Non 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 special Dura Additives that are non-fluoride (non-etching) along with generic chromic acid; there are no proprietary blends to buy. The advantage of this bath over a fluoride based Thin – Dense bath is avoiding the need to provide a dedicated tank for this work. You can now do flash, heavy build-up and Thin – Dense plating in the same tank.
The Thin – Dense process is user friendly, easy to control and inexpensive to operate.
Some of the typical applications for Thin – Dense Chromium include:
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 enhanced by applying Micro Tuff Polymer after plating.
Following are the benefits of using the Dura Thin – Dense process for these applications:
• Surface Hardness of 72-74 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.
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 120:1 is generally used with a bath temperature of 140 degrees F. The Dura-3000 and 3500 additives are used to control the deposit properties, buffer the bath, chelate impurities and improve throwing power. Following are the control points:
|Chromic Acid||30.0 oz/gal.||25 – 40|
|Sulfate||0.25 oz/gal.||(by ratio)|
|Ratio||120:1||80 – 140:1|
|Dura-3000 & 3500||(see below)|
|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||130 – 150|
|Current Density||4.0 ASI||1 – 10 ASI|
Depending upon the bath type, either the Dura-3000 Additive or Dura-3500 Booster is used for conversions and maintenance. While analysis may be used, the Dura levels are best controlled by the plating speeds obtained.
This powdered additive is used to convert standard hard chrome baths to High Speed High Efficiency usage. Dura-3500 Booster provides a drastic improvement in bath performance and plating speed. Do not add Dura-3500 Booster while plating; always add it when the tank is empty of work. Sprinkle the addition evenly into a hot bath and agitate until fully dissolved. The Dura-3500 Booster is a one-time addition used only for the initial conversion. For bath maintenance see below.
Do not add the Dura-3500 Booster to baths that plate parts with highly stressed base metals or those with surface hardness exceeding 45 RC, without first stress relieving the parts by baking and then shot peening prior to plating, see QQ-C-320. This is especially important for deposits thicker than 0.001” because the base metal stress can transfer to the chrome deposit causing macro-cracking,
and in severe cases a loss of adhesion, especially after grinding. The only downside for not using Dura-3500 in these baths is a slightly slower plating speed. You should, however, still use the Dura-3000 Additive.
This liquid additive is used to convert other high efficiency baths. It’s also used for bath maintenance once a bath has been converted.
Conversions to this process are easy and inexpensive.
These baths are converted by simply adding 2.5 oz/gal. of the Dura-3500 Booster.
High Efficiency Baths (others)
These baths are converted by adding 3% by volume of Dura-3000 liquid. The Dura-3500 Booster is not normally needed, unless the catalyst level is low.
High Fluoride Baths
High fluoride baths should not be converted to this process.
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 0.00125″
3 ASI 0.00188″
4 ASI 0.00250″
6 ASI 0.00375″
|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.|
This bath is much more resistant to the effects of impurities like trivalent, copper and iron than other High Efficiency or Thin – Dense baths are. As such, the plating speed doesn’t slow down as much or as fast. Baths with extreme impurity levels, however, will likely be around 0.0002”/hour slower than the ideal rate, depending on the voltage & current density used. For best results the impurity levels should be kept as low as possible. 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.
Most applications require vapor blasting, liquid honing or glass bead blasting prior to plating, as a microscopically roughened, but clean, surface is best. A fairly close anode spacing is preferred. Using 4 ASI is preferred, along with the blasting to prevent stress in the deposit. Parts that require heat treating should be hardened prior to Thin-Dense plating.
Stop-Off & Masking
This bath is non-etching so no special stop-off is needed except where used to control the deposit. The fixtures made of steel, copper or aluminum should be PVC coated to prevent them drawing the DC current.
The typical stick or conforming type anodes can be used. The best alloy is 7% tin-lead, but 6% antimony-lead is suitable and may be preferred if rigidity is a concern.
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 also be used, but are not recommended because of stray currents. 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 impurities build-up. The tank should be ventilated and PVC systems are commonly used. The heating and cooling systems can be made of bare titanium, or Teflon. Mild air agitation is desirable for mixing in chemical additions and avoiding temperature stratification in deeper 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.
Regulations & Fume Suppressants
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.