Surface Preparation – Stainless Steel

As stainless steels are corrosion-resistant, usually they are not finished further by electroplating. However, they are occasionally electroplated for color matching, lubrication during cold heading and drawing operations, improvement of heat and electrical conductance, jewelry decoration, and other specialized purposes.

Stainless steels are difficult to plate because of their passivity. The problem is to activate the surface and keep it active until plating has started.

The corrosion resistant property of stainless steel is the result of a thin transparent film of oxides form on the surface. This film rapidly reforms after it has been stripped off. An adherent electrodeposit cannot be obtained over this oxide film. The principal aim of cleaning operations is to remove the film by surface activation and start the electroplating before reformation of the film. Any of the commonly electroplated metals may be electrodeposited successfully on activated stainless steel surface.

The preparation of stainless steel for electroplating involves three basic steps in the following order –

• Removal of scale,
• Removal of oil, grease, or other foreign material by cleaning, and
• Activation immediately before electroplating.

Pre-cleaning and alkaline cleaning sequences are conventional as for carbon steels. After cleaning activation must be accomplished before plating to remove the oxide film.

Cleaning

Pre-cleaning — Removal of fabricating lubricants and finishing compounds from the stainless steel should be undertaken immediately following the fabrication or finishing operation before any heat-treatment. Spray cleaning with a nozzle pressure of 200–400 kPa (30–60 psi) in a power washer, using an alkaline or emulsion-type cleaner, is the generally preferred method, especially for the removal of heavy drawing, buffing, or polishing compounds. Soak cleaning or vapor degreasing may also be used.

Descaling — If scale removal is necessary, one of the chemical methods (like pickling in sulfuric acid or nitric and hydrofluoric acids or treatment in molten alkali or salt baths) or mechanical methods (like abrasive blasting, sanding, grinding, power brushing), or by a combination of these may be used.

Electro-cleaning — Anodic cleaning is generally preferred. When brightness is important, alkalinity, current density, and temperature should be kept as low as the part will permit.

Metal Lubricants — Metal lubricants such as copper, lead, or cadmium, applied to stainless steel wire for cold heading, wire drawing, or spring forming are removed by immersion in a solution of 20% (vol) nitric acid at 50–60 °C.

Activation

After cleaning and before electroplating, the parts must be completely activated by removing the oxide film from the surface. This film will reform if the parts are allowed to dry or are exposed to oxygen-containing solutions. The part should be immediately transferred to the plating bath, or preferably, a simultaneous activation-electroplating bath should be used. The suitable activation procedure depends on the nature of the part and preceding or subsequent processes.

Immersion Treatments — Immersion pickling can be used for activating some stainless steels. Immerse the part in a solution of 5–50% (vol) sulfuric acid maintaining at 65–80 °C for at least 1 min after gassing starts. If gassing does not start within 1 min, touching it with a carbon-steel bar will start the pickling action. This activation treatment will produce a dark, adherent smut that is removed in the electroplating bath. A cathodic current of at least 0.54 A/dm2 may be used to accelerate activation. Lead anodes are suitable for this solution.

Cathodic Treatments — Cathodic treatment in sulfuric or hydrochloric acid is successfully applied for activation of many stainless steels. One of the following procedure may be applied –

• Cathodic activation in 5–50% (vol) sulfuric acid at current density 0.54 A/dm2 for 1–5 min at room temperature using pure lead anode.
• Cathodic activation in 5–50% (vol) hydrochloric acid at current density 2.15 A/dm2 for 1–5 min at room temperature using nickel anode.
• Immersion pickling in 10–30% (vol) hydrochloric acid at room temperature for 0.5–1 min, followed by cathodic treatment in 5–50% (vol) sulfuric acid at current density 0.54–2.7 A/dm2 at room temperature using pure lead anode.

Simultaneous Activation-Electroplating Treatments — The most successful method for plating stainless steel is to activate it in an activation-plating bath. The bath maintain at room temperature contains nickel chloride at 230–260 g/L with enough hydrochloric acid at 8–10% (vol) for activation. Bath iron content should not exceed 7.5 g/L. Nickel is used as electrode. The part is etched by anodic treatment for 1–2 min at a current density of 2 A/dm2. The current is then reversed and the part is plated with nickel at a current density of 2 A/dm2 for 2-20 min.

The principle of this process is that the part is activated in the bath and remains active until plating is started. A sufficient deposit of nickel protects the part from passivity and it is then easily rinsed and transferred to another bath for plating. As the bath builds up in nickel and uses acid, portion of the bath must periodically discarded and acid added to maintain the quality. Sometimes a cathodic activation in sulfuric acid may precede the simultaneous activation-plating in the NiCl2–HCl bath to ensure a high degree of adhesion.

Rinsing

The parts should be transferred to the cold-water rinse and to the plating solution as rapidly as practicable after the activation procedure; otherwise the surface will passivate itself and the electrodeposit will not be adherent. The rinse water should be kept slightly acidic (pH 2.5–3.5). The acid carryover from the activation operation will maintain this pH in many instances.

Electroplating

An adherent electrodeposit of commonly electroplated metals (cadmium, copper, brass, chromium, gold, nickel, or silver) may be electrodeposited directly on stainless steel provided the surface is active.  Where practical, the parts should have the current applied during entry into the electroplating solution.

• Nickel may be electrodeposited at normal current densities directly on properly activated stainless steel from standard nickel-electroplating solutions at pH 2–4, preferably 2.
• Decorative chromium electroplating with better coverage and a wider bright range is obtained in solution containing 400 g/L of chromic acid operating at a current density of 16.2 A/dm2 and at about 49 °C.
• A bright nickel electroplate under chromium, preceded by a simultaneous activation-electroplating treatment, may often be used for better color matching and elimination of chromium buffing.

Stripping

Nitric acid is the preferred stripping solution. Decorative chromium electrodeposits can be stripped in a solution of 50% (vol) hydrochloric acid at 45–50 °C for 1 min; it may also be stripped anodically in any alkaline solution.

Post Electroplating Operations

Post electroplating operations such as stress relieving, buffing or coloring, and forming or drawing may be applied to stainless steel in the same manner as to any other basis metal, as long as the natural differences in the characteristic of the stainless steel are taken into consideration.

Md. Nazmul Hasan Rasel

Materials Engineer

Email: hasanrasel.mse@gmail.com

Cell: +8801724512021