PTFE fluoropolymer coated bolts after exposure to corrosive environments can be removed with hand tools. The easy on/easy off properties exhibited by these fasteners provide safe has been demontarted after Salt spray corrosion testing up to 4,000 hours. The advantage is lower plant maintenance costs and increase safety using
PTFE fluoropolymer coated fasteners.
Xylan 1000 Series
Xylan is a fluoropolymer topcoat manufactured by Whitford Corporation, which in the 1014 and 1070 coatings afford high lubricity (co-efficient of friction as low as .02), and corrosion resistance of approximately 1000 hours(ASTM B117) when applied over phosphate, and up to 2000 hours when applied over electroplating. These are solvent based thin film topcoats. These Xylan products have a maximum operating temperature ranging up to 500 deg. F. Xylan 1000 and 1400 series may also be applied over other basecoats, for added corrosion protection.
Xylan 1400 series:
The 1400 series coatings by Whitford (typically 1424), are low voc coatings. They have a coefficient of friction of .05 to .10, and a maximum constant operating temperature of 400 deg. F. The corrosion resistance of the 1400 series is approximately 1500 hours, when applied over a suitable basecoat such as zinc electroplating.
Sermagard 1105 (ASTM F1428)
Sigma Fasteners is a licensed applicator for Sermagard products. 1105 is a ceramic-metallic sprayed basecoat that at 1 mil of thickness or more affords 3000-4000 hours of ASTM B117 salt spray resistance, with an appropriate topcoat. It can be used at high temperatures. It is extremely effective in salt atmospheres. The Sermagard 1105 process involves curing at high temperatures (Minimum of 750 deg. F) and subsequent burnishing with appropriate blast media to achieve conductivity. Sermagard 1105 contains aluminum flake.
Sermagard 1280 is a fluorocarbon topcoat, which when applied to Sermagard 1105 basecoat gives superior corrosion resistance and UV protection.
Moly coating is typically used as a bonded lubricant, and has some corrosion resistance. It is spray coated to a dry film thickness of .4 to .7 mil, and cured in ovens.Everlube Coatings
Sigma also applies bolting coatings from other manufacturers, per customer specifications.
Other options include:
Cadmium Plating (ASTM B766)
An electrolytically deposited, silver-gray plating which provides exceptionally good protection against corrosion, particularly in a salt atmosphere. Chromate Clear Dip is a protective film dip applied after electroplating, and gives additional corrosion protection and a bright, shiny appearance which resists staining and finger marks. Cadmium is a known carcinogen, and has been banned in many countries. Commercial grades of cadmium plating are typically applied a minimum of 5 microns thick(.0002”). Cadmium plating with a chromate dip must not show white corrosion products at 96 hours of salt spray testing per ASTM B117.
Zinc Electroplating (ASTM B633)
Zinc electroplating is also a protective coating against corrosion, blue gray in color, non-toxic, and can be used around food. Zinc plating performs better than cadmium in an industrial atmosphere, but not in a salt atmosphere. A clear Chromate dip is a protective film applied after electroplating, adding corrosion protection. At Sigma, we also use a “silicon dip,” to aid in assembly. Commercial grades of zinc electroplating are typically applied a minimum of 5 microns thick (.0002”). Zinc electroplating with a
chromate dip must not show base metal corrosion products at 96 hours of salt spray testing per ASTM B117, but may show white corrosion at the edges. Thickness conversion factors: microns x .0394=mils, mils x 25.4=microns, mils x .0254=millimeters, mils x .001=inches
Clear chromate dip as used by Sigma is applied to all zinc plated parts unless otherwise requested. We use an excellent product produced by MacDermid, which contains no hexavalent chrome.
Olive Drab Dichromate- Electroplated work is dipped in solution of chrome, nitric and acetic acids and a dye. This produces additional corrosion resistance, and is thicker than standard chromate dips.
Yellow Iridescent Dichromate – Electroplated work is dipped in solution of sodium dichromate, takes on surface film of basic chromium chromate which resists corrosion. Finish is yellow to brown in color.
Parts to be electroplated that are RC38 or higher in hardness, should be further processed to relieve hydrogen embrittlement. This process should include baking at 375 deg. F for 8 hours or more within 4 hours of the plating process. If the parts are over 40 RC, the baking should be for 24 hours minimum.
The baking should be done before the chromate treatment. The parts are re-dipped in acid very briefly to
reactivate the surface prior to chromating.
Supplementary Phosphate – Zinc-plated parts are dipped in a solution of acid phosphates, and catalytic agents at 200 degrees F. Adherent, porous coating makes an excellent bond for paint or oil, and is performed on all Sigma parts to be coated. Several types of phosphate coatings are available: manganese, zinc, and iron. At Sigma, we prefer zinc phosphate as the best preparation for coating.
Black oxide is a thin film of iron oxide formed on the metal. It will not chip, peel, crack, or crumble. The pleasing finish provides only mild rust protection.
Passivation gives stainless steels added resistance to corrosion. The nitric acid or sodium dichromate dip removes any embedded metal particles on the surface, and leaves a protective film of stainless only, to prevent rusting.
Hot Dip Galvanizing (ASTM A153):
Hot dip zinc galvanizing is available for many fasteners. Threads in the nuts must be tapped oversized after galvanizing, or the parts will not assemble. This coating is very effective in salt atmosphere because it is applied approximately 2 mils thick. Because of the need to tap the nuts after galvanizing, the inside of the nut forms corrosion that will not allow disassembly after a period of time.
Sigma does not recommend Hot Dip Galvanizing for medium to high strength alloy parts used in the
production of oil and gas, because of the chance of hydrogen embrittlement, and the tendency for the
thickness to vary, causing uneven torque readings upon application.