Treatment and Polishing Process for Stainless Steel Flanges
Large stainless steel flanges can be susceptible to rusting in ordinary environments, primarily due to the presence of free iron on any surface, which can lead to corrosion of the stainless steel. It's essential to remove these impurities. Loose particles like powders can be removed along with dust. Some particles may be deeply embedded and require treatment to deal with the iron contamination.
In addition to dust, there are various sources of surface iron contamination, including cleaning with regular carbon steel wire brushes, using sand, glass beads, or other abrasives for shot blasting, which were previously used on common carbon steel, low-alloy steel, or cast iron components. All these activities may introduce iron contaminants into stainless steel flanges and equipment near the mentioned non-stainless steel products during repairs and grinding.
Rust Removal Methods for Flanges: One method for rust removal is to immerse the flange in hydrochloric acid. The principle is to reduce rust, but this method carries some risks, as hydrochloric acid is a corrosive substance harmful to humans. During the operation, caution is essential to avoid contact with the skin and to handle it with alkaline substances rather than direct contact with hands.
Post-fabrication inspection can help prevent and detect the presence of free iron. ASTM standard A380 outlines the method for corrosion testing of stainless steel surface iron or steel particles. This test method should be used when absolute freedom from iron is required. If the results are satisfactory, the surface should be rinsed with clean distilled water or nitric acid until the deep blue color is entirely removed. As per standard A380, if the rust cannot be entirely removed using the rust inhibitor test solution, it is not recommended for surfaces that come into direct contact with products consumed by humans. A simple test is to expose the flange in water for 12-24 hours to check for rust. It's an insensitive and time-consuming experiment. These are inspection tests and not cleaning methods. If iron is found, it will require chemical and electrochemical cleaning to address it.
In the polishing process of stainless steel flanges, some black oxide scales or ANSI flanges are prone to occur on the surface of stainless steel flanges, affecting their application value. The application of various post-treatments, including mechanical polishing, chemical polishing, and electrochemical polishing, is necessary to enhance their value. The following process is recommended for obtaining better results:
Chemical Degreasing → Rinsing → Pickling → Rinsing → Chemical Polishing → Rinsing → Electropolishing → Rinsing → Passivation → Rinsing → Drying → Inspection.
Stainless steel flange gaskets can be divided into soft gaskets and hard gaskets. Soft gaskets are typically used for heating, gas, and medium- to low-pressure industrial pipelines. If there are no specific requirements for gasket thickness, the general specifications are as follows: 1.6mm for flanges with a diameter smaller than 125mm, 2.4mm for flanges larger than 125mm but smaller than 500mm, and 3.2mm for flanges larger than 500mm. High-temperature, high-pressure chemical industry pipelines commonly use hard gaskets, which are metallic gaskets.
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