JPH01279787A - Corrosion-resistant coating - Google Patents

Abstract

PURPOSE: To impart high corrosion resistance to a corrosive water-base medium to a metallic structure, by protectively coating a metallic structure contg. substrate metal corroded in a corrosive oxidizing environment with oxide of metallic Ti or the like.

CONSTITUTION: On the surface to be exposed to a corrosive oxidizing environment of a structure, which is the metallic one exposed to corrosive oxidizing conditions in the use and in which the greater part is composed of substrate metal to be corroded, protective coating is formed with tight adhesion. This protective coating is composed of at least one kind of oxides of metallic Ti, Zr, Ta and Nb, and e.g. in the case of TiO₂, it is preferable that coating whose thickness is substantially less than about 100 micrometer is formed and Ni is not contained. In this way, the coating suitable for oxidizing treatment for contaminated industrial water can be formed.

COPYRIGHT: (C)1989,JPO

Description

DETAILED DESCRIPTION OF THE INVENTION This application is a continuation in part of our pending application: 1981
No. 4,483,720, filed Nov. 27, 1984, entitled "Method of Applying Thermal Barrier Coatings to Metals and Resulting Products," No. 1,984.
No. 662,253 filed on October 17, entitled “Method of applying coatings to metals and products obtained”; 19
Application S N662,252 of October 17, 1984, entitled "Method of applying hard coating etc. to metal and products obtained" and Application S N11 of October 21, 1987
1, 210, entitled "Method of Applying Coatings to Metals and Resulting Products 7" This invention relates to a method for preventing corrosion of structural steel exposed to corrosive aqueous environments. Representative examples of such metals and conditions are industrial Treatment of wastewater by air oxidation in the presence of catalysts that remove or reduce toxic components from the wastewater. Such treatment is highly corrosive to structural steel.

It is customary to use Hastelloy for equipment used in such processes, but it is very expensive.

Titanium may be used, but when applied as a melt,
Its high melting point and reactivity are disadvantages.

When applied by the kise method (talading), its use is limited as tarading is difficult or impossible to apply to complex shapes.

It is known to provide an oxide coating that protects alloys against oxidation at high temperatures and/or provides thermal insulation properties to the alloys, eg British Patent No. 1,439,947;
See 1,086.708 and 1,396,878.

For example, oxidation at high temperatures in gas turbines is quite different from corrosion in aqueous media at lower temperatures. Low temperature corrosion in highly corrosive aqueous media does not require thick coatings and would benefit from impermeable coatings if uniformity is not required.

We believe that if titanium dioxide is properly applied to structural steel,
It has been found that it provides a high degree of corrosion resistance in highly corrosive aqueous media. Other metal oxides such as zirconium oxide, tantalum oxide or niobium oxide may be applied as well to produce similar results. Mixtures of two or more of these oxides may also be used.

Examples of base metals for which titanium dioxide is commonly used are pressure vessel steel,
Carbon steel, tool steel, etc.

The following description describes the application of titanium dioxide.

It will be appreciated that oxides of zirconium, tantalum or niobium may also be used, as described above, and that mixtures of two or more oxides may also be used.

The titanium is applied in the form of an alloy with a metal that serves to bond the titanium to the base metal, such as iron, nickel or cobalt, preferably nickel. The alloy is preferably preformed, i.e. it is made of titanium Although applied as an alloy of pulverized titanium and bond metal, a mixture of finely ground titanium and bond metal may be applied and heated to form an alloy in situ.

Suitable binder metals include nickel, cobalt, steel and iron, with nickel being preferred.

The proportions of titanium (or substitute metal) and binder metal, represented by Ml and M8, respectively, range from Ml 90% to 1
Preferably the proportion of Ml is greater than 55%, which can be in the range of 0% (the remainder Mz).

All percentages are weight percentages.

The alloy of M and M can be applied by dipping the metal substrate into the molten alloy, or the alloy of M in fine powder form can be applied by dipping the metal substrate into the molten alloy.
, /M, the metal alloy can also be applied in the form of a slurry in a volatile solvent. Such slurries are disclosed in U.S. Pat.
483,720 No. 41! It is described in lines J15-36. The alloy can also be applied by plasma spraying, vapor deposition or flame spraying.

As mentioned above, the M 1/M z metals can be applied as preformed alloys or as mixtures of individual metals, for example by slurry coating methods, and can also be formed into alloys in situ by heating.

After applying the alloy coating to the base metal, it is preferably annealed by heating. The zero-order annealed coating is subjected to selective oxidation at elevated temperature, with the oxygen partial pressure P However, the bonding metal Ml is such that it is not oxidized.

The procedure described in US Pat. No. 4,483,420 can be used.

This results in selective oxidation of M+ and results in a coating consisting of an outer layer of an oxide of M, such as Ti0z, bound to the base metal and the intermediate interaction zone by an inner layer of M8, such as nickel.

Alternatively, the alloy coating may be anodized, for example by a purpose-designed electrochemical process or by the conditions of use, in the latter case the metal with the alloy coating is exposed to an oxidizing environment during use, and metal M,
will undergo oxidation.

That is, it is sufficient to apply the alloy and then use the coated metal for its intended purpose, for example as a container, pipe or tube in an industrial process, such as the treatment of industrial wastewater by air oxidation in the presence of a catalyst. . This will expose the coating to oxidation to convert the Ti to T i O*, thus forming a protective layer of oxide.

However, it is preferred to oxidize the titanium to titanium dioxide by selective thermal oxidation; such oxidation selectively oxidizes the titanium and also transfers M8 metal, and iron extracted from the substrate, into the substrate. will drive towards M.
A thin skin coating of Ti01 without gold metal can be applied.

Such a coating is preferred because it is less prone to failure or spalling due to differences in the coefficients of thermal expansion of the coating and the substrate than thicker coatings. The difference is less destructive in thin coatings than in thicker coatings, with M1 oxide coatings no thicker than about 100 micrometers being preferred.
The absence of M2 metal from exposed surfaces is also advantageous. This is because it is attacked by a corrosive environment. Ml
If present on the exposed surface, this leaching will result in a porous coating, which will be further attacked and the base metal will be attacked.

Use of eutectic or near-eutectic alloys with lower melting points than the base metal to avoid melting or other destructive effects on the base material surface and undesirable degree of migration of base material components into the protective coating. is preferred. Also, eutectic alloys deposit a solid phase of uniform composition when they solidify.

A suitable oxidizing atmosphere is a cot/co mixture which undergoes the following equilibrium reactions at elevated temperatures.

(1) Co□dekoCot + 1/20sH O/H
30 mixtures are also suitable, which undergo the following equilibrium reactions at elevated temperatures.

(2) HIO tantit +1/20g However, preferably the oxygen atmosphere is provided by using a noble gas such as argon containing a very small proportion of oxygen. This avoids the formation of undesired hydrides.

As mentioned above, various methods of applying the alloy coating may be used. Slurry coating by dipping, spraying or brush chambers has been found to be particularly preferred for large or complex geometries, such as inner surfaces of tubes, tubes and tanks, and reaction vessels.

Application of the alloy coating and annealing of the coating are preferably carried out in an inert atmosphere such as deoxygenated argon.

If dip coating is used, it is preferable to heat the metal before dipping to avoid or minimize cooling of the alloy; in dip coating, the dipping time is long enough to apply a uniform and smooth coating. preferably, but not so long as to extract large amounts of metal from the substrate.

The temperature used in selective thermal oxidation should be high enough to avoid oxidation of metals other than M1.

If the coating is by a slurry method, the coated substrate is first heated to evaporate the solvent and M + /M
The zero-order alloy coating is annealed and selectively oxidized by melting the z-alloy to form a continuous surface coating.

The surface to be coated is preferably cleaned before coating by ultrasonic cleaning, for example with acetone, and then air dried;
It is then soaked in HCI solution to remove surface oxides and then rinsed with deionized water.

For coating by dipping the substrate into the molten alloy, it is preferred to bring the temperature of the substrate to or near that of the molten alloy. If permitted, this may be accomplished by holding the substrate over the molten alloy for a sufficient period of time to bring it to or near the temperature of the alloy. Such a procedure minimizes metal extraction from the substrate. In the typical case, the substrate is held above the molten alloy at 1150'C for 2 hours;
It was then dipped into the alloy for 15 seconds, then removed and held on the molten alloy for 3 seconds and re-dipped for 15 seconds.

The following example provides a detailed explanation of the invention. Temperatures are in degrees Celsius.

The air in the chamber, which was sealed except for the gas line, was replaced with argon. The temperature was raised to 1000°C or more. This chamber contains a molten eutectic alloy (Ti-28,5Ni,
(i.e., 71.5% Ti and 28.5Ni), all percentages are weight percentages.

The room was heated electrically. The samples (A315 degree plain steel in one case, 612 carbon steel in the other case) were gradually lowered into the molten alloy, dipped for 10 seconds, withdrawn, and similarly dipped and withdrawn again. Annealing was performed by holding the sample above the molten alloy in a chamber.

The coated samples were removed from the immersion chamber and placed in another chamber where they were exposed to an atmosphere of argon containing oxygen at a partial pressure of IQ-16 atmospheres for 5 hours.

1. In the immersion apparatus described in Example 1, the sample was heated to a TI of 1150"C.
28.5Ni alloy melt for 2 hours, then 1
Immersed for 5 seconds, then removed and annealed on the melt for up to 1 hour, then the furnace was cooled to room temperature. Oxygen partial pressure is 1
The pressure was maintained below 04S atmospheric pressure.

Examination of samples coated in this way showed four distinct areas in the following order: (1) fil substrate; (2) pearlite-free area; (3) interaction area and (4) dip coating. y1
layer.

Table 1 below shows the corrosion results of two types of steel respectively coated with Ti-Ni alloys. The letters "0" and "D" represent the thermal oxidation of the coating and the as-is dip coating, respectively. i.e. ``O'' coaching is 1150
immersed at °C and p(Ox) 10-' 100 at
The "D" coating was oxidized for 20 hours at 0'C, while the "D" coating was soaked at 1150C but otherwise untreated.

The aqueous test solution is as follows: (1) 0.01 N MCI + 0.099 M
NaCl (2) 0.0 in neutral water. I M NaxS*0
s (3) 0.1 M N13 + Sulfur (4) 0.1 M Na5CN in neutral water (blank below) An annular cycle corrosion test was performed on a similar coated steel sample using the HCI/
using KCI solution and also PH1O,5NH,
Performed using OH solution. The test involved subjecting the coated sample to 20 cycles between 270 and 100'C in an autoclave, with a heating section (100'C) for each cycle.
°C to 270'C) took about 30 minutes, and the cooling part of the cycle (starting with switching off the heat source) was about 5 hours.

The results are shown in Table 2.

(Left below) As described above, in the selective thermal oxidation of the metal M1 of the coating, the temperature and oxygen partial pressure should be selected so that an oxide of only the metal M1 is produced.

The attached diagram shows nickel oxide (NjO) and iron oxide (Fe).
d) and the stability of titanium oxide (T i Ot ). The N-axis represents the logarithm of the oxygen pressure and the horizontal axis represents the temperature.As an example, assuming the absence of iron during the coating of titanium/nickel alloys and other more noble metals such as iron during the coating. Assuming no oxygen is present, an oxygen partial pressure of less than about 10-3 atmospheres at 500'C will ensure that no nickel oxide will form and that the only oxide that will form will be TtOz, whereas 10
At 00'C, an oxygen partial pressure of about 10-16 atm would suffice.The curves in Figure 0 are based on available thermodynamic data and are intended to serve as a rough guide. . If iron is likely to be present, e.g. because it is extracted from the substrate before solidification of the coating alloy, then F
The eO curve should also be considered. Other oxides such as FeTiOs may also be present, but the curves in the figure are nevertheless useful as an indicator.

The selection of temperature and p(Ox) should be well within the ranges indicated by the diagram.

It should therefore be apparent that there has been provided a new and useful protective coating for metals, such as structural steel, that is resistant to corrosion in corrosive oxidizing environments, and that there has been provided a new and useful method of making the coating.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the stability of oxides of nickel, iron and titanium. Agent's name Kawahara 1) - Ho 300 4oo si 700 1
000 2000 U1 Hiki (6C)

Claims (13)

[Claims] (1) A metal structure that is intended to be exposed to corrosive and oxidizing conditions during use, the structure forming a major part of the structure and normally corroding under the intended use conditions. the structure has a protective coating on the surface exposed to the corrosive oxidizing environment, the coating being an oxide of at least one of the metals titanium, zirconium, tantalum and niobium; , the metal structure wherein the coating adheres to the substrate and provides a barrier to corrosive components of the aqueous solution. (2) The structure according to claim 1, wherein the base metal is a ferroalloy. (3) A structure according to claim 2, wherein the base metal is structural steel and the structure is suitable and intended for use in the oxidation treatment of contaminated industrial water. (4) The structure according to claim 3, wherein the metal oxide is TiO_2. 5. The structure of claim 4 wherein the TiO_2 forms a coating substantially no thicker than about 100 micrometers and is substantially free of nobler metals. 6. The structure of claim 5, wherein the thin coating of TiO_2 is bonded to the substrate by a nobler metal. (7) The structure according to claim 6, wherein the nobler metal is nickel. (8) A method of applying a protective coating to the surface of a metal structure intended to be exposed to a corrosive oxidizing aqueous environment, the method comprising: (a) applying an alloy or physical coating of M_1 and M_2 to the surface; M_1 is at least one of the metals titanium, zirconium, tantalum and niobium; M_2 is a metal that produces an oxide with lower thermal stability than M_1;
and (b) heating the applied coating to melt the metal if M_1 and M_2 are a physical mixture, and then causing oxidation of the M_1 component of the coating without substantial oxidation of M_2. Method of applying. (9) A method according to claim 8, in which the M_1 and M_2 metals are applied in the form of molten alloys. (10) Using the M_1 and M_2 metals in a finely divided state dispersed in a volatile liquid, evaporating the liquid and heating the resulting M_1/M_2 coating to
Claim 8, in which the alloy is applied to the surface of a metal structure
The method described in section. (11) A method according to claim 9 or 10, wherein the alloy coating is annealed after it is applied and before it is oxidized. (12) The method of claim 8 in which the alloy coating is subjected to selective thermal oxidation that oxidizes the M_1 component without oxidizing the M_2 component. (13) A method according to claim 8, wherein the alloy coating is oxidized by the environment during its intended use.