
No adhesion 
Poor No adhesion 
6 65% H.sub.2 SO.sub.4 
Anodic, 31 A/dm.sup.2, 1 min. 
Poor Poor Poor 
7 65% H.sub.2 SO.sub.4 
Anodic, 31 A/dm.sup.2, 2 min. 
Good Poor Poor 
8 Conc. H.sub.2 SO.sub.4 (Sp. gr. 1.84) 
Anodic, 31 A/dm.sup.2, 1 min. 
No adhesion 
Poor Poor 
9 Conc. H.sub.2 SO.sub.4 (Sp. gr. 1.84) 
Cathodic, 31 A/dm.sup. 2, 1 min. 
No adhesion 
Poor No adhesion 
10 30% H.sub.2 SO.sub.4 + 5% HF 
Anodic, 31 A/dm.sup.2, 2 min. 
Excellent 
Poor Poor 
11 30% H.sub.2 SO.sub.4 + 5% HF 
Cathodic, 31 A/dm.sup.2, 2 min. 
No adhesion 
No adhesion 
No adhesion 
12 65% H.sub.2 SO.sub.4 + 5% HF 
Anodic, 31 A/dm.sup.2, 1 min. 
Excellent 
Excellent 
Excellent 
__________________________________________________________________________ 
TABLE 3 
______________________________________ 
ADHESION OF HC CHROMIUM ON U-700 AS 
A FUNCTION OF ETCHING TIME AND HYDROFLUORIC 
ACID CONCENTRATION* 
Time (sec) 
HR (%) 15 30 60 120 
______________________________________ 
7 Good Ex Ex -- 
5 Poor Ex Ex Ex 
2.5 Ex Ex Ex Ex 
1.25 Poor Ex Ex -- 
______________________________________ 
*All solutions contain 65% sulfuric acid. Etching was conducted anodicall 
at a current density of 31 A/dm.sup.2.