Backgraund of the invention 1. Field of the invention.

This invention relates to the field of product protection against the influence of environmental factors and particularly to protection of metallic and metalliferous surfaces of products made of metals and alloys. This method can find application in machine-building, instrument engineering, power generation, aircraft, chemical and medical industries.

2. Description of the Background.

There is a known method to treat steel products in aqueous solutions containing hexavalent chrome and molybdenum with addition of fluoborates, phosphates. acetates and other catalytic anions to obtain iridescent coatings at 90-100°C temperature (US patent No. 365 7079. Cl. 204-51).

It is impossible to get a protecting or decorative coating on the surface of products made of stainless steel whereas iridescent films obtained on the surface of carbon steels are insufficiently solid.

There is a known method of coloring components made of stainless steel (Switzerland patent No. 572988, Cl. C23F 7/26).

According to this method components of Cr- ; Cr-Ni- ; Cr-Ni-Mo-steels are immersed into a solution heated up to 50-120°C temperature which contains sulphuric acid (400 parts bv volume in 600 parts by volume of water) and ammonium chromate 30-700 g/l.

After being kept in this solution products acquire colours from grey to yellow-green depending on the time of staying in the solution.

The above described method in some modifications is also applied to get coloring films on zinc. cadmium, aluminium and other metals.

The general disadvantage of the so called"conversion chromate films"is lack of thermal stability (in case of temperature rising above 250°C they experience destruction), small thickness (less than I mkm) and low corrosion resistance in humid environment.

A method which is most similar in its technical nature and the result obtained to the claimed one is the method of steel chemical treatment under pressure in aqueous solutions on the basis of molybdates of alkaline and alkaline-earth metals or in alkaline solutions containing NaOH, KOH, NazS, MgO as the main component with addition of molybdates (Japan patents No. 44127 and No. 44416, cl. 12 A 41, published 8.11.72) as well as respective (Great Britain patents No. 1195990, cl. C 74), (France patents No. 1549749. No. 1554777, cl. C23 F 7/00) and (US patents No. 3701693, cl. 148-6.24).

According to this method a steel product surface is treated in one of the said solutions at a temperature of 240-400°C and a pressure from 40 to 300 kg/cm2. As a result of the treatment, protecting films containing compounds of FeO, Fez03, Mo02, FeMo03, FeS, etc. as it is proved by the data of X-ray analysis are formed on steel surfaces.

Chemical compounds in aggregate which are constituents of a protecting coating impart high corrosion resistance to steel. However presence of pores in the protecting coating brings about a necessity of additional treatment, particularly, in a solution of chromic acid. An important factor is concentration of chromic acid used to increase film corrosion resistance.

The type of coatings studied herein may be referred to the cathode one, therefore additional treatment of films in chromic acid solutions is required as it extends the induction period, i. e. the time period till the occurence of the first traces of corrosion effect on the protecting coating surface. Additional treatment in oxide medium does not provide a durable protection on the steel surface, but only blocks for some time active pore centers of the coating.

On the other hand film treatment in chromic acid solutions is useless when it is necessary to preserve products working under the conditions of sharp temperature gradient or cycling heating and cooling processes, as chromic films are subjected to destruction at a temperature above 200°C.

Heat treatment of products made of austenitic steel, e. g. in the weld region, raises considerably sensitivity of these zones to tubercular corrosion and coating porosity in this case is impermissible.

Another significant disadvantage of this method is the following: the majority of chemical compounds (mainly ferric oxides) which are incorporated in the coating composition even in the aggregate are characterized by limited protecting properties. e. g. in humid environment with the presence of chlor-ions.

Summary of the invention. Detailed description of the invention.

The target of this invention is to obtain a sound thermal resistant protecting coating on a metallic or metalliferous surface durable in corrosive liquids and gas surroundings which can be created on a product surface with an irregular shape.

This target may be accomplished by treatment of metallic or metalliferous product surfaces in a solution containing 5-55 gr/1 of metallic compounds of the Y, YI and YII groups of the Periodic system of elements and additionally 0.3-15 go/1 of fluorine compounds (in conversion to fluorine ions).

The process of coating application is realised at a temperature of 180-375°C and a pressure of 20-220 kg/cm2 within the time sufficient to complete the reaction.

Application of such processes allows to obtain on the surface unifoorm, solid films of 6 mkm containing desired metals (according to the X-ray analysis).

Up to 50 % metal contents in films combined with the strength of their cohesion with the base impart them high thermal resistance in corrosive surroundings and low vacuum.

As the process takes place at a higher temperature of the solution, i. e. under the conditions when simple fluorides dissosiiation speed increases which intensifies interaction of fluoride ions with metal, including formation of an insufficiently solid layer of the protecting coating, in this case complex fluorides are added to the solution of metalliferous compounds.

Complex fluorides of alkaline and/or alkaline-earth metals, e. g. fluosilikates, fluoborates, fluotitanates, etc. separate fluoride ions into the solution slower. At the same time they are complexing agents and buffers allowing to keep the process solution pH constant for a long time which contributes to the process optimization.

It should be noted that introduction of fluorides into process solutions makes it possible to lower the maximum process temperature as distinguished from GB patent No. 1180951, for example, for chrome-nickel steel from 320-350°C to 280-300°C thereby to halve pressure which is very important for treating pressurized and thin-wall products. Fluorides addition increases reactivity of process solutions and contributes to a deeper interaction of an alloying component with steel surface.

An advantage of the so-called method of"oxide alloying"is intensification of reduction- oxidation and transfer reactions which take place at a higher temperature at the metal- solution phase interface and results in formation of practically nonporous protecting coating.

Microchemical analysis of films by layers revealed that as a protecting coating recedes from the base it is enriched to a large degree by alloying elements.

Spent molybdate-fluoride solutions have an intensive blue coloring caused by "molybdenum blue"which is indicative of an active molybdenum participation in reduction-oxidation reactions and it is a characteristic difference from the solutions described in the GB patent where spent solutions are colorless and display an alkaline reaction.

In molybdate-fluoride solutions exchange reactions at higher temperatures are accompanied by formation and accumulation of minor amounts of difficulty soluble compounds which contain iron and molybdenum. After filtration and settling process

solutions may be used anew with sunsequent correction of the composition, i. e. with addition of minor amounts of initial components.

Experiments carried out to establish possible multiple use of process solutions after their correction have proved that qualitative and quantitative properties of coatings obtained in this case do not differ from those received in"fresh"solutions.

Example 1 After surface cleaning and preparation samples in the form of chrome steel and titanium plates as well as chrome-nickel steel tubular products were placed on a frame and put into an autoclave. A process solution was prepared in a separate container with prior heating of deionized water to boiling and first fluosilicate portion and then a hexavalent tungsten compound were dissolved in the following amounts (gr/1): Na2WO42H2O-10 (in conversion to tungsten 5.5 gr/1) Na2SiF6-0.5 (in conversion to fluorine 0.3 gr/1) An autoclave was filled with the prepared solution. To check air tightness and to avoid boiling up of the solution in the process of heating small excessive pressure of 3-5 kg/cm2 was created, e. g. from an inert gas cylinder. After that the solution temperature was gradually raised up to 300°C. The samples were kept at this temperature within 6 hours, then heating was stopped and the autoclave was cooled. The samples were taken out, washed and dried.

As a result a dark glossy film 4-5 mkm thick without pores was formed on the product surface.

Example 2 Samples identical to those described in Example 1 were put into an autoclave filled with a process solution of the following concentration: Sodium tungstate 40.00 gr/l; Fluosilicate 4.00 gr/1 The solution was heated up to 300-310°C and the samples were kept at this temperature within 4 hours. After washing and drying the samples were visualized and subjected to a metallographic analysis.

The external appearance of the samples and the protecting coating thickness corresponded to those given in Example 1.

Example 3 Samples identical to those given in Example 1 were put into an autoclave filled with a process solution of the following concentration: sodium molybdate 20.00 gr/l; sodium fluosilicate 3.00 gr/1

Samples were kept at 300-310°C temperature within 2.5 hours. Following cooling up to 90°C the solution was discharged, samples taken out and washed. According to the metallographic analysis data thickness of the protecting coating was 3-4 mkm.

Example 4 Tubular samples of chrome-nickel, chrome and carbon steel as well as plates of niobium, zirconium, after respective surface preparation, e. g. washing off in the solution of complexing agents, were put on a frame into an autoclave.

To prepare a process solution water (preferably distilled water) was heated in a separate vessel and first a portion of fluosilicate was dissolved followed by a chrome compound in the following amounts (gr/1): K2Cr207 (potassium bichromate) (in conversion to chrome) 3.5; Na2SiF6 (sodium fluosilicate) (in conversion to fluorine-ion) 0.3.

An autoclave was filled with the prepared solution. To check air tightness and to avoid boiling up. excessive pressure of 3-5 kg/cm2 was created in the chamber, e. g. from an inert gas cylinder.

The temperature was raised to approximately 180°C. Samples were kept at this temperature within 6 hours. After that heating was stopped and the autoclave cooled. Upon reaching 80-90°C the solution was discharged, samples taken out, washed with water and dried.

As a result of the process smooth uniform coating of dark-grey colour was formed on a stainless steel surface, a black one-on a carbon steel surface and a dark one with a greenish shade one-on a niobium and zirconium surfaces. Thickness of the protecting coating on the product surface was from 2 to 4 mkm.

Example 5 Tubular products of chrome-nickel, chrome and carbon steel as well as plates of niobium and titanium were placed on a frame, put into an autoclave and treated in a process solution containing (gr/1): potassium bichromate (in conversion to chrome) 7.0; sodium fluosilicate (in conversion to fluorine ion) 1.8 at a temperature of 320-330°C and a pressure of 120 kg/cm2 within 4 hours.

As a result of the treatment a dark glossy or dull coating 2-2.4 mkm thick was formed on the surface of all samples.

Example 6 Tubular samples of chrome-nickel, chrome and carbon steel as well as samples of titanium, zirconium and niobium were treated in a process solution containing (gr/1): potassium bichromate (in conversion to chrome) 17.50; sodium fluosilicate (in conversion to fluorine-ion) 4.50 at a temperature of 320-330°C and a pressure of 120 kg/cm2 within 1 hour.

As a result of the treatment a dark uniform coating was formed on steel surfaces and a greenish one-on titanium, zirconium and niobium surfaces.

Example 7 Tubular samples of chrome-nickel, chrome and carbon steel as well as plates of niobium and zirconium were treated, as is described in Example 6, in the solution of the following composition (gr/1): A mixture of manganese, tungsten, chrome and iron compounds (at the ratio of components in the mixture 2: 1: 1: 3) 20.0; sodium fluosilicate 3.0.

As a result of the treatment a dark glossy or dull coating of mkm was formed on the sample surface.

Besides the salts mentioned in the Examples LiW04, MgW04, Licr04, NaLiW04, Cr2 (WO4) 3, Cr2 (Mo04) 3, were used as metalliferous compounds and MgTiF6, MgSiF6, Li3AIF6, Cr2 (SiF6), Na3AIF6-as fluorine containing ones.

Example 8 (The method in accordance with GB patent No. 1180951).

Some variants of the known method were tried out. Sodium molybdate solutions containing 20 and 40 gr/1 of salt as well as molybdate solutions with sodium hydroxide at the ratio of 0.05: 0.5; 0.01: 1.0; 0.1: 0.5 gram-molecule/1 were used as process solutions.

Chrome-nickel, chrome, carbon low-alloy steel samples were treated in an autoclave at 280-380°C.

Smooth dark films 1.5-2 mkm thick were formed on the surface of carbon steel as a result of this treatment. Chrome and especially chrome-nickel steels oxidized with more difficulty. At best a bluish film was formed in this case.

Introduction of sodium hydroxide into sodium molybdate solutions significantly improves coating formation rate and at a temperature above 300°C dark films 1 mkm thick are formed on chrome-nickel samples.

Assessment of coating reliability was carried out according to the following parameters: 1. Cohesion strength between a coating and a base Tubular samples of 12 mm diameter and 0.4 mm thickness were subjected to deformation by a metal cone of 1: 10 conicity> Tests were conducted with various degrees of deformation up to breaking the product integrity.

2. Coating resistance to oxidizing destruction Samples were heated within a short period of time in 95% humid air up to 800°C.

3. Thermal resistance in low vacuum Samples were heated in argon for a long time up to 700°C in low vacuum (2 x I o-2 mm of the mercury column).

4. Coating resistance in long storage Tests were performed with alternating daily and seasonal temperature variations within two years.

Test results by parameters are shown in Table 1.

(Product state assessment after tests by 5 points is given in the Table).

I-Coating has practically lost its protecting functions 2-Coating has significant damages 3-Coating has negligible external damages 4-Insignificant rupture of the coating internal structure is noted 5-Coating has no external or internal changes Table 1 Example No. Test parameters and product state assessment after tests 1 2 3 4 Example 1 5 4 4 4 Example 2 5 5 5 5 Example35555 Example 4 5 _ 4 5 Example 5 5 5 _ 5 5 Example 6 5 5 5 Example 7 5 5 5 5 Example 8 4 3 Application of the product surface protecting method proposed herein provides as compared with the existing methods the following advantages:

-possibility to obtain solid nonporous coatings on the surface of stainless steel and alloys without addditional treatment; <BR> <BR> <BR> <BR> <BR> -possibility to obtain simulteneously coatings on products of various dissimilar metals in combination with other materials (titanium, zirconium, etc.); <BR> <BR> <BR> <BR> <BR> -durable protection and reliable preservation of valuable products against corrosion in different climatic conditions; <BR> <BR> <BR> <BR> <BR> -increasing of product reliability and efficiency at higher temperatures in corrosive medium; <BR> <BR> <BR> <BR> <BR> -possibility to control properties of coatings by introducing some alloying metals into saltsolutions; <BR> <BR> <BR> <BR> <BR> -significant widening of the list of products to be used for protecting coating application; <BR> <BR> <BR> <BR> <BR> -possibility to obtain protecting coatings with minimal adhesive capacity in relation to various materials.