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Application of Phytic Acid in Metal Surface Treatment

Zhou Heping (Technical Development Department of Hengyang Chengxin Phytic Acid Factory)

Phytic acid, also known as cyclohexane hexaphosphate, is a natural non-toxic chemical product extracted from food crops.Its special molecular structure and physical and chemical properties determine its important application value in metal surface treatment.Especially in today's increasingly strengthened environmental protection, phytic acid products will play an extremely important role in metal surface treatment.
1.Protective mechanism of phytic acid
    The molecular weight of phytic acid is 660, and the molecular formula is C618246 .Phytic acid contains six phosphate groups, so it is easily soluble in water and has strong acidity.Phytic acid has 24 oxygen atoms, 12 hydroxyl groups and 6 phosphate groups which can coordinate with metals.Therefore, phytic acid is a rare metal multidentate chelating agent.When complex with metals, it is easy to form multiple chelating rings, and the formed complex is extremely stable.Even in a strong acidic environment, phytic acid can form a stable complex.
    In the molecular structure of phytic acid, only one of the six phosphate groups is located at a position, and the other five are located at e position, among which four phosphate groups are on the same plane.Therefore, when phytic acid is complex with metal on the metal surface, it is easy to form a dense monomolecular protective film on the metal surface, which can effectively prevent O2 from entering the metal surface, thus resisting the corrosion of metal.The monomolecular organic film formed on the metal surface after phytic acid treatment has similar chemical properties to the organic coating, and the active genes such as hydroxyl group and phosphate group contained in the film can react with the organic coating chemically.Therefore, the metal surface treated with phytic acid has stronger bonding ability with organic coating.Based on the above special properties of phytic acid, we applied phytic acid to metal anticorrosion, phosphating at room temperature, chromium-free passivation and other processes, and achieved good results.
2.Application of phytic acid in metal surface treatment
2.1 Additives for low cyanide alkaline zinc plating bath
    Phytic acid can combine with all divalent or above metal ions except monovalent metals to form phytate, which has the characteristics of complexing with various metal ions in a wide range of pH values.At the same time, phytic acid or its salts can replace sodium cyanide for low cyanide or cyanide-free electroplating, which can not only greatly improve product quality, reduce production cost, but also significantly reduce environmental pollution.Phytic acid instead of sodium cyanide for alkaline low cyanide zinc plating is a good example.In alkaline zinc plating bath, one or more phytic acid or phytate, one or more water-soluble polymer compounds and one or more aromatic aldehydes are added to form a composite additive.The bath was composed of zinc oxide 10g/L, sodium cyanide 10g/L, caustic soda 70g/L, phytic acid 0.15 g/L, polyethyleneimine 0.2 g/L, piperonal 0.3 g/L, 25 ℃, 2A/dm2 .The coating with good finish, compact coating, wide current density range and comprehensive excellent performance can be obtained.
2.2 for chromium-free zinc plating passivation solution
    Phytic acid compound corrosion inhibitor replaces chromate for passivation treatment, and the film has good corrosion inhibition performance.
(1) Formula of passivation solution
    Sodium silicate (40%) 40g/L, sulfuric acid (98%) 3g/L, hydrogen peroxide (98%) 40g/L, nitric acid (10%) 5g/L, pH 2 ~ 3.
(2) Corrosion inhibition additive
    HEDP (50%) 5g/L, NTP (50%) 5g/L, EDTMP (50%) 5g/L, phytic acid 5g/L.
(3) Preparation of passivation solution
    Add 40% sodium silicate into water slowly, stir while adding, then slowly add dilute sulfuric acid, and then add hydrogen peroxide, nitric acid and corrosion inhibition additive in turn.
(4) Process flow
    Zinc coating → cold water washing → dipping in passivation solution for 10 ~ 20s → cold water washing → drying
    Passivation treatment of zinc coating can obviously delay the corrosion of passivation film, which is due to the fact that phytic acid has six active phosphate groups, which can chelate with ions to form a single molecule dense protective film, thus enhancing the corrosion resistance of passivation film.
The appearance of passivation film is bright, uniform and fine.After being impregnated with 3% sodium chloride and 0.005 mol/L sulfuric acid solution, the corrosion spots or rust spots appeared in 1% area of the test piece for more than 70 hours.
2.3 Complexing Agent for Electroless Plating
    In electroless plating, the use of complexing agent directly affects the stability, service life and deposition rate of plating bath.The deposition rate of Ni-P electroless plating bath prepared with phytic acid as complexing agent is ≥ 20 μ m/h, and the service life of solution can reach more than 8 cycles.The composition and technological conditions of plating bath:NiSO4·6H2O 23g/L,NaH2PO2·H2O 35g/L,Na2657·2H2O 35g/L, phytic acid 5g/L, auxiliary complexing agent 10g/L, accelerator 16g/L, wetting agent 0.05 g/L, pH 4.5 ~ 5.0, loading capacity 1.7 dm2/L.
2.4 Low temperature rapid phosphating solution
2.4.1 Formulation and process conditions
    Zinc oxide 1.8 ~ 2g/L, phosphoric acid 10g/L, sodium dihydrogen phosphate
15g/L, oxalic acid 0.2 g/L, citric acid 0.2g/L, sodium hexametaphosphate 0.3 g/L, diammonium hydrogen phosphate 3g/L, thiourea 0.02 g/L, hydroquinone 0.5 g/L, phytic acid 0.5 g/L, pH 2 ~ 3.
    Diluted 10-15 times according to the above formula, the phosphating time is 5-15min at 20-35 ° C.
2.4.2 Main Performance Indicators of Phosphating Film
    Copper sulfate resistance time ≥ 300s, phosphating area 45 ~ 60m2/kg, film weight 2.0 ~ 3.4 g/m2 .
2.5 Anticorrosive coating agent for metal surface
    When the coating agent is applied on the surface of iron, zinc, aluminum and alloy, the corrosion resistance and the corrosion resistance after painting, as well as the adhesion of the coating film can be greatly improved.
2.5.1 Formula
    Ammonium fluorotitanate 10g/L, phytic acid 16g/L, silica gel 30g/L, PVA (polymerization degree 1400) 50g/L, pH 5.3.
2.5.2 Usage
    The coated metal parts were wiped with organic solvent to remove the dirt on the surface layer, and then coated with anticorrosive coating agent with the dosage of 10mg/m 2 , and then dried in circulating hot air at 120 ° C for 35s.
2.6 Metal Preservatives
    It is suitable for anticorrosion of metal and alloy surfaces in contact with gas or liquid water.
2.6.1 Formula
    Phytic acid 3g/L, sodium benzoate 7.5 g/L, proper amount of triethanolamine, pH ≥ 7.2
2.6.2 Anticorrosion performance
    The aluminum sheet was immersed in aqueous solution containing 50mg/L of the above preservative for 7 days, and the metal surface was bright without corrosive holes.
2.7 Rinsing agent for phosphating film on metal surface
    Zinc-based phosphating film rinsing agent can effectively improve the corrosion resistance of phosphating film and paint layer, improve the adhesion between phosphating film and paint layer, and prevent the color change of sprayed paint film.In this formula, phytic acid is used instead of chromate to effectively reduce pollution and improve environment.
2.7.1 Formula
    Phytic acid 0.5 g/L, sodium hydroxide, pH 3.0.
2.7.2 How to use the
    The phosphating metal parts were soaked in rinsing solution for 10s, washed with deionized water, dried, then coated with a 20 μ m thick water-soluble resin film by electrophoretic painting process, and finally baked at 170 ° C for 30min.After comparative test, the comprehensive surface properties are better than those of chromate treatment.
2.8 Passivation treatment of aluminum and its alloys
    Aluminum and its alloy are treated in alkaline aqueous solution containing two or more kinds of titanium, zirconium and iron ions with pH value of 9, then washed and dried with water, and then treated with acidic solution containing phytic acid, so that it has good anticorrosion, compact coating film and excellent appearance.
2.7.2 Formulation and process conditions of alkaline metal aqueous solution
    Hexafluorotitanic acid 0.2 g/L, zirconium sulfate 0.2 g/L, ferric hydroxide g/L, organic acid 1g/L, proper amount of sodium hydroxide, pH 11.5 ~ 13.8, 70 ℃.
2.8.2 Methods of Operation
    After soaking aluminum and aluminum alloy in the above alkaline aqueous solution for 6s, washing with water, putting them into aqueous solution containing phytic acid 20g/L (pH 3.5), soaking at 60C for 6s, cleaning with deionized water and drying.
2.9 Metal Corrosion Inhibitors in Water Soluble Media
    Phytic acid is an excellent corrosion inhibitor for metal materials such as copper, tin, zinc and steel in water-soluble medium.
    The electrochemical determination shows that phytic acid and its salts are more effective corrosion inhibitors for copper than benzotriazole and its derivatives.Pitting corrosion of copper is very common in water supply system and refrigeration system, and adding phytic acid can effectively prevent this corrosion.Some people simulated the hot water supply system and measured the corrosion potential of the inner surface of copper pipe.When the water contains Cl - 3mg/L and 60 ° C, the pitting potential of copper is + 150mV (vsSCE).If phytic acid is added to the water, the corrosion potential suddenly decreases, and the same is true when Cl - is added again.At this time, the surface of copper tube is covered with a layer of blue-green substance.When the concentration of phytic acid in hot water is maintained at 0.01% ~ 0.1%, the corrosion of copper pipe in hot water supply system can be inhibited.Phytic acid and its salts can not only prevent the corrosion of copper in water supply system, but also effectively prevent the corrosion of copper in refrigeration system.For example, 0.1% N 2 H 4 and 1% phytic acid can prevent the corrosion of copper, and the corrosion loss rate is only 1.28 × 10-7g/m2h.
    Phytic acid and its salts also have strong corrosion resistance to copper alloy.The corrosion behavior of brass was studied.It was found that in 0.5 mol/LNaCl air saturated aqueous solution buffered by HAc-NaAc with pH 4.43, the ability of preventing copper and zinc dissolution was phytic acid> thioglycolic acid> benzotriazole.
    The corrosion inhibition of phytic acid and its salts on copper and its alloys is related to the pH value of the medium.In boric acid and its sodium salt aqueous solution, phytic acid has corrosion inhibition effect on copper and copper-zinc alloy when pH> 0.2.When pH<9.2, phytic acid only has corrosion inhibition effect on copper, but has no corrosion inhibition effect on copper-zinc alloy, but will accelerate corrosion.If calcium phytate is used as corrosion inhibitor, it can inhibit the corrosion of copper and copper-zinc alloy in various pH ranges.
2.10 Corrosion Protection of Galvanized Sheet
    As early as the early 1980s, phytic acid was studied for surface treatment of hot dip galvanized sheet, cast iron and steel.It was found that the treatment solution composed of heterocyclic compounds (such as mercaptobenzotriazole), fluoride, chloride or borate and phytic acid or phytate (in which phytic acid is 30g/L) was impregnated at 80 C and dried at 130 C.After salt spray test for 24 hours, no corrosion was found in the treated metal materials.
    Phytic acid and its salt can also be used for blackening galvanized sheet.The galvanized sheet was immersed in the treatment solution with phytic acid as chelating agent (pH <3.03, NO 3 - 0.1 mol/L, PO 4 3  3 - 1.5), and the blackened alloy elements were Ni, Co, Fe, Cu, Cr, Mo or Sn.The treated galvanized sheet has a uniform dark black color and strong corrosion resistance, which is an ideal material for automobiles, electronic equipment, furniture and buildings.
    After phytic acid treatment, the corrosion resistance of galvanized sheet is improved, and the adhesion between its surface and organic coating is improved.U.S.Patent US4341558 discloses a non-chromium metal surface treatment agent, which consists of titanium or zirconium compound, phytic acid, silica gel or film forming agent, and can be directly dried at 120 ℃ without water washing, and finally coated with alkyd resin melamine paint.Through salt spray test, it is found that the adhesion of the organic coating and the corrosion resistance of metal materials are far better than those of conventional chromate treatment.The surface treatment agent for galvanized sheet provided in this patent is (NH4 )2 TiF 6 1.0%, 50% phytic acid solution is 1.6%, silica gel 200 3.0% and deionized water is 94.4%.
2.11 Corrosion Protection of Tinplate
    The tinplate treated with various planting acid salts has good oxidation resistance, wear resistance and weldability, can resist black spots caused by sulfur, and has excellent appearance.
    Japan's Yoshio Futian and others put forward a surface treatment method for chromium-free tin plating cans.The treatment solution consists of phosphoric acid 0.5 ~ 10g/L (calculated by PO4), phytic acid or phytate 0.2 ~ 2g/L, sodium chloride, calcium chloride, oxide, etc.The corrosion resistance of the treated tin can is better than that of chromic anhydride method.The patent introduces a surface treatment method for tinplate.
2.12 Additives for Anticorrosive Coatings
    Adding a small amount of phytic acid and its salt into anticorrosive coatings can improve the adhesion and corrosion resistance of coatings.
2.13 Others
    Phytic acid can also be used for surface treatment of aluminum and aluminum alloys.European patent EP78866 introduces a treatment solution formula.The aluminium and aluminium alloy treated with this solution have been proved to have strong corrosion resistance by salt spray and damp-heat resistance tests.In addition, phytic acid can also be used for zinc plating on one side of steel plate.Japanese patent JP8263671 reported a method of zinc plating on one side of steel plate.
    Phytic acid is widely used in metal surface treatment because of its unique physical and chemical properties, which can form a strong and dense protective film on metal surface and inhibit metal oxidation corrosion.Improving product quality, reducing production costs and reducing environmental pollution play an inestimable role in improving economic and social benefits.

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