Process for forming adherent chromium electrodeposits from high energy efficient bath on ferrous metal substrates

An improved activation solution is provided for forming adherent chromium electrodeposits on metal substrates from high energy efficient chromium baths. The solution is a substantially neutral alkali metal sulfate solution.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to electrodeposition of chromium on basis metals, 
and, more particularly, it is concerned with an activation process by 
which an adherent chromium electrodeposit is formed on ferrous metal 
substrates using a high energy efficient chromium plating bath. 
2. Description of the Prior Art 
The use of high efficiency chromium plating processes has been hampered by 
the inability to obtain adequate coating adhesion to certain basis metals. 
Chromium plating baths containing halides have exhibited adhesion problems. 
Such baths are of the types disclosed in Mitsui, J7B-33941 (September, 
1978); Dillenberg U.S. Pat. No. 4,093,522; Perakh et al., U.S. Pat. No. 
4,234,396; and Chessin, U.S. Pat. Nos. 4,450,050 and 4,472,249. 
The problem has been attributed to the presence of a halide which may 
interfere at the initiation of deposition. 
In order to obtain an adequate bond as measured by ASTM B 571-79, with 
typical chromium plating solutions, such as those using a solution of 
chromic acid and catalysts such as sulfate or sulfate in combination with 
various fluorides, the usual technique is to reverse or anodically etch a 
ferrous workpiece in the plating solution or in a separate chromium acid 
containing solution at a predetermined current density for a predetermined 
time. 
A table which lists the time lengths for such an etching process is found 
in "Metal Finishing" 80 (5) 65-8 (1982) by C. H. Peger. The use of certain 
sulfuric acid and hydrofluoric acid etches for specific stainless steels 
is also suggested in the publication. 
Anodic chromic acid treatments for 400 stainless steel alloys and for low 
and high carbon steels is disclosed in "48th Metal Finishing 
Guidebook-Directory" 78, 188-202 (1980) by A. Logozzo. Also recommended 
are cathodic treatments in sulfuric-fluoride solutions for 300 stainless, 
for nickel alloys and for cast iron. 
A further alternative which is found in the prior art is the use of a Woods 
nickel strike for certain special nickel and cobalt-based alloys. (Boeing 
Aircraft) BAC5709-5.2j(1). 
The use of a ferric chloride-hydrochloric acid solution as a clearing agent 
for the smut produced by anodic sulfuric etches is disclosed at page 137 
of "Hard Chromium Plating" Robert Draper Ltd., Teddington, 1964 by J. D. 
Greenwood. 
Brune and McEnally in "Plating"42, 1127-32 (1955) describe the use of a 
magnesium sulfate sulfuric acid anodic etch solution for preparing ferrous 
parts for plating. Similarly, ASTM Specification B-242-49T suggests the 
application of an anodic etch using a sulfuric acid solution containing 
sodium sulfate. ASTM B177-68 described the use of sulfuric acid or chromic 
acid as activators for chromium electroplating on steel for engineering 
use. 
Chessin in U.S. Pat. No. 4,450,050 described an activation pretreatment for 
bonding high efficiency chromium electrodeposits on a metal substrate 
which is characterized by the step of first plating the substrate metal 
with iron or an iron alloy from an iron salt containing bath. 
Hermann, in U.S. Pat. No. 4,416,758, activates metal substrates in an 
aqueous alkaline cyanide containing solution using current which is 
periodically reversed, followed by rinsing and chromium plating. 
It has been found that when these procedures are employed with the halide 
containing chromium plating baths, most of the ferrous metal substrates 
are not adequately plated in that the chromium deposits from these baths 
have inadequate adhesion. 
It can be speculated that the reducing conditions at the cathode at the 
initiation of deposition cause the halide ion to be reduced to a form 
which interferes with the molecular bonding of the chromium to the 
substrate. In any event the use of high efficiency chromium plating is 
limited by the problem of inadequate adhesion. 
Accordingly, it is an object of this invention to provide an improved 
process for forming adherent chromium electrodeposits, particularly from 
high energy efficient baths, on ferrous metal substrates. 
Another object herein is to provide an activation solution for pretreating 
a ferrous metal substrate before electrodepositing chromium thereon, which 
solution does not require the use of strong acids such as sulfuric acid, 
which would effect the ferrous metal substrates. 
Still another object is to provide an activation solution for 
electrolytically, preferably anodically, etching a ferrous metal substrate 
in preparation for electrodepositing an adherent chromium metal deposit 
thereon from a high energy efficient bath, where the degree of adherence 
of the chromium deposit is relatively independent of the transit time 
between activation and rinsing. 
Yet another object herein is to provide adherent and smooth chromium 
deposits on a ferrous metal substrate from a high energy efficient 
chromium electroplating bath. 
SUMMARY OF THE INVENTION 
It has now been found that the adherence problems encountered with the 
prior art systems for electrodepositing chromium on metal substrates from 
high energy baths can be overcome through the use of a process in which a 
metal substrate is subjected to an activation step in an activation 
solution which is a substantially neutral (pH 5-10) alkali metal sulfate 
solution. The activation solution does not require sulfuric or chromic 
acids to perform its activating function. 
In accordance with the present invention an adherent and smooth chromium 
electrodeposit is obtained on a metal substrate by means of a process 
which comprises the steps of electrolytically etching the metal substrate 
in said activation solution, and thereafter electrodepositing chromium 
metal thereon. 
The process herein is particularly useful in that it is capable of forming 
adherent chromium electrodeposits from a high energy efficient chromium 
plating bath, that is, one which includes a halide ion therein. As an 
additional feature of the invention, the degree of adherence is 
substantially independent of the transit period between activation and 
rinsing the metal substrate, in advance of the actual electroplating of 
chromium thereon. 
DETAILED DESCRIPTION OF THE INVENTION 
While the process of the invention can be utilized for electrodepositing 
chromium from any chromium bath, it can be used advantageously for forming 
adherent chromium on a metal substrate from a high energy efficient 
chromium bath which includes a halide ion as an essential constituent. The 
presence of the halide ion can cause problems with adherence of the 
chromium deposit unless the metal substrate is given the pretreatment of 
the invention. Such high energy efficient baths are described in Chessin, 
U.S. Pat. No. 4,472,249. A prior art less advantageous pretreatment for 
bonding such high efficiency chromium electrodeposits is described in 
Chessin et al., U.S. Pat. No. 4,450,050. 
The process of the present invention comprises subjecting the metal 
substrate to electrolytic, preferably anodic, etching in a substantially 
neutral (pH 5-10) solution of an alkali metal sulfate, and 
electrodepositing chromium thereon. 
The electrolytic etching step is carried out for about 10 seconds to 10 
minutes at a current density of about 0.1 asi to 10 asi, and at a 
temperature from about room temperature to 50.degree. C. 
After the activation step the current is turned off and the activated metal 
substrate is transferred to a rinsing vessel where it is rinsed free of 
the activation solution. Then the metal substrate is placed in the 
chromium electroplating bath and chromium metal is deposited thereon. 
Optionally, a reverse (i.e. the workpiece is made the anode) in a chromic 
acid-containing solution, for example, the chromium plating solution 
itself, may be used preceding the actual chromium plating operation. 
The activation solution, while free of added sulfuric acid or chromic acid, 
may contain one or more of the following additives: a buffer, such as 
borax, to maintain the pH of the solution at the desired level; an 
oxidizing salt, such as sodium molybdate or sodium chromate, for faster 
polarization; an attack metal compound, such as sodium chloride or sodium 
nitrate, to aid in attacking the surface of the substrate; a complexor 
such as citrate or gluconate, to stabilize the products of the 
electrolytic reaction; and a surfactant to reduce surface tension between 
solution and substrate. 
Typical metal substrates include ferrous metals such as cast irons and 
alloy and high carbon steels.

The invention will now be described with reference to the following 
examples, which are not to be construed as limiting of the invention. 
EXAMPLE 1 
An activation solution was prepared from 45 g/l of sodium sulfate and 20 
g/l of sodium molybdate hexahydrate. The pH was 7. A cast iron substrate 
was placed in the solution at room temperature and the substrate was 
anodically etched at 0.5 asi for 30 seconds. After transfer to a rinsing 
bath, the activated metal was chromium plated in a high energy efficient 
bath containing iodide ion at 5 asi for 60 minutes. The chromium deposit 
exhibited excellent adherence to the substrate and was smooth. 
A similar process in the absence of application of any reverse activation 
current provided only very poor adhesion of chromium on the cast iron 
metal substrate. 
EXAMPLE 2 
The activation solution consisted of 120 g/l sodium sulfate and 40 g/l of 
sodium nitrate. The pH was 6. The substrate was strut steel, 1024, an 
induction hardened steel. Activation was carried out at 23.degree. C. at 1 
asi for 3 minutes, followed by rinsing and high energy chromium reverse at 
5 asi for 3 minutes, and high energy chromium plating at 5 asi for 15 
minutes. The adherence of the chromium to the substrate was excellent and 
the deposit was quite smooth. 
EXAMPLE 3 
The process of Example 2 was repeated using additionally 10 g/l of boric 
acid in the activation solution, a pH of 7, and anodic etching at 2 asi 
for 2 minutes. A similar excellent adhering deposit was obtained. 
EXAMPLE 4 
The process of Example 2 was repeated using an activation solution 
comprising 40 g/l sodium sulfate, 20 g/l sodium nitrate, 20 g/l sodium 
citrate dihydride, 5 g/l sodium chloride, and 20 g/l sodium tetraborate 
hexahydrate. The pH was 8. Activation was carried out at 1 asi for 4 
minutes, reverse etching in a high energy bath at 5 asi for 3 minutes and 
chromium plating at 5 asi for 15 minutes. An adherent, smooth chromium 
deposit was obtained. 
EXAMPLE 5 
The process of the above examples was repeated using potassium sulfate in 
place of sodium sulfate with similar advantageous results. 
While the mechanism of action of the activation solution and process of the 
present invention is unknown at present, it is believed to reside in the 
formation of a passive layer during the anodic etching step, which layer 
remains intact in the absence of any strong acid in the activation 
solution. 
Although the invention has been described with reference to certain 
preferred embodiments thereof, it will be understood that changes and 
modifications may be made which are within the skill of the art. 
Accordingly it is intended to be bound only by the appended claims. 
EXAMPLE 6 
The process of Example 2 may be repeated using additionally 10 g/l of boric 
acid in the activation solution, a pH of 7 and etching employing 
alternating current at 2 asi for 2 minutes. A similar excellent adhering 
deposit will result.