DE2218175A1 - Process for improving the resistance of ferrous metal objects to corrosion - Google Patents

Description

2218175 8 MUNICH "22 1.0 ..'... April. IQ72

r - ^ THIERSCHSIRASSE 8

DR-BRANDES ₇ DR ₁ -INCHELd TELEPHONE: (0811) 293297 25/2

Dipl.-Phys. Wolff

Registration number. 123 357

CHROMALLOY AMERICAN CORPORATION, 120 Broadway, New York,

New York, U.S.A.

Process for improving the resistance of ferrous metal objects against corrosion.

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The invention relates to a method for improving the resistance of ferrous metal objects Corrosion by applying a protective coating, in which the object to be protected with the formation of cise aluminide is thermally aluminized.

In particular, the invention relates to a method of improvement the resistance of objects made of ferrous metals, the effects of strong salty atmospheres and / or the Exposed to marine atmosphere by applying a protective Coating that is produced by thermal aluminizing and the application of a non-metallic coating layer.

It is well known that parts of nozzles and gas turbines and compressors in a highly salty atmosphere to the Air inlet openings / easily corrode and that these parts are also often exposed to direct contact with abrasive particles, for example coral dust. In addition, for example, compressor blades or blades are strong mechanical Are exposed to stresses due to centrifugal forces, thermal shock, vibrations and others Sources of tension. Corrosion that also occurs can lead to catastrophic consequences as corrosion centers E.g. corrosion holes and other corrosion effects such as stress centers can act.

For the manufacture of compressor blades and other machine and nozzle parts, for example in the aviation industry therefore, as is generally known, high-strength iron alloys are used (for example those with the alloy designations AMS 5508, AMS 5616 and AMS 6304 of the American Society of Automotive Engineers). Due to their low resistance

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en
against corrosion caused by salts /, however, these alloys are generally coated with a protective layer. A method of protecting the metal objects is to protect the iron objects
a so-called pack aluminization at coating temperatures
of up to about 538 ⁰ C, with on the too
protective object is applied an aluminum protective layer produced by a kind of diffusion process. Preferably, no temperatures above about 5 38 C are used in order to avoid undesired crystallographic or metallurgical changes occurring in the substrate to be protected during the coating process, which could have an adverse or undesirable effect on the mechanical properties of the parts to be protected. Protective layers of this type produced on an aluminum basis improve the resistance of the iron parts to oxidation
and erosion and have further reduced the formation of powdery corrosion products, thereby significantly improving the life of the nozzle components
could.

Although the method described to improve the
Corrosion resistance of the iron parts has led, it has been shown that the achievable improvements in the
Corrosion resistance, especially for nozzle parts
not yet meet all requirements and that it is desirable to further reduce the maintenance and overhaul work required to protect the parts by further improving the corrosion resistance.

The object of the invention is therefore a method for improving the resistance to ferrous metal objects Indicate corrosion, which makes it possible to

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to protect exposed parts in such a way that the geic.hr of occurring corrosion is further reduced.

Ls has been found to increase the resistance of objects made of ferrous metals to significantly increase corrosion by adding a so-called protective Duplex coating protects, which is composed of a thermal aluminizing in combination with a special non-metallic top coat.

The invention was based on the surprising finding that there! ^1st the corrosion resistance of metal objects can be unexpectedly improved by first thermally aluminizing the objects and applying an adhesive, non-metallic coating of a silicate salt as the most essential component to the surface produced by aluminizing.

The invention therefore relates to a method for pre-improvement the resistance of ferrous metal objects against corrosion by applying a protective coating, in which the object to be protected with the formation of Iron aluminide is thermally aluminized, which is characterized in that on the initially aluminized surface of the object at least one layer of sodium, potassium, Lithium or ethyl silicate is applied.

The invention also relates to the protected ferrous metal objects produced by the process of the invention.

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The improved resistance to corrosion that can be achieved by the method of the invention is apparently a synergistic effect between the layer produced by thermal aluminization and the one not due to the metallic cover layer, the layer produced by thermal aluminization being replaced by an intermetallic Eiseri-Alumiriidphase is marked.

From US Pat. No. 2,529,206 it is known per se that applied to the surface of iron objects Alkali metal silicates are able to improve the resistance of ferrous metal objects to corrosion. It has been found, however, that this manner of treating the ferrous metal objects does not lead to the desired corrosion resistance required for the conditions to which nozzle machine parts are exposed, which are in operation in particularly corrosive, salty environments are.

Surprisingly, experiments showed that the intermetallic iron-aluminide phase, which is obtained by pack aluminization, contains iron Articles in a bed containing essentially alumina, some aluminum powder, and a small but effective one Amount of a halide, e.g., AlCl, and the like, characterized in that it is easily a silicate liquid, for example an aqueous solution of a silicate such as sodium silicate able to absorb. It is believed that the good affinity of the layer produced by aluminizing for the silicate is based on the physico-chemical character of the surface produced by aluminizing. So it was found that the iron aluminide produced on the surface of the objects to be protected has a particularly high upper

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area, a high degree of chemical purity and chemical activity. Hs is therefore assumed that the properties described for the occurrence of the synergistic effects are responsible, which lead to the unexpectedly favorable improvement in the corrosion resistance to lead. Under a thermally aluminized surface or a thermally aluminized surface is to be understood here as such a ferrous metal surface in which, through thermal diffusion of aluminum, iron aluminide has been generated.

According to an advantageous embodiment of the invention, a solution of a soluble silicate is applied to the thermally aluminized surface of the object to be protected, expediently at a temperature of up to about 10O ⁰ C, for example at temperatures of 70 to 95 ⁰ C, whereupon the excess liquid from the Surface is removed, which can be done, for example, by blowing with air, wherein a uniform layer of the silicate salt is produced, whereupon the layer is dried on the surface.

Whether the surface produced by aluminizing itself has a certain resistance to corrosion caused by salt, as a typical salt spray test shows that corrosion products form after a test duration of about 15 hours, whereas when the In the method of the invention, the articles subjected to the test withstood test times of 200 hours and more.

In particular, they are suitable for carrying out the method of the invention Silicate solutions, for example a wide variety of sodium silicate solutions. For example

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Prepare suitable silicate solutions for carrying out the method of the invention from solutions of 50 to 100 % concentrations of Na2U «3.22 SiO ^. Furthermore, a wide variety of other sodium silicates can be used, for example, for the production of silica solutions suitable for carrying out the process of the invention, for example for the production of solutions of 40, 45, 47 and 50 ° trees. Potassium silicates, lithium silicates and organic silicates such as ethyl silicate can be used in a corresponding manner.

A particularly advantageous solution for creating a uniform Layer or pre-layer on the intermetallic iron aluminide phase consists for example of a solution with 0.05 to 2 wt. I SiC 4 equivalents, for example one Solution of a soluble silica in the form of Na2Ü 3.22 SIOt The temperature of the substrate is preferably at Apply the silicate solution at around 70 - 95 ° C.

A particularly advantageous method for applying the silicate solution to produce the layer or preliminary layer consists in bringing the thermally aluminized object, for example a compressor blade, into a tank which is filled with the silicate solution and which is at a temperature of about 70 to 95 ° C having, the object to be protected being left in the bath until the object has reached the specified temperature, so that good absorption of the solution from the aluminized surface is ensured. The excess liquid can then be blown off with air, for example, after which the coated object can be dried. This process can be several times wi ^e be repeated until to achieve the desired layer Turk: It has been shown that ER in the above-air drying technology for a short dive times

2 O 9 8 U ^9/1 Π 3 1 BÄD

are necessary in order to achieve a continuous silicate layer structure. Keeping the object to be protected for too long in the bath can possibly lead to the applied layer being dissolved or dissolved again in the solution will.

In order to improve the wettability of the object to be coated, The coating solution used for coating can be an anionic surfactant or wetting agent may be added, for example a phosphate-based anionic surfactant such as des Triton QS-30 (manufacturer Rohm & Hass, USA).

According to a further particularly advantageous embodiment of the invention, which leads to particularly stable silicate layers, the applied silicate layers are heated or hardened, which can be done, for example, in an oven. For heating or curing of the silicate layers in an advantageous manner with infrared ovens or heated air circulation furnaces can be used, in which the layers are heated to temperatures of about 150 to 43o C ^0.

According to the method of the invention, for example uniform layers of silicate about 0.0254 cm / + thick produce. When applying the silicate layers, it is expedient to ensure that the formation of areas is excessive Silicates on the surface of the object to be coated is avoided as far as possible.

It has proven to be particularly advantageous initially to carry out at least one type of precoating using a dilute silicate bath, which is 0.05 to 2

/ + (e.g. 0.0001 inch ") and above

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_{Contains 2} equivalent by dipping the object several times into the bath, drying it and curing it in the manner described, followed by at least one spray coating using a more concentrated solution, e.g. 2.5 to 17.5% by weight ( for example 6.8% by weight) SiO ₇ equivalent.

The main advantage of hardening an applied layer of silicate, where / by applying a plurality of layers is made possible, consists in the fact that the hardened coating can survive, for example, 10 oxidation-corrosion cycles, one cycle being to heat the coated article to about 538 ° C for one hour and then to about 5 hours Subject to salt spray test. In contrast, the one described offers an item made from stainless steel intermetallic layer without silicate coating only one

from

Protection for about 3 cycles, with occurrence / pitting in the to be protected Stainless steel item.

By applying the silicate coating and hardening it, for example by heating to 205 ^° C., greatly improved protection is achieved, in many cases only extremely small amounts of corrosion products (sacrificial products) after 10 cycles (heating to 538 ^° C. and carrying out the salt spray test ) were found.

The tests described are eminently suitable for determining the quality improvement produced by a silicate coating before further coating treatment is carried out.

The corrosion resistance of the intermetallic layer can optionally be further improved by

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that the object to be protected is subjected to a so-called conversion coating with at least one hardened silicate layer (conversion coating). Following such a conversion coating, the layer produced in this way can be applied again a layer of silicate can be applied.

The so-called conversion coating can be carried out by customary known methods, for example by spraying on corresponding coating solutions, for example spray guns or spray devices with piston compressors (reciprocating guns) can be used.

. After the so-called conversion coating further silicate solution may then be applied, for example, one having 6.8 wt ° ä ^ SiO -A'quivalent that 0.002 wt advantageously example -.% Of an anionic surfactant composed of a phosphate compound may contain, wherein such a solution can be sprayed onto the layer produced by the conversion coating. The surface temperature of the layer produced by the conversion coating is expediently not above 65 C. After the silicate layer has been applied, it can be hardened again, for example by heating to temperatures of about 150 to 430 ^° C., with hardening being carried out in an infrared-heated oven, for example and can take, for example, 10 minutes. On the other hand, the layer produced by the conversion coating can also be coated by immersing the object in a hot silicate solution, for example sodium silicate solution. This solution can advantageously have a temperature of about 82 to 93.degree. For example, silicate solutions, for example sodium silicate solutions with a concentration of 0.9 to 2.4% by weight SiO2 equivalent with 0.002% by weight of an anionic surface-active compound consisting of a phosphate, can be used for the coating.

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The object to be protected is immersed in the bath until it has reached the desired temperature, whereupon it is removed from the bath and excess liquid is quickly removed, for example by means of an air jet. The object can then be immersed again in the bath, pulled out of it and dried with air. This method can advantageously be repeated a third time. It has proven to be useful to remove excess liquid in order to prevent foaming during the hardening process. The purpose of repeated dipping and drying is to ensure that a uniform coating is achieved on the surface. The hardening is advantageously carried out at temperatures of, for example, 42 7 ^° C. in an oven, for example an infrared-heated oven.

An advantageous, simple method of applying more uniformly Silicate layers consists, for example, in the following process:

Compressor blades made of steel (type AMS 5616) were fastened in a frame, which i & iSxä8KX »tX« «X into the silicate bath was dived. Immediately after the rack was immersed in the solution, the rack came out of the solution again pulled out. The adhering liquid was allowed to drain off, whereupon the frame was renewed after about 15 seconds was immersed in the solution. After the second dip, an annular stream of air with an air pressure of about

2
4.90 kg / cm directed at the frame, whereby excess liquid was vented. For this purpose, the frame was moved by the air flow so that all parts of the leaves were

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Schüssiger solution could be freed. The procedural measures described formed a cycle. To achieve the desired silicate layer, the cycle described was repeated three more times, so that a total of four process cycles were carried out. If necessary, a jet of air can be directed at the base of the blades to remove excess liquid. After carrying out the fourth cycle, the blades were freed from moisture by drying, which can be done, for example, by blowing air on them. The temperature at which the silicate layers can be applied can vary. Especially advantageous has proved in this case to work at temperatures of about 70 to about 90 ^0C. After the four cycles have been carried out, the coating produced can be cured on the dried blades, which can be done, for example, by placing it in an infrared-heated oven which has been heated to a temperature of 427 ° C., for example.

As already stated, the synergistic effect is quite evident through the iron aluminide of the aluminized Surface and the applied silicate coating reached, so that the tendency of the aluminized surface to form of corrosion products to corrode (to corrode sacrificially) is noticeably inhibited, whereby the service life the protective coating that produces corrosion products (sacrificial coating) in an unexpected manner in a saline environment is extended, compared to the life of such objects, which only have an aluminized surface exhibit.

The implementation of a conversion coating, through which the service life of the aluminized and ompr Si 11 c; it coating provided objects can be increased even further,

can be done using solutions in much the same manner as described for the silicate coating became.

Advantageously, suitable for carrying out the conversion coatings solutions containing 5 to 30 wt -.% Th from phosphoric acid, in particular from 10 to 30 parts by Hen of phosphoric acid, 3 to 8 parts by weight Hen of chromic acid (CrO ₃₎ as well as at least one metal forming a phosphate and a chromate and the remainder of water.

Particularly advantageous solutions for the implementation of conversion coatings therefore exist, for example 5 to 30 parts by weight, in particular 10 to 30 parts by weight of phosphoric acid, 3 to 8 parts by weight of chromic acid (CrO,), 0.0235 to 3 Hen by weight of aluminum, 0.75 to 6 Hen by weight of magnesium and the remainder of water. In an advantageous manner the solution can optionally also contain a surface-active agent.

A particularly advantageous solution for the implementation of conversion coatings has, for example, the one below specified composition:

Weight I

Phosphoric acid 15.0

powdered aluminum 0.225

Chromic acid (CrO ₃ ) 5.0

Magnesium shavings 1.5

a non-anionic surface-active compound consisting of the condensation product of ethylene oxide and
an alkylphenol
(Triton X-IOO, manufacturer Rohm & Hass) 0.1

Water 78.175

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Aluminum and magnesium are thereby dissolved due to the presence of free acid. In the conversion coating, the blades provided with a silicate coating are advantageously sprayed, dried and cured on both sides at the same time, whereby the same oven that is used for curing the silicate layers can be used, with the blade surfaces again, for example, to a temperature of 427 ⁰ C can be heated after the Ilärtungsvargang the blades can be cooled prior to application of the next layer. Spraying on, heating or hardening and cooling represent a cycle. It has proven to be useful to carry out the conversion coating in three cycles.

The conversion coating described enables uniform surface layers to be achieved which have an excellent Offer oxidation-corrosion protection without the need for additional surface treatment or Surface treatment exists.

When performing a wide variety of silicate and conversion coatings in the manner described, for example, layer thicknesses of up to 0.000254 cm and more can be achieved.

The following examples are intended to illustrate the invention in more detail.

example 1

A group of compressor blades was first aluminized. For this purpose, an aluminizing pack was first produced from 363 kg of aluminum powder mixed with 91 kg of Al ₇ O. The particle size of both the aluminum powder and the aluminum

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2218775

oxides corresponded to -60 +140 mesh according to the US standard sieve series. 13.6 kg of dry AlCl were then added to the 454 kg mixture at a humidity of preferably not more than 45 % .

The mixture ingredients were mixed together in a vibrating mixer for 5 to 10 minutes. When using Fresh packaging mixes are expediently burned out first, for example by heating for 36 hours to a temperature of, for example, 425 to 44O ° C can be done. When using a pack mix that has already been used however, such burnout is unnecessary. The packing was then placed in a muffle or under dry conditions Retort, with the compressor blades (made of AMS steel 5616) were completely embedded in the packing mix. The muffle was then made by attaching the muffle cover or of the retort lid closed, the two muffle or retort parts by means of several layers of aluminum foil were sealed in the form of a seal. The seal was designed in such a way that air could enter the muffle or the Retort prevents gaseous by-products from escaping from the muffle or the retort.

.However

The muffle was then placed in a room temperature oven which was heated to the desired temperature. When the temperature was increased, an endothermic reaction at -176 ⁰ C due to the evaporation of AlCl? established. The temperature was raised to about 4 25 to 44O ⁰ C, which were about 36 hours lan} adhered '. At the end of the heating phase, the muffle was removed from the furnace and cooled to about 205 C j'clu.sscu, whereupon it was removed from the furnace and left on P; j: imt empeicit ui ablleii.

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The cooled muffle was then put into a room whose moisture content could be controlled, whereupon the cover of the muffle was removed and the compressor blades off removed from the cementation pack. The compressor blades were then removed from adhering materials by blowing over them cleaned in dry air. The compressor blades were then immersed in water to remove dust and other debris to remove. In this way a very clean aluminum deposit, containing an intermetallic iron aluinide compound such as FeAl, visible.

The surface produced by aluminization has like other aluminum coatings produced by thermal diffusion protective properties (sacrificial properties) because these layers are in front of the iron object on which they have been applied corrode in a salty environment and consequently the ferrous metal object Protect against corrosion. It has been shown that the aluminum coatings produced by thermal diffusion absorb slightly dilute silicate solutions, due to the microporosity of the aluminum layers and that these Layers appear to be synergistic with the silicate to form a tightly adherent protective layer or React pre-shift.

After the compressor blades were aluminized, were these? fixed in a frame, after which the frame was immersed in a tank which contained an aqueous sodium silicate solution, corresponding to an SiC ^ equivalent of 0.1 to 2.0% by weight. The temperature of the sodium silicate ^{solution was 70 to 95 °} C. The solution also contained an effective concentration of an anionic surface-active compound obtained from a phosphate. The shovels were long

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immersed in the bath until the blades reach the temperature of the
Bades had accepted. They were then removed from the bath, whereupon excess liquid was blown off with air and the
Shovels were dried.

The process described was repeated several times until the desired layer thickness had been achieved, for example a layer thickness of 0.00127 mm (0.05 mil).

The compressor blades coated in this way exhibited considerably improved resistance to this Corrosion. This resilience could continue atiix can be improved in that the thermally aluminize and silicate blades a hardening process were subjected, for example by placing in an oven and heating to a temperature of about 150 to 43O ° C.

To illustrate the process of the invention
Corrosion protection achieved, several compressor blades (made of AMS 5616 steel), which were coated in various ways, were subjected to a series of oxidation salt spray corrosion tests.

In order to illustrate the effects achievable according to the invention, compressor blades were prepared according to the methods A to G given below, which differ from the method of the invention
distinguish, coated. In addition, some compressor blades were coated according to the method of the invention (tests 1 to 3).

In detail, the following were described in more detail
Compressor blades exposed to testing:

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(A) blades with exposed surface;

(B) blades coated with a hardened sodium silicate coating;

(C) blades with hardened sodium silicate layers and a cured conversion coating;

(D) blades with only one hardened conversion coating;

(E) blades with a hardened conversion layer having about 0.1 weight ⁹ o SiO ₂ equivalent (as sodium silicate);

(F) Steel blades with only one surface aluminized by thermal aluminization with iron aluminide and

(G) Steel blades with a cover layer produced by thermal aluminizing and a hardened one by conversion coating generated top layer.

The blades coated by the method of the invention had the following structure:

(1) The blades had a surface produced by thermal aluminizing and a hardened sodium silicate top layer;

(2) the steel blades exhibited thermal aluminization generated surface which had been coated as described under (C);

(3) the steel blades had a surface generated by thermal aluminizing, followed by conversion coating with a top layer 0.1 - ₉ -XqUivalent was applied as sodium silicate% SiO..

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In the corrosion test carried out, the specimens were heated to 482 ° C. for one hour, after which they were cooled and subjected to the salt spray test according to ASTM B 117-64 for 5 hours. Heating and spray testing formed one Cycle. This cycle was repeated several times, for example ten times or more, to determine the relative quality of each of the produced To determine protective layers.

The device used to carry out the ASTM salt spray test B 117-64 consisted essentially of a fog chamber, a reservoir for a saline solution, a supply line for conditioned compressed air, an or several nebulization nozzles, holding devices for the test items to be tested and a device for heating the chamber and control and control devices. The specimens to be tested were placed in the chamber at an angle of 15 to 30 of of the vertical without contact with each other, preferably parallel to the main direction of the horizontal the chamber moving fog stream. The saline solution consisted of 5 + _ 1 part salt to 95 parts of distilled water that was not contained more than 200 ppm solids. The test device was operated in such a way that the condensed Fog had a pH of 6.5 to 7.2. The temperature inside the chamber was 35 C + 2 or -3 °. The duration of the salt spray test was 5 hours. Care was taken to ensure that no condensed saline solution from a test object dripped onto the other.

The results obtained when carrying out the tests are summarized in the following table:

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«Ο

Oxidation-corrosion tests carried out with compressor blades made of steel (AMS 5616) with and without duplex coating

Attempt corrosion protection

A unprotected compressor blades

B Steel surface with a preliminary layer of sodium silicate, applied from a 1/2 to 1 volume solution (about 0.15 to 0.3% by weight SiO ₂ equivalent) and subsequent hardening

C the blades were like

follows coated: (a) Pre-layer of sodium silicate from a 1/2 to 1 complete solution (about 0.15 to 0.3 wt. I SiO ₂ ) and curing; (b) spray coating with a 25 vol. sodium silicate solution ( 7.3% by weight SiO ₂ ) and curing; (c) spray coating for the purpose of generating a conversion layer * and curing; (d) spray coating with a 25 vol. Sodium silicate solution (7.3% by weight SiO ₂ ) and curing

D steel surface with a conversion coating *

E steel surface with

a conversion layer '*' with 0.1% by weight »» SiO ₂ equivalent as sodium silicate % corroded surface

100% surface corrosion after the first cycle

25% surface corrosion after 2 cycles and 100% corrosion after the 4th cycle

25lige surface corrosion after the 5th cycle; over 50% corrosion after the 8th cycle and 85lige corrosion after the 12th cycle

Surface corrosion of more than 45 ° after the 3rd

Cycle and over 80Hge corrosion after the 8th cycle

45% surface corrosion after the 3rd cycle and over 90% corrosion after the 8th cycle

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aluminized steel surface with iron aluminide alone

25% surface corrosion after the 8th cycle, 55% corrosion after the 10th cycle and 90 ° corrosion after the 12th cycle

The conversion layer was created using a solution containing 15% by weight of phosphoric acid, 5% chromic acid, 0.2251 dissolved aluminum, 1.51 dissolved magnesium, 0.11 of an anionic surface-active compound based on phosphate and the remainder water (78.175!) contained.

attempt

Corrosion protection

1. Aluminized steel surface as described under F with a top layer made from hardened sodium silicate, made from a 1/2 to 1 total Sodium silicate solution (generated from a solution at 41.5 ° Baume). This corresponds to 0.15 to 0.3 wt. I SiOo equivalent.

2. Aluminized steel surface as described under F with a coating layer as described under C.

3. Aluminized steel surface as described under F with a top layer, obtained by conversion ^{coating "1} ", containing 0.1% by weight SiO ₂ equivalent as sodium silicate

I corroded surface

only 5% of the surface corroded after the 8th cycle and 101 after the 12th cycle

no surface corrosion after the 8th cycle and only Hige surface corrosion after the 13th cycle

no surface corrosion after the 8th cycle, 101 corrosion after the 10th cycle and 251 after the 13th cycle

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S3.

The conversion coating was performed with a solution containing 15 parts by weight I phosphoric acid, 5 wt ^Λ 1 chromic acid, 0.225 part by weight I dissolved aluminum, 1.5 wt -..% Dissolved magnesium and 0.1 I of an anionic surface-active compound Contained phosphate base and the remainder consisted of water (78.175 wt ^{. 9.} ).

In the test (G) that was also carried out, the aluminized surfaces of the steel blades were treated with a Conversion coating using a solution of the composition given in the table above. The results obtained after performing 13 cycles were inferior to the results of the experiments No. 1, 2 and 3.

The curing of the silicate coatings was carried out by heating the test specimens for 10 minutes in an infrared-heated one Oven at 2O5 ° C. The layers produced by conversion coating were hardened in an infrared-heated oven by heating the specimens to 425 ° C for 10 minutes.

As can be seen from the table above, the test specimens which were tested according to tests A to F (outside the scope of the invention) were attacked significantly faster than the test items, which had been coated in the manner according to the invention (experiments 1 to 3). Of particular importance is that when experiment No. 1 (aluminized surface and hardened sodium silicate coating) was carried out a nine time stronger corrosion protection compared to test no. F was achieved with an aluminized test specimen after 8 cycles. from It is also of particular importance that when carrying out tests No. 1 and No. 2, only very little corrosion after the 12th cycle occurred.

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The attached figure shows a 500-fold enlargement of a section through the surface of a test piece, which the Experiment No. 2 was subjected. The enlargement shows the iron aluminide layer and that due to the surface microporosity firmly bonded to this non-metallic top layer.

It has been shown that the so-called "duplex" coating system of Experiment 2, is particularly advantageous. In this system, the non-metallic coating (as in experiment no. C specified) in a series of process stages as follows:

(1) A type of precoat of sodium silicate is applied to the aluminized steel surface by dipping the test specimen several times in a solution containing 0.15 to 0.3% by weight of SiO ₇ equivalent and hardening;

(2) a layer of silicate is then applied by spray coating using a 25% by volume Sodium silicate solution (obtained from a 41.5 Baume solution), corresponding to 7.3 wt. I SiO ^ equivalent, and then Hardening;

(3) applying a conversion layer to the hardened silicate layer and

(4) Spraying an approximately 25% by volume silicate solution (7.3% by weight SiO _? Equivalent) onto the conversion layer and then hardening it.

To use the sodium silicate solutions, 41.5 ° Baum6 strong solutions used.

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If the silicate layers are applied by immersion **, it has proven to be advantageous to use silicate solutions, for example sodium, potassium, lithium and ethyl silicate solutions, the concentration of which is about 0.05 to 2.0% by weight SiO ^ equivalent. If, on the other hand, the silicate solutions are used for spraying on, it has proven to be expedient to use solutions which _{contain about 2.5 to 17.5% by weight of SiO 2} equivalents.

Example 2

According to the method described in Example 1, compressor parts (vane shrouds) made of stainless steel were aluminized with 17 % Cr, 4% Ni, 1% Cu and the remainder iron (stainless steel of the type 17-4PH). With regard to the design of the compressor parts used for the tests, particular care was taken when preparing the parts for pack aluminization. The same cementation pack and cementation conditions as described in Example 1 were used to carry out the aluminizing process. After the pack aluminization, the parts were freed from adhering packing material and then provided with a layer of silicate salt, which was then hardened. A conversion coating was then carried out and hardened again.

The thermosetting or baking of the duplex silicate conversion coating is believed to result in a reaction product which offers a new and improved resistance to corrosion in saline environments.

While the silicate layer or the silicate layers are preferably applied to the surface produced by thermal aluminization

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surfaces are applied before a conversion coating takes place, it is also possible to apply the silicate layers and the conversion layers together, using coating solutions that are intended for the production of conversion layers, but which also contain a silicate of the type described , for example sodium, potassium, lithium or ethyl silicate. As conversion coating solutions can be applied, for example, the I part by weight SiO contain from 0.05 to 2 ₂ "equivalent For example, a conversion coating solution is applied before the spraying, the 0.05 to 2 wt -..% SiO2 ~ equivalent in the form of of sodium, potassium or ethyl silicate.

In the individual case, advantageously in the conversion coating solution soluble amount of silicate depends somewhat on the pH of the solution, which is why the concentration of the in the solution soluble silicates is limited in some respects. However, it has been found that the amount of silicate soluble in the solution is sufficient to improve the corrosion resistance of the ferrous object to be protected, especially since the silicate tends to migrate into the aluminized surface during the coating process and has a kind of separating function there Takes over and reacts with the conversion layer during the curing cycle. In this context, refer to experiment No. 3 referenced, in which the conversion layer applied to the test specimen also contained sodium silicate.

According to the invention, a wide variety of corresponding coating compositions can be used to produce the conversion layers Phosphate chromate type can be used in compounds with the soluble silicate salts. The in the method of the invention

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The conversion coatings produced consist of phosphates and chromates of at least one metal such as Al, Mg, Zn, Be, Ba, Sr and Ce group metals and others Metals. It has proven to be particularly advantageous to produce conversion coatings made from phosphates and Chromates are composed of aluminum and magnesium.

For example, some coating compounds for producing conversion layers are given below:

200 ml of 85 Hgen phosphoric acid (1.6 g / ml) is diluted with water to a pH of 1.01. 62.5 g of chromic acid (991 CrO ₃ ) and 40 g of beryllium phosphate (Be ₃ (P ^) ₂ ) are then added to the resulting solution. The resulting solution has a density of about 1.2 g / ml.

The phosphate-chromate solution obtained can be, for example to create conversion layers by spraying the solution onto an aluminized surface, with hardening temperatures from about 150 to 425 to 482 C can be used. The spraying process can be used in an advantageous manner and repeating the hardening process several times, for example three times or more if necessary.

The beryllium phosphate chromate solution prepared in the manner described can also be used as a starting solution for the production of further coating solutions, which other soluble metal salts can be added.

Advantageously, for example, 0.69 g of another metal salt can be added to 120 g of the solution described will.

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In the following, for example, several conversion solutions are given that are based on the beryllium phosphate described Chromate solution can be prepared:

(1) 0.69 g of magnesium chromate dissolved in 120 g of the described Solution at pH 1.8;

(2) 0.69 g of magnesium phosphate dissolved in 120 g of the solution described at a pH of 1.3;

(3) 0.69 ρ aluminum phosphate, dissolved in 120 g of the solution described at a pH of 2 ;.

(4) 0.69 g of Ba (OH) ₂ - 3H ₂ O dissolved in 120 g of the solution described at a pH value v-on 1.8;

(5) 0.69 g Ce ₂ (CO ₃ ) ₃ * 5H ₂ O, dissolved in 120 g of the solution described at a pH of 1.9;

(6) 0.69 g Ce (PO ₃ ) i »dissolved in 120 g of the solution described at a pH of 1.4.

It has been shown that layers produced and dried and cured by spraying on from the solutions described Steel surfaces result in excellent conversion coatings, as evidenced by performing oxidation and salt spray test cycles.

It has proven to be particularly advantageous to use those conversion solutions which contain at least 0.5% by weight of at least one metal which forms a phosphate or a chromate, for example 0.5 to 10% by weight or 5 to 30% by weight Phosphoric acid, 3 to 8 wt .-% chromic acid (CrO ₃ ) and essentially consist of water.

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Vf

Conversion solutions that have proven to be particularly advantageous are those containing 0.02 to 3% by weight of dissolved aluminum, 0.75 to 6% by weight of dissolved magnesium, 5 to 30% by weight of phosphoric acid (preferably 15 to 30% by weight). -%), 3 to 8% by weight of chromic acid and the remainder consist essentially of water. A special conversion solution consists, for example, of 0.25% by weight of aluminum, 1.5% by weight of magnesium, 15% by weight of phosphoric acid, 5% by weight of chromic acid and the remainder essentially of water.

Silicate salts such as sodium silicate, potassium silicate, lithium silicate are suitable for carrying out the process of the invention and organic silicates such as ethyl silicate.

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Claims (5)

PATENT CLAIMS 1. Process for improving the resistance of ferrous metal objects against corrosion by applying a protective top layer, in which the object to be protected is thermally aluminized with the formation of iron aluminide, characterized in that the initially aluminized surface of the object at least one layer of sodium, potassium, lithium or ethyl silicate is applied. 2. The method according to claim 1, characterized in that is applied to the aluminized surface of the article a solution of the silicates, excess liquid is removed, drying the applied layer of silicate and, optionally, by heating to a temperature of 150 cures to 43o ⁰ C and d
if repeated several times. 150 to 430 C hardens and that this process is given 3. Process according to Claims 1 and 2, characterized in that at least one silicate layer is produced after production this creates an additional conversion layer. 4. The method according to claim 3, characterized in that for the production of the conversion coating the object with "an aqueous solution containing 5 to 30 wt -.% Phosphoric acid, 3 to 8 wt .- $ chromic acid (CrO ₃₎ and at least 0.5 wt -.% of at least one treating a chromate and a phosphate-forming metal, dried and heated. 209849/1031 5. The method according to claim 4, characterized in that an aqueous solution is used to produce the conversion layer
which contains 0.02 to 3.0% by weight of aluminum and 0.75 to 6.0% by weight of magnesium as a chromate and a phosphate-generating metal. 209849/1031