Fluoride Thin - Dense Chrome

Thin – Dense Hard Chrome is a special plating process that offers significant benefit to both users of the coating as well as to the shops applying it. Thin – Dense Hard Chrome can be an extremely profitable process for job shops due to the high production capability and the short plating times involved. End users benefit as the Thin – Dense Process does not alter the part dimensions or the mechanical properties of the substrate to any appreciable extent. It also eliminates secondary steps such as grinding and polishing. Thin – Dense chrome can be applied to any base metal.

Thin – Dense hard chrome is a proven technology that is applied under several trade names. This process uses two special Dura Additives along with generic chromic acid; there are no proprietary blends to buy.

The Thin – Dense process is user friendly, easy to control and inexpensive to operate.

Part Types
Some of the typical applications for Thin – Dense Chromium include:

Aerospace Cutting Tools Forms Robotics
Aircraft Dies Gauges Rollers
Bearings Drills Gears Splines
Blowers Electronics Molds Taps
Bolts Engines Printing Threads
Bottlers Files Pumps Tools

This is by no means a complete listing of all Thin-Dense applications. It can be used wherever thin deposits are preferred, the deposit structure is critical and where corrosion resistance is important. The corrosion resistant properties of the deposit can be even further improved by applying Micro Tuff Polymer after plating.

Following are the benefits of using the Dura Thin – Dense process for these applications:

  • Surface Hardness of 72 Rockwell C.
  • Deposit is void of micro-cracks which enhance corrosion resistance.
  • Friction and Wear are reduced with improved lubricant retention.
  • Bond Strength exceeds 125,000 psi and will pass a severe bend test.
  • Coating can be easily stripped with conventional methods.
  • 95 % Fatigue Limit reduction over conventional hard chrome.
  • 93 % reduction in deposit internal stress.
  • Eliminates build-up and nodulization on sharp corners and edges.

Bath Control
The Thin-Dense bath can operate with a chromic acid level of 25 – 40 oz/gal. Most operations control the chrome at 30 oz/gal. A sulfate ratio of 215:1 is generally used with a bath temperature of 140 degrees F. Dura-100 and Dura-76-F are used to control the deposit properties, buffer the bath, chelate impurities and improve throwing power. Following are the control points:

Item Optimum Range
Chromic Acid 30.0 oz/gal. 25 – 40
Sulfate 0.14 oz/gal.
Ratio 215:1 190 – 260
Dura-100 3 % Vol. 2 – 4
Dura-76-F 2.5 oz/gal. 2.0 – 5.0
Trivalent Cr 1 % of Hex. 0.70 – 2.50
Metallic Impurities < 5.0 g/l
Chloride < 20 ppm
Insolubles < 0.04 g/l
Temperature – F 140 120 – 150
Current Density 4.0 ASI 1.5  –  6  ASI

The initial addition of Dura-100 is 3 % of the bath volume and the Dura-76-F is 2.5 oz/gallon. Maintenance additions should be made as follows. Also, send a bath sample to Plating Resources on a monthly basis for a complete analysis.

Item Per 100,000 Ampere Hours Approx.  Per 100 lbs. of Chrome
Dura-100 0.5 gals. 2 gallons
Dura-76F 2.5 lbs. 10 pounds

Deposition Rates and Current Density
The Thin-Dense bath also allows operation at high current densities for faster plating speeds. Higher current densities are favored due to a reduction in internal stress. The following plating speeds are typical (inches/hour/side), but may vary with higher impurity levels.

2 ASI 0.0012″
4 ASI 0.0024″
6 ASI 0.0036″

Deposit Thicknesses

Range: 0.000025 – 0.0005″      Typical: 0.0002″
Typical Deposit Variance: +/- 0.000025″ up to 0.00025″ thickness

+/- 0.000050″ from 0.00025 – 0.0005″

Typical Plating Time: 5 minutes at 4 ASI for a 0.0002″ deposit.

Surface Preparation
Most applications require vapor blasting, liquid honing or glass bead blasting prior to plating. A fairly close anode spacing is preferred. Using 4 ASI is best as, along with blasting, this further reduces stress in the deposit. Parts that require heat treating should be hardened prior to Thin-Dense plating.

Stop-Off & Masking
Masking is not needed for short runs or where the entire surface is plated, except to control the deposition. Stop-Off should be used, however, in critical unplated areas to avoid unnecessary cathodic etching, especially for thicker deposits, highly machined or polished areas. Bare steel, copper or aluminum fixtures should not be used with this bath. These should be coated with adequate stop-off to prevent their corrosion and subsequent bath contamination.

The typical stick or conforming type anodes can be used. The best alloy is 7% tin-lead, but a small amount of antimony can be added for rigidity if needed.

Bath Impurities
Bath impurities should be kept as low as possible for best results. Of particular concern is trivalent, copper and iron. High impurity levels require additional voltage to maintain the desired current density with a greater tendency for burning, pitting and a reduction in throwing power.

The best removal method for trivalent and chloride is to dummy the bath using CR-3 Reducer and an anode ratio of at least 20:1. Porous pots used with CR-3 Reducer can be used to remove copper and iron; this process can be slow if the levels are high though

The equipment normally used for hard chrome plating is satisfactory for the process. Typical tank linings like PVC, Koroseal or molded HD polyethylene are best. Lead linings should never be used due to corrosion. The rectifier(s) can be the typical SCR type (or other control types) and should have sufficient amperage capacity at up to 9-12 volts, with a maximum of 5% ripple at the actual voltage used. This slightly higher voltage can be beneficial when contaminants build-up. The tank should be ventilated and PVC systems are commonly used. The heating and cooling systems should be made of Teflon. Titanium may be used, but it will eventually corrode. Mild air agitation is desirable for mixing in chemical additions and avoiding temperature stratification in deep tanks. The air agitation should be provided by a low pressure blower and controlled to avoid excess chrome mist. Pumps, filters and plumbing should be schedule 80 CPVC with glued fittings.

Like all chrome plating solutions, this bath produces misting and this can be reduced if lower chrome levels are used. This mist contains Cr(VI) which is regulated by the EPA for environmental and OSHA for worker safety issues. Be sure to follow all federal, state and local regulations for safe operation and hazardous disposal.

This bath contains chromic acid, sulfuric acid and the various Dura additives outlined above. These are all industrial chemicals and must be handled carefully and in accordance with the directives provided in the individual SDS forms.

Read and understand the SDS on all of these chemicals before handling or using. Ensure that all regulatory standards are followed and limit personal exposure as required for Cr(VI) by OSHA.

Avoid personal contact with these chemicals, avoid splashing and avoid breathing any fumes released during operation. Do not inhale any dust, mist or vapors from these chemicals. Do not allow these products to contact the skin or eyes. In case of contact, flush immediately with large amounts of fresh water and seek immediate medical attention. Wear protective clothing such as aprons, gloves, face masks and respirators. Be sure that adequate eyewashes and emergency showers are available nearby before handling or using any of these chemicals. Designated work clothing should be worn while using these chemicals and the worker(s) should thoroughly shower and change into fresh-clean street clothing before going home. Decontaminate all work clothing before reuse.

The user is responsible for providing adequate work clothing, personal protection, limiting personal exposure, and providing any required clean-up, decontamination as well as any needed medical attention.