Coating Technology For Advanced Applications

The finishing industry is familiar with generic hard chromium, thin-dense chromium, titanium nitriding, electroless nickel and various nickel/Teflon-plating applications. The industry is also familiar with the limitations of each type of plating; however, alternative plating technologies that meet the functional and economical demands of industry are limited. That is why Electrolizing, Inc. (Providence, RI) has developed coating processes to meet specific engineering demands.

Each coating is based on high-grade chromium materials. Major differences are in the application procedures, control levels and costs. The coatings are Micro-ETM , Electrolizing, Al-Coat and hard chrome. In addition to these coatings, the company provides advanced metal surface preparation. This multi-clean procedure is used, after extensive research on the part and its soils, to carefully and completely clean metal prior to processing. The process cleans the surface of microscopic residual contaminants left from machining, oils or other debris-generating media.

These combined coatings meet an engineer’s five most critical needs: hardness, lubricity, corrosion, adhesion and precision.

Hardness: A combination of factors enables these coatings to achieve a Rockwell C hardness of 72 and greater. The company engineers proprietary alloys and pure chromium with a consistent, controlled and unique deposition processing procedure. The coating density provides a surface that has fewer cracks, inclusions, voids and other surface irregularities compared to conventional chromium plating.

Lubricity: This characteristic is the “benchmark” of providing beneficial coefficient of friction properties. PTFE is the polymer of reference. “Our coatings,” stated Chris Bejbl, general manager, “provide a dry lubricant surface. When discussing ‘steel against steel,’ the frictional values range from about 0.20 or greater. Our coatings usually have values of 0.09 - 0.12, or about a 50% improvement in lubricity.”

Corrosion: The company subjects its products to corrosion testing using ASTM-B117. The coatings are also evaluated using humidity tests, salt water, DI water, bleaches, copper-sulfate, mold cleaners and various commercial reagents, acids, alkalines and salts. In each case, the coatings provide substantial protection against corrosion.

Adhesion: Adhesion is the most important aspect of the coatings. Without adhesion, neither metallic nor polymer-type coatings have any benefits. The coatings are subjected to 180–degree bend tests without signs of chipping, spalling or separation. This meets ASTM standard B-489-85. The technology focuses on the cohesive properties of the molecular elements of the coatings, assuring that these molecules form an absolute cohesive bond to themselves and to the base material.

Precision: The company engineered these coatings to be precise and thin. Normal coating thickness ranges from 0.00005 - 0.001 inch per side; however that varies depending on material, engineering applications and expected performance criteria. However, there is not “best” thickness. Each application is different, and many variables determine a coating thickness. Mr. Bejbl explained, “A 0.001 - 0.004 inch thickness will perform well; however, we prefer that our customers talk with our engineering department in order to tailor the deposit thickness to their specific application.

“Because we can provide these thin, precisely controlled coatings,” noted Mr. Bejbl, “the customer can eliminate the need for traditional hard chromium plating and grinding.”

Aluminum sample coated with Al-Coat.

Rather than simply focusing on a coating process, the company provides a technology. The company’s strength is in its chromium coating applications, technology and research and development. All this occurs in a clean, modern 20,000-sq-ft facility that has been tested and certified compliant to all state and federal regulations. In addition to local and federal compliance, Electrolizing has also recently obtained approval for the new standard ISO 9001-2000. The coatings the company has developed since its inception more than 50 years ago, include the following:

Electrolizing was developed in 1945. Since then, the company has refined the process to keep it an ultra-pure, versatile chrome coating. The coating is a blend of pure chromium that produces a hard surface, measuring 70-72 on the Rockwell C scale.

The coating is applied at a temperature below 180F. A “cold” process is used to protect precision-machined parts from hydrogen embrittlement, distortion, annealing and changes in the base metal’s tensile, fatigue and microstructure properties.

The coatings can be applied from 0.00005 - 0.001 inch per surface, according to customer specifications. Precision tolerance requirements can be engineered for any deposit thickness. The final finish is a smooth, fine-grained surface that is free of blisters, nodules, porosity and excessive edge buildup.

Micro-E coating is an engineering extension of the Electrolizing coating. This coating incorporates a multi-step cleaning process along with modified coating application techniques that result in a harder surface finish, somewhere near 75 Rc. The deposit thickness ranges from 0.00005 - 0.0003 inch per surface, depending on customer requirements. This thin, precise deposit eliminates dimension change, part distortion and edge buildup problems.

Certified technicians at the company give parts receiving this finish preferred handling, inspection and processing. Inspection includes fluorescent x-ray and laser micrometers. This coating is preferred when size, fit and after coating tolerance are the primary concerns, or when additional hardness is required.

Al-Coat is a newer coating developed by Electrolizing. It is designed to meet the challenge of protecting all aluminum alloys. The coating deposit is a dense, non-magnetic, high-chromium alloy deposited directly on the aluminum substrate, no matter what the alloy. However, the company does need to know what the alloy is, in order to tailor the application procedures to that specific alloy.

Link To Graphic
Hardness testing on a 6061-T6 aluminum sample using a hardened steel scribe with a 10-lb point load.

According to Mr. Bejbl, “Aluminum is difficult to coat with most conventional electroplatings and/or conversion coatings because aluminum oxidizes rapidly. Also, it acts as a cathode during the coating process, which leaves voids in the coating, causing corrosion. Another issue is that the thermal expansion of aluminum differs from most metal alloys used in plating. This factor causes most conventional plating products to peel, chip and flake.”

Al-Coat, as with the other company coatings, has an extremely hard surface that provides an unusual combination of bearing-type properties, such as low friction coefficients, longer wear life and excellent corrosion protection. It has also withstood a 180-degree bend test.

The deposit thickness for most applications can be engineered up to 0.001 inch per side. The deposit thickness is uniform and consistent, without staining or discoloration. The coating will not chip, flake or peel. The finish can be either a non-reflective, matte satin gray, or it can duplicate a high polished finish. The coating meets specifications QQ-C-320, AMS-2438 and is USDA compliant.

These coatings have applications for bearing steels, aluminum, brass, bronze, beryllium copper, copper, stainless steel, tool steels, Inconel and Monel. Industrial uses include bearing and sliding surfaces; plastic and rubber molding; metal-to-metal surfaces; high-pressure valves and pumps; cryogenic and high-temperature applications; high-vacuum applications; aluminum machined parts; and fasteners and threads.

“Our competition comes from many sources,” noted Mr. Bejbl, “including small and medium-sized platers, both regional and local. However, we are a stand-alone company. Our business of ‘only chrome applications’ makes us different. All of our experience and success is in the applications of chromium.”

Standard Hard Chromium Plating on Aluminum

Aluminum cylinders, pistons, bearings and brake drums are often hard chromium plated to produce hard, wear-resistant and, sometimes, oil-retaining surfaces. Hard chromium plated aluminum has good corrosion resistance, which is important for cylinders, particularly in atmospheres where sulfuric acid may form. Since the coefficient of expansion of aluminum is about three times that of chromium, heating hard chromium-plated aluminum parts will cause the deposit to crack. This is not necessarily harmful to the adhesion because the crack structure ensures that no stress is present in the coating, which could cause it to flake off.

A single zincate is sometimes applied to hard chromium plating. Parts are polished, degreased, etched in an alkaline solution, rinsed and dipped in an alkaline sodium zincate solution. Parts are then rinsed prior to entering the chromium-plating bath. In the bath, the zinc dissolves before the current is applied. A double zincate process is better than a single zincate process when preparing chromium for hard chromium plating.

In a double zincate process, the first zinc coating is stripped in nitric acid and a new zinc deposit is applied. This helps when plating many difficult-to-plate alloys. The second zinc coating is more compact and has more fine grains. This provides for a more uniform chromium finish.

The hard chromium plating process that follows zincating does not differ much from the process used on other substrates. Plating baths have 150 g/liter chromic acid and 2.5 g/liter sulfuric acid. They are operated at 300-500 amp/sq ft at 4-8v, producing deposits with 1,000 VPN hardness.