PowerPoint Presentation TCP Passivation process Aluminium Aluminum naturally forms an oxide film that protects it from further oxidation in many environments, but not for saltwater spray environments. Aluminum naturally forms an oxide film that protects it from further oxidation in many environments, but not for saltwater spray environments. The only way to prevent this from occurring is to use a more robust conversion coating on aluminum surfacesto uniform high quality protective layer
TCP Passivation process Aluminium Aluminum naturally forms an oxide film that protects it from further oxidation in many environments, but not for saltwater spray environments The only way to prevent this from occurring is to use a more robust conversion coating on aluminum surfaces to uniform high quality protective layer Aluminium Passivation Passivation is the process of making a material “passive”, usually by the deposition of a layer of oxide on its surface Three main aluminum passivation methods: Alclading Chromate conversion coating Anodizing The deposition of a layer of chromium oxide on substance Chromium Oxide Chromium : Generally exists in Cr(III) and Cr(VI) oxidation states in the environment Chromium is well documented as a harmful element and its toxicity depends strongly on the oxidation state Cr(VI) is classified as carcinogenic and mutagenic and can be exposed to the human body through breathing, eating, drinking and skin contact Cr(III) is not known to cause cancer and its membrane permeability is poor, making it less toxic than Cr(VI) Tri Chrome Passivation Chromate conversion coating: Increase corrosion resistance Hexavalent chrome, Cr(VI) Trivalent chromium, Cr(III) TCP process Cleaning Mild Alkaline cleaner Alkaline Etching Etching additives and scale inhibitors Desmut Nitric acid free Acid Emulsion Desmutting TCP passivation Cr(III) passivation (dipping) Drying Hot air Water rinse *2 between each steps This diagram does not represent the process of TCP passivation Pretreatment: Cleaning Principles of cleaner formulation It must break the covalent bond formed between the air-formed aluminium oxide film and soil present on surface It must can remove the contaminants, greasy and soil on part surface by a combination of reaction ex saponification, solubilisation, suspension Non-foaming or very low form Must be rinsed off readily Stable during usage Pretreatment: Cleaning Types of cleaner Solvent cleaner: Only remove contaminants that are soluble in the solvent Emulsion systems: Effective but danger of carry-over kerosene into subsequent process Alkaline cleaner: Widely used method for cleaner Generally, using mild alkaline cleaner with inhibitors added to prevent etching Acid cleaner: Need to add some additive to be effective nonetching cleaner Pretreatment: Etching Principle of etching aluminium: If aluminium is immersed in a hot caustic soda solution there is a vigorous reaction in which aluminium is dissolved and form H2 The effect on the surface of the aluminium is to produce a fine etch pattern, and leave a smooth semi-matt surface Pretreatment: Etching Factors affecting Etch rate: Free caustic soda content: More progressive increase in amount dissolved up to around 250g/l But since make up cost control, most plants operating at range of 50-100g/l at 50-60 oC Aluminium content: Aluminium content builds up the etch rate is lowered Temperature: The rapid rise in metal dissolved with increase in temperature 10 Pretreatment: Desmut The function of the desmut is to remove the thin inter-metallic particles layer on surface which call “smut” after alkaline etching Types of Desmutting: Conventional acid desmutting: 10-25%(by vol) nitric acid solution Chromate-containing desmut solution: Cause toxicity of chromates Sulfuric-fluoride mixtures: Capable for desmut on difficult alloy 11 TCP Passivation Mechanism of TCP passivation 12 TCP Passivation Operating condition: Solution concentration (Tri chrome chemical): 200 -250 ml/l Temperature: 35-40 oC Time: 30 -300 sec pH: 3.8-4.0 13 TCP Process 14 Quality Test Method Corrosion testing: Salt Spray test Exposed time shall be mutually agreed upon btw purchaser and the seller 15 Thank you for your attention 16