A method of inhibiting corrosion of duplex stainless steel by aqueous acidic solution, comprises carrying out a preliminary treatment which is passivation of the steel while exposing the steel to a first aqueous acidic solution containing at least one organic corrosion inhibitor able to adsorb to a steel surface, so as to deposit a passivating and corrosion inhibiting film on the steel surface; and thereafter during a second period of time exposing the steel to a second aqueous acidic solution different from the first aqueous acidic solution and also containing at least one organic corrosion inhibitor able to adsorb to a steel. The second aqueous acidic solution maintains the film on the steel surface, such that weight loss through corrosion in the second period of time is lower than the weight loss which would be observed with the same second aqueous acidic solution but without carrying out the preliminary treatment.

A method of inhibiting corrosion of duplex stainless steel by aqueous acidic solution, comprises carrying out a preliminary treatment which is passivation of the steel while exposing the steel to a first aqueous acidic solution containing at least one organic corrosion inhibitor able to adsorb to a steel surface, so as to deposit a passivating and corrosion inhibiting film on the steel surface; and thereafter during a second period of time exposing the steel to a second aqueous acidic solution different from the first aqueous acidic solution and also containing at least one organic corrosion inhibitor able to adsorb to a steel. The second aqueous acidic solution maintains the film on the steel surface, such that weight loss through corrosion in the second period of time is lower than the weight loss which would be observed with the same second aqueous acidic solution but without carrying out the preliminary treatment.

A surface treatment method for aluminum heat exchangers including (a) a step wherein a chemical conversion coating film is formed on the surface of an aluminum heat exchanger by subjecting the aluminum heat exchanger to chemical conversion using a chemical conversion agent; (b) a step wherein the aluminum heat exchanger, the surface of which has been provided with a chemical conversion coating film in step (a), is brought into contact with a hydrophilizing agent that contains a hydrophilic resin; and (c) a step wherein a hydrophilized coating film is formed on the surface of the aluminum heat exchanger by baking the aluminum heat exchanger, which has been subjected to a contact treatment in step (b). The chemical conversion agent used in step (a) contains zirconium and/or titanium in an amount of 5-5,000 ppm by mass in total, vanadium in an amount of 10-1,000 ppm by mass and a metal stabilizer in an amount of 5-5,000 ppm by mass. In addition, the chemical conversion agent used in step (a) has a pH of 2-6.

A trivalent-chromium chemical conversion coating from which substantially no hexavalent chromium is released. The trivalent-chromium chemical conversion coating is one formed on the surface of a zinc or zinc-alloy deposit. In a brine spray test, the chemical conversion coating has unsusceptibility to corrosion (time required for white-rust formation) of 96 hours or longer. The chemical conversion coating has a hexavalent-chromium concentration less than 0.01 μg/cm.sup.2 in terms of metal atom amount. The amount of hexavalent chromium released after 30-day standing in a thermo-hygrostatic chamber at a temperature of 80° C. and a humidity of 95% (amount of hexavalent chromium released when the coating is immersed in 100° C. water for 10 minutes) is smaller than 0.05 μg/cm.sup.2.

Provided are a chromium-free surface-treated tinplate, a production method and a surface treating agent thereof. By coating, on the surface of a tinplate, an environmentally friendly aqueous surface treating agent containing 0.1-5 wt % of a zinc salt, 0.1-5 wt % of a zirconium salt and/or a molybdenum salt and 5-30 wt % of siloxane or polysiloxane, a layer of chromium-free passivation film having uniform and dense ingredients and a good performance and being stable is formed on the surface of a tin layer. The passivation film contains 0.1-20 mg/m.sup.2 of zinc, 0.1-20 mg/m.sup.2 of zirconium and/or molybdenum and 0.5-100 mg/m.sup.2 silicon. The passivation film can impart an excellent surface stability, corrosion resistance and paint film adhesion performance to the surface of the tinplate; in addition, contact with food is safe. The tinplate is comparable to chromium passivation in performance, and the production process thereof does not use a chromate, so that a truly green production process of a tinplate is achieved, complying with the requirements of increasingly strict environmental protection laws and regulations.

Provided are a chromium-free surface-treated tinplate, a production method and a surface treating agent thereof. By coating, on the surface of a tinplate, an environmentally friendly aqueous surface treating agent containing 0.1-5 wt % of a zinc salt, 0.1-5 wt % of a zirconium salt and/or a molybdenum salt and 5-30 wt % of siloxane or polysiloxane, a layer of chromium-free passivation film having uniform and dense ingredients and a good performance and being stable is formed on the surface of a tin layer. The passivation film contains 0.1-20 mg/m.sup.2 of zinc, 0.1-20 mg/m.sup.2 of zirconium and/or molybdenum and 0.5-100 mg/m.sup.2 silicon. The passivation film can impart an excellent surface stability, corrosion resistance and paint film adhesion performance to the surface of the tinplate; in addition, contact with food is safe. The tinplate is comparable to chromium passivation in performance, and the production process thereof does not use a chromate, so that a truly green production process of a tinplate is achieved, complying with the requirements of increasingly strict environmental protection laws and regulations.

A surface treatment method capable of imparting exceptional corrosion resistance and moisture resistance to an NB heat exchanger. The method includes subjecting an NB heat exchanger to a chemical conversion treatment to form a chemical conversion film on the surface thereof using a chemical conversion treatment agent that contains zirconium and/or titanium in a total amount of 5-5,000 ppm by weight, vanadium in an amount of 10-1,000 ppm by weight, and has a pH of 2-6; bringing the NB heat exchanger on whose surface the chemical conversion film is formed into contact with a hydrophilization agent containing a hydrophilic resin and a guanidine compound and/or a salt thereof; and baking the NB heat exchanger subjected to the contacting process, whereby a hydrophilic film is formed on the surface thereof.

Provided is a surface treatment solution composition comprising: 30 to 51 wt % of a trivalent chromium compound comprising chromium phosphate (A) and chromium nitrate (B) and having a content ratio of A/(A+B) that satisfies 0.3 to 0.6; 5 to 15 wt % of silane coupling agent; 0.2 to 3 wt % of vanadium-based anti-corrosive rust inhibitor; 3 to 12 wt % of colloidal silica; 0.5 to 5 wt % of polysiloxane copolymer; and 14 to 61.3 wt % of water, a hot dip galvanized steel sheet surface-treated using same, and a manufacturing method thereof. The hot dip galvanized steel sheet treated with the surface treatment solution composition containing trivalent chromium has an excellent corrosion resistance, blackening resistance, pipe-forming oil reactivity, and alkali resistance.

Provided is a solution composition for steel sheet surface treatment, comprising 30 wt % to 60 wt % of a trivalent chromium compound containing chromium phosphate (A) and chromium nitrate (B); 0.2 wt % to 0.4 wt % of a rust-inhibiting and corrosion-resisting agent; 0.1 wt % to 0.3 wt % of a molybdenum-based compound; 5 wt % to 10 wt % of a water-soluble cationic urethane resin; 0.5 wt % to 2.0 wt % of a silane coupling agent; and 27.3 wt % to 64.2 wt % of water, a zinc-based plated steel sheet surface-treated with the same, and a manufacturing method therefor, the zinc-based plated steel sheet surface-treated with the solution composition for steel sheet surface treatment containing trivalent chromium may have an excellent effect on corrosion resistance, blackening resistance, fingerprint resistance, oil resistance, and alkali resistance.

Provided is a solution composition for steel sheet surface treatment, comprising 30 wt % to 60 wt % of a trivalent chromium compound containing chromium phosphate (A) and chromium nitrate (B); 0.2 wt % to 0.4 wt % of a rust-inhibiting and corrosion-resisting agent; 0.1 wt % to 0.3 wt % of a molybdenum-based compound; 5 wt % to 10 wt % of a water-soluble cationic urethane resin; 0.5 wt % to 2.0 wt % of a silane coupling agent; and 27.3 wt % to 64.2 wt % of water, a zinc-based plated steel sheet surface-treated with the same, and a manufacturing method therefor, the zinc-based plated steel sheet surface-treated with the solution composition for steel sheet surface treatment containing trivalent chromium may have an excellent effect on corrosion resistance, blackening resistance, fingerprint resistance, oil resistance, and alkali resistance.