Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 396 of 707

simplest form consists of five steps: These are: (1) hot alkaline soak clean to remove grease and oils, (2) overflowing cold water rinse, (3) blackening, (4) overflowing cold water rinse, (5) final seal with a water displacing oil, a wax, an acrylic, or a soluble oil. Should rust or light heat treat scale or oxidation be present, the use of an acid pickle to remove it may be required. If heavy heat treat or forging scale is present it is most effectively removed by mechanical means such as vibratory finishing or sand or vapor blasting. There are instances when specialized procedures are required. When the black oxide is to be used as a paint base or when strict adherence to Military Specification C-13942C, Class 1, is required, the inclusion of a dip in a dilute solu- tion of chromic acid after the blackening step becomes necessary. When powdered metal or sintered metal parts are blackened by the hot alka- line nitrate method it becomes imperative to include a hot oil dip after the black oxide step to extract residual alkali that has been absorbed by the part to prevent white alkaline bleed-out at a future time. The hot oil is normally used at a temperature of 230 to 250O F with a dwell time of 1 to 2 minutes or possibly longer. The tank used for the hot alkaline nitrate blackening solution should be properly designed to assure efficient operation and safety. The tank should be insulated to minimize heat loss. The material of con- struction should be mild steel. The black oxide tank should be gas heated for most efficient operation. If natural or bottled gas is not available the solution may be heated by electric immersion heaters. If electric immersion heaters are used it is important that they are properly installed and positioned to assure a uniform rolling boil. Since the proper boiling range is critically important to the proper function- ing of a hot alkaline nitrate black oxidizing solution, it is advisable to have a tem- perature controller that is integrated with a motor operated valve, which adds water to the blackening solution to maintain the proper boiling point. As water is boiled off the boiling point of the oxidizing solution increases and must be low- ered by the addition of water. The tank should be vented to remove the alkaline aerosol that is given off. The black finish produced by the hot black oxidizing solution is a deep jet black finish, which offers moderate abrasion resistance and will yield up to 96 hours of salt spray resistance per ASTM Specification B 117 when sealed with a moder- ately dry supplemental oil. The black oxide in and of itself does yield some corro- sion resistance above that of the unblackened steel; however, it rarely finds appli- cation in its unsealed state. The blackening solution is primarily consumed by drag-out; therefore, a solution should never stop blackening as long as it is kept up to volume. Although all hot alkaline black oxide formulations on the market today con- tain varying proportions of the three main ingredients, i.e., sodium hydroxide, sodium nitrate, and sodium nitrite, all salts are not equal. The proprietary for- mulations that offer the lowest applied cost are those that have rectifiers that effec- tively remove, either by floating or precipitating, the red iron oxide (colloidal iron) that tends to accumulate in the oxidizing solution. These formulations also contain unique wetting agents that lower the viscosity and surface tension thus resulting in less drag-out and lower applied costs. The square foot cost to apply the hot alkaline nitrate black oxide finish is 395

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