Rod Electroplating – Porous and Non-porous Hard Chrome Plating

Electroplating procedures, such as chromium plating, have been widely used for many years. The combination of high hardness, corrosion resistance, and low coefficient of friction have made chrome plating a commonly used reconditioning procedure. Basically, the chrome plating process involves depositing chromium on the rod surface by setting up the part (rod) as the cathode in an electrolytic bath. The bath consists of a solution of chromic acid, water, and one more acid radicals (usually sulfate and fluoride). The gap between anode and cathode is controlled to ensure that chrome is deposited evenly along and around the circumference of the rod. Time, current density, bath temperature, and proprietary chemical additives are critical parameters which must be carefully regulated. To achieve more rapid plating rates, bath temperature is normally increased.

Cleanliness and integrity of the base material is critical in assuring good bonding. Extremely good adhesion to the base metal is required for hard chrome deposits to perform acceptably in service.

In general, two types of chrome plating are available, non-porous and porous. Nonporous platings are used not only in restoring piston rods but also to restore such components as crankshaft journals, crosshead pins, bearing journals, etc. In lubricated service, non-porous chrome platings provide minimal lubricant retention capability. This in turn causes additional friction and the need for increased lubrication. For non-lubricated services, non-porous chrome is a poor choice, because packing material does not adequately deposit on the rod surface. Again, this causes increased friction, heating, and packing wear.

The difficulty of assuring adequate wettability led to the development of porous chrome having a high degree of porosity. Porous chrome platings are etched after the plating has attained a predetermined thickness. For a short time, chromium is removed selectively from the plated rod surface through an electroetching process. Small pores or channels are thus produced. These act as lubricant reservoirs. Pores do not extend entirely through the chrome plating. This process is a patented development of the Van der Horst Corporation under the trade name of “Vanderkrome.”

Although the patent has since expired, few chroming shops have demonstrated a capability to duplicate the electroetching process. A detrimental effect of chrome plating is hydrogen occlusion. During plating, hydrogen penetrates the base metal, causing a reduction in mechanical properties, most importantly, poor resistance to crack propagation. Many chrome plating control procedures incorporate a final baking to remove this hydrogen. Common baking temperatures employed are in the range of 350 to 370°F (177 to 191°C).

Approximately 50 to 60% of the total hydrogen present is removed at these temperatures with minimal effect on plating hardness. Higher temperatures result in removal of a greater amount of hydrogen at the expense of decreasing plating hardness and corrosion resistance.

Advantages include:
• Low base metal temperatures are maintained during plating. Original heat treatment of the rod is unaffected.
• Good lubricant retention and wettability (porous chrome plating only).
• Good bonding strength. Molecular type bond.
• Minimal distortion or warping.
• Corrosion resistant (reduces pitting susceptibility of rods in standby service).
• High thermal conductivity. Aids in maintaining low surface temperatures.
• Moderately thick coatings can be applied (up to 0.015 inch).
• Moderately hard coating.
• Can be applied to a wide variety of base materials (ferrous and nonferrous).
• Moderate cost.
• Ease of application and control.
• Low coefficient of friction.

Disadvantages include:
• Quality of workmanship varies widely from shop to shop.
• Bond is highly dependent on proper cleaning and surface preparation.
• Surface finish of chrome plating is highly dependent on smoothness of the base metal before plating (should be 20 micro-inches RMS or better).
• Fair to poor lubricant retention and wettability (non-porous platings).
• Hydrogen penetrates base metal during coating process causing base metal hydrogen embrittlement and reduction of fatigue strength. Final baking is required.
• In services badly corrosive to base metal, chromium plating tends to flake off.

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