Historical Articles

April, 1952 issue of Plating

 

Conservation of Nickel in the Plating of Chemical Process Equipment

A. Kenneth Graham
Graham, Crowley & Associates, Inc., Jenkintown, PA

 

IN THE LAST 10 YEARS, steel equipment with a protective electrodeposited nickel coating has been used to an increasing extent in the chemical industry in lieu of all-nickel or nickel-clad equipment. Electrodeposited coatings for this purpose must meet specifications for minimum thickness or minimum porosity or both. There will be a minimum thickness required to pass the porosity test. For the more severe chemical applications, an added thickness of coating is necessary to provide the required service life, and this additional thickness depends upon the rate of corrosion of nickel in the specific environment. Because the coating thickness required by the porosity test is dependent -upon a large number of factors, it is important as a means of conserving nickel that such conditions be established that the test can be passed with the minimum coating thickness. In view of the current critical shortage of nickel, a brief discussion of the factors that influence the minimum coating thickness might prove of value.

STEEL QUALITY
The quality of the steel is a factor of first importance. Steel can be classified with respect to its cleanliness, i. e., the amount and distribution of such inclusions as slag and oxide. Oxide produced during hot rolling is embedded in the surface of the steel and varies greatly in amount depending upon the care exercised in controlling the oxidation and in scale removal during rolling and subsequent pickling. Unfortunately, the consumer-has no control over these factors, and with the current shortage of steel, one must usually accept steel from whatever sources are available.

PRECLEANING CYCLE
To compensate for the lack of cleanliness and to render the steel surface more uniform and satisfactory as a basis for a plated coating for chemical uses, it is common practice to sand-blast the steel to remove surface defects and inclusions. The subsequent cleaning and acid pickling are designed to remove soil and further remove oxide inclusions, as well as embedded sand or grit particles.

The first requirement of a good precleaning cycle for plating is that the coating have good adhesion. The data in Table I illustrate variations in the adhesion of nickel applied to a sand-blasted steel surface that has been subjected to several different precleaning cycles. The adhesion was tested by a pealing test after nickel plating to thicknesses of 4.5 and 6 mils, the heavier nickel coating giving the more severe test. It may be seen that the adhesion is very sensitive to the method of precleaning.
Data are given in Table TI showing the effect of precleaning cycles 1 and 4 on the behavior of nickel of different thicknesses in the hot-water-corrosion test. It may be seen in the third column that a 3-mil (76-µ) nickel coating was sufficient to pass successfully the corrosion test* when applied over a good sand-blasted surface following the good precleaning cycle 4. With precleaning cycle 1, however, the 3-mil (76-µ) coating was not sufficiently thick to pass the test. On the other hand, a poorly sand-blasted surface from which the oxide had been incompletely removed required a nickel coating 4.5 mil (114 µ) thick in order to pass the test.

*Three-hour immersion in air-agitated tap water at 185 ± 5° F.

IRON STRIKE
An undercoat of electrodeposited iron has been shown to reduce the porosity of the nickel coating(1), and Thon(2) has demonstrated that a very thin preliminary iron deposit would reduce the permeability of electrodeposited nickel foil. It was considered impractical to deposit a heavy iron coating prior to nickel plating of the chemical equipment specifically involved at the time this matter was under consideration. An attempt was made, however, to determine the value of a thin strike coating of iron (0.005.05 mil; 0.131.3 µ) under nickel deposits 3.0.0 mils (7102 µ) in thickness as a means of improving the hot-water corrosion-test rating. The data presented in Table III indicate that nothing is gained by the use of these thin iron undercoats.

 

OTHER TECHNIQUES
Experience has shown that brushing the nickel surface with a rotating stainless-steel crimped-wire brush will improve the corrosion-test rating. It is assumed that there is sufficient burnishing or flowing of the nickel over minute capillary openings or pores to account for this improvement. It is known that there are pores in electrodeposited nickel, and other metal coatings, and that the porosity is reduced as the thickness of the coating is increased. If one should brush the surface of a nickel deposit and subsequently apply a second nickel coating, it is logical to expect that any pores in the second coating would not necessarily occur at the same sites as the pores in the first coating, especially if the pore diameters were small. The data presented in Table IV not only confirm this view, but show that it is possible to reduce the required thickness of nickel further by the use of this technique to improve the corrosion-test rating.

DESIGN LIMITATIONS
The findings reported in this article were based on tests on flat panels on which the metal distribution was extremely uniform. The irregularly shaped objects met in practice are, however, more difficult to plate uniformly. The minimum thickness must be deposited on the most inaccessible portion of the surface in order that the object pass the corrosion test. Good engineering with proper spacing and location of anodes with respect to the surface to be plated will improve the metal distribution and reduce the average amount of nickel required. However, these steps cannot be expected to correct difficulties arising out of poor or complicated design. It has been necessary, for example, to use average nickel thicknesses of 5-7 mils (127-178 µ) on large objects to obtain the minimum thickness of 3 mils (76 µ) in recessed areas. Much can be and has been done to reduce the required average thickness by modifying the design.

PLATING PRIOR TO FORMING
Because of the limitation imposed by design on the ‘distribution of electrodeposited coatings, attention has been directed to the possibility of electroplating flat sheets and then forming and fabricating the object. This is not as simple as it first appears. Adhesion of the coating must be perfect, and even then the properties of electrodeposited coatings are such that they may fail on severe deformation. The most serious limitation is the possibility of mechanical damage to the coating during forming, which can only be corrected by replating. Furthermore, heavy plated coatings, 3 mils (76 µ) and more in thickness are particularly limited in their forming characteristics, possibly because the structure changes with increasing thickness. Internal stress in the electrodeposited coating becomes an important factor and must be controlled. It does not appear likely, therefore, that plating prior to forming will offer an easy solution to the problem of metal distribution and conservation of nickel.

GOOD HOUSEKEEPING
Continuous electrolytic purification-of nickel plating baths is recommended as a means of controlling the purity and quality of the deposited nickel. Even though the current density employed in electrolytic purification is low, the amount of nickel consumed may be considerable. It is important, therefore, that care be exercised to prevent contamination of the plating bath with metallic and organic impurities so that the amount of electrolytic purification may be reduced to the minimum. The number of rejects requiring replating may also be reduced thereby. Proper maintenance of rack insulation is important for similar reasons. In other words, good housekeeping is a means for conservation of nickel.

ANODE SCRAP
In plating large and irregularly shaped objects, anodes of different lengths and shapes must be used. The amount of anode scrap produced may, therefore, be much larger than in the ordinary plating of decorative and protective coatings. However, much has been done to reduce the amount of anode scrap by using scrap anodes where short-length anodes are required and by using anode-saving baskets.

REFERENCES CITED
(1). A. W. Hothersall and R. A. F. Hammond, Trans. Electrochem. Soc. 73, 449 (1938).
(2). N. Thon, L. Yang and D. Kelemen, Plating 37, 749 (1950).