Metal coloring process and solutions therefor

A process for forming a hybrid conversion coating on a ferrous metal substrate, comprising the steps of:

• (a) applying to the substrate an intermediate coating rich in molecular iron and oxygen;
• and
• (b) contacting the coated substrate of step (a) with a reagent of
• (1) an aqueous solution of oxidizing agents containing alkali metal hydroxide at a concentration of about 20-1000 grams per liter;
• (2) a sequestrant;
• and
• (3) an accelerator selected from organic and inorganic nitro compounds, alkali metal compounds of citrate, molybdate, polyphosphate, vanadate, chlorate, tungstate, thiocyanate, dichromate, stannate, sulfide and thiosulfate, stannous chloride, stannic chloride, ethylene thiourea, benzothiazyl disulfide, thiourea, alkyl thiourea, dialkyl thiourea, cysteine, cystine, and mixtures thereof;
• to form a surface that is predominantly magnetite, Fe3O4.

A ferrous metal article having a surface formed by two treatments, wherein the first treatment comprises an iron/oxygen-enriched intermediate oxidized coating formed on the ferrous metal article with an aqueous solution containing a reagent of

• (a) oxalic acid at a concentration of about 0.5-35 grams per liter, a pH of about 0.5 -6.5, a temperature of about 50-150° F., and a contact time of about 0.5-10 minutes;
• (b) an accelerator selected from organic and inorganic nitro compounds, a hydroxylamine accelerator, and mixtures thereof;
• and
• (c) a wetting agent;
• optionally by a slurry deposition, and
• the second treatment comprises a further oxidation of the first coating to convert the first coating to a magnetite coating on the ferrous metal article.

A ferrous metal article having a surface formed by two treatments, wherein

• the first treatment comprises an iron/oxygen-enriched intermediate oxidized coating formed on the ferrous metal article, and
• the second treatment comprises a further oxidation containing a reagent of
• (a) an alkali metal hydroxide at a concentration of about 20-1000 grams per liter;
• (b) a sequestrant;
• and
• (c) an accelerator selected from organic and inorganic nitro compounds, alkali metal compounds of citrate, molybdate, polyphosphate, vanadate, chlorate, tungstate, thiocyanate, dichromate, stannate, sulfide and thiosulfate, stannous chloride, stannic chloride, ethylene thiourea, benzothiazyl disulfide, thiourea, alkyl thiourea, dialkyl thiourea, cysteine, cystine, and mixtures thereof;
• to convert the first coating to a magnetite coating on the ferrous metal article.

An oxidation solution for oxidizing at least a portion of an iron/oxygen enriched intermediate coating on a ferrous substrate to magnetite comprising an aqueous solution containing a reagent of

• (a) alkali metal hydroxide at a concentration of about 20-1000 grams per liter;
• (b) a sequestrant;
• and
• (c) an accelerator selected from organic and inorganic nitro compounds, alkali metal compounds of citrate, molybdate, polyphosphate, vanadate, chlorate, tungstate, thiocyanate, dichromate, stannate, sulfide and thiosulfate, stannous chloride, stannic chloride, ethylene thiourea, benzothiazyl disulfide, thiourea, alkyl thiourea, dialkyl thiourea, cysteine, cystine, and mixtures thereof.

A process for forming a hybrid conversion coating on a ferrous metal substrate, comprising the steps of:

• (1) subjecting the ferrous metal substrate to treatment selected from cleaning, degreasing, descaling, and mixtures thereof;
• (2) rinsing the substrate from step (1) with water;
• (3) subjecting the substrate from step (2) to a first oxidation containing a reagent of
• (a) oxalic acid at a concentration of about 0.5-35 grams per liter, a pH of about 0.5-6.5, a temperature of about 50-150° F., and a contact time of about 0.5-10 minutes
• (b) an accelerator selected from organic and inorganic nitro compounds, a hydroxylamine accelerator, and mixtures thereof;
• and
• (c) a wetting agent
• optionally by a slurry deposition to form a molecular iron/oxygen enriched intermediate coating;
• (4) rinsing the substrate from step (3) with water;
• (5) subjecting the substrate from step (4) to a second oxidation to form a predominantly magnetite, Fe3O4 coating;
• (6) rinsing the substrate from step (5) with water;
• and
• (7) sealing the substrate with an appropriate topcoat.

A process for forming a hybrid conversion coating on a ferrous metal substrate, comprising the steps of:

• (1) subjecting the ferrous metal substrate to treatment selected from cleaning, degreasing, descaling, and mixtures thereof;
• (2) rinsing the substrate from step (1) with water;
• (3) subjecting the substrate from step (2) to a first oxidation to form a molecular iron/oxygen enriched intermediate coating;
• (4) rinsing the substrate from step (3) with water;
• (5) subjecting the substrate from step (4) to a second oxidation with a reagent selected from
• (a) an aqueous solution containing alkali metal hydroxide at a concentration of about 20-1000 grams per liter;
• (b) a sequestrant for hard water salts;
• and
• (c) an accelerator selected from organic and inorganic nitro compounds, alkali metal compounds of citrate, molybdate, polyphosphate, vanadate, chlorate, tungstate, thiocyanate, dichromate, stannate, sulfide and thiosulfate, stannous chloride, stannic chloride, ethylene thiourea, benzothiazyl disulfide, thiourea, alkyl thiourea, dialkyl thiourea, cysteine, cystine, and mixtures thereof;
• to form a predominantly magnetite, Fe3O4 coating;
• (6) rinsing the substrate from step (5) with water;
• and
• (7) sealing the substrate with an appropriate topcoat.

A ferrous metal article prepared according to a process for forming a hybrid conversion coating on a ferrous metal substrate, comprising the steps of:

• (a) applying to the substrate an intermediate coating rich in molecular iron and oxygen with a reagent selected from
• (1) an aqueous solution of oxalic acid at a concentration of about 0.5-35 grams per liter, a pH of about 0.5-6.5, a temperature of about 50-150° F., and a contact time of about 0.5-10;
• (2) an accelerator selected from organic and inorganic nitro compounds, a hydroxylamine accelerator, and mixtures thereof;
• and
• (3) a wetting agent;
• optionally by a slurry deposition;
• and
• (b) contacting the coated substrate of step (a) with an aqueous solution of oxidizing agents to form a surface that is predominantly magnetite, Fe3O4.