DE2210720A1 - Blackening bath - contains iron sequesterant to give intense black oxide coating on ferrous metal - Google Patents

Abstract

Bath has compsn. (by wt) - 32-55% alkali metal hydroxide, 1-8% alkali metal nitrite, 0.5-6% alkali metal nitrate, 0.05-10% iron sequestrant, bal. water, the sequestrant being chosen from alcohol amines, sorbitol and its derivs, metal gluconates and derivs, metal nitriloacetates and derivs. Pref. sequestrants are triethanolamine and sodium (hepta)gluconate used in amts. 0.05-2%. The sequestrants can be used to regenerate spent baths and gives increased corrosion resistance of coatings, lower operating temps. and ability to coat a wide range of alloys.

Description

blackening agent The invention relates to a blackening agent, and in particular, improved chemical agents and their use in manufacture of black oxide coatings on ferrous metals.

It is already known that protective, black oxide silver coatings on Ferrous metal items are produced when these items are in highly alkaline Solutions or salt baths based on alkaline metal salts, in particular based on nitrates, nitrites and hydroxides (US-PS 3,085,035 and US-PS 2,960,420).

So that these and other similar previously known baths work satisfactorily, they must be kept at relatively high temperatures that are sufficient can be to affect the properties of heat treated articles.

These baths also have to work in a much narrower temperature range are kept as the agents according to the invention. Furthermore, the known means often limited in terms of the alloys they can black.

Well-known baths can only tolerate small amounts of iron before they turn red There are deposits on the surfaces of the treated articles, and they are after Olonomic facial spots are less desirable because they are more chemical Have consumption.

The invention is based on the object of specifying means, the oxide coatings result in better depth of color and better corrosion resistance demonstrate. The blackening agents according to the invention should either be dry or as Liquid can be stored, in the latter case the mixing of highly alkaline Powder with water and the associated risk of boiling over or a other danger is avoided. The blackening agents according to the invention are intended as liquid baths have a greatly increased lifespan compared to known baths to have. Furthermore, the agents according to the invention should be used in a much larger temperature range be applicable and still achieve good results, with the blackening times are relatively shorter than in known baths. After all, through the Invention blackening agents are specified that are useful in baths that at lower temperatures than known baths, for example in the region of 134 ° C up to 160 ° C or 11 to 33 oC lower than the working temperature ranges known Baths, can be used.

This is achieved according to the invention by means that essentially consists of sodium hydroxide, sodium nitrate and sodium nitrite, triethanolamine or other specified additives are added, the agents as dry Mixtures or as solutions can be stored in water.

The inclusion of an iron scavenger or a combination of iron separating agents in a strongly alkaline bath, the sodium nitrate and sodium nitrite contains, gives a better black oxide coating on ferrous metals. It it is important that a separating agent (sequestrant) that special iron separates in highly alkaline media, used in the present invention will. It has been shown that separating agents, these in highly alkaline media at bath temperatures above 134 oC deposit according to the basic idea of the invention, certain alcohol amines, sorbitol and its derivatives, certain metal gluconates and theirs Derivatives, certain metal nitrile acetates and their derivatives and combinations of the foregoing Means are. Examples of iron separating agents useful in the invention are triethanolamine, sodium gluconate, sodium heptagluconate, sorbitol, diethanolamine, Monoethanolamine, sodium nitriloacetate etc

Of these remedies, triethanolamine is the most suitable and due the bath stability, the absence of foams and the better separation of iron the preferred separator, cations other than sodium are also effective. The alkanity of the bath can be achieved with sodium or halium hydroxides. The superiority of the black oxide coatings that results is shown both in the icu vision as well as in the icorrosion resistance. The superiority lies also at the lower working temperatures and in the greater variety of iron alloys, which can be treated with success.

The use of the agents according to the invention is in liquid form an advantage over known means, because a means approaching the working bath has the advantage that additions can be made at the optimal bath temperature, without the risk of boiling over or other dangers. This latter Consideration is important because the bath is used at high temperatures.

Although sodium salts are referred to as preferred in the following examples it is possible to use potassium or lithium salts instead.

The broad ranges of the three salt components based on weight the actual blackening bath, d. H. including of the water and the iron separator are as follows: Sodium hydroxide 32-55 percent by weight Sodium nitrate 1 - o percent by weight sodium nitrite 0.5 - 6 percent by weight.

The said composition is used when it is made by adding water is brought to 100 ß, as a blackening bath for many ferrous metals. It has however, it has been shown that its effectiveness is enhanced by the addition of an iron separating agent, especially triethanolamine, can be greatly expanded.

Although triethanolamine is the preferred release agent in the inventive The following iron removal agents can also be used: Diethanolamine, monoethanolamine, sodium gluconate, sodium heptagluconate, sodium nitriloacetate and sorbitol.

Rs can contain up to 10% of an iron separator as an additive alone or used in a mixture of two or more components. For triethanolamine, Sodium gluconate or sodium heptagluconate is sufficient in a concentration of 0.05 to 2 p. For other isene deposition additives, the concentration should be in the range between 1 and 10%.

If iron separating agents, as indicated above, to the alkaline ones Blackening baths of this invention are added, the following advantages are compared known baths achieved: 1. A more intense black coloration, 2. Coating with increased corrosion resistance, 3. lower working temperature ranges, 4. greater simplicity and safety in loading and adding additives in the tanks, 5. the possibility of coating alloys in a larger area, and 6. wider operating temperature ranges.

Table I gives examples of bath compositions.

Table I Bath No. 1 2 3 4 5 6 7 8 9 10 Component by weight NaOH 32 36 36 36 36 36 36 38 46 55 NaNO2 6 8 8 4.5 4.5 4.5 4.5 7 1 4 NaNO3 4 2.5 6 2.5 2.5 2.5 2.5 0.5 2 0.4 additives * 1 0.7 0.7 2.0 0.05 10 1.0 0.7 0.5 0.7 water 57 52.8 49.3 55 56.95 47 56 53.8 51.9 38.5 * Additive baths 1, 2, 3, 8, 9, 10 triethanolamine Baths 4, 5 total amount of triethanolamine, sorbitol, sodium gluconate or sodium heptagluconate (alone or in mixtures) Baths 6, 7 total amount of diethanolamine, monoethanolamine and sodium nitriloacetate (alone or as mixtures) The common The properties of the additives in the baths given above are that of any separating agent Iron in a highly alkaline solution binds, deposits or thus organometallic complexes Forms connections.

Preferred compositions of the present invention boil at Temperatures between about 134 OC and 160 OC and contain in percent by weight: Sodium hydroxide 34 - 38/4 sodium nitrite 4.5 - 6.0 'O sodium nitrate 2.2 - 4.0 c, l and 0.05 to 2 i triethanolamine additive and water to supplement 100 '^ 0.

The composition given immediately before, in the case of triethanolamine is used as a separating means, the preferred embodiment of FIG Invention.

In practicing the invention, a mixture of the dry Ingredients (sodium hydroxide, sodium nitrite and sodium nitrate) and can then be stored as a separate product or can be mixed with the required amount of water be mixed to achieve the desired bath concentration. The triethanolamine or the other separating agent is added up to the solubility limit. then it can be added in increments as the bathroom wears out.

As stated above, the manufacture and storage of these forms Compositions in liquid form are of considerable benefit to the user there is no need to dissolve a strong alkali in high concentration. hen the invention Compositions are used in liquid form, the user need only Pump the liquid into the tank and raise it to the boiling point before the articles to be blackened.

In test trials with working blackening baths, there is an improvement the quality of the blackening is already noticeable with the addition of 0.09% triethanolamine. A very considerable improvement was observed with the addition of 0.5% triethanolamine. However, when 2% triethanolamine was used, soot developed which did develop easily removed by conventional cleaning.

Previously used baths based on known baths contaminated with iron and which have reached the end of their service life as a result usually brownish red. These baths were brought back to working order, with the red-brown color was eliminated by adding triethanolamine or another of the specified iron removal agents.

The blackening bath is used in the usual manner; Ii.

once the components have been brought together, the bathroom becomes lJis heated almost to boiling point. Then the articles to be blacked out are put into the heated composition immersed and held in it to the desired depth blackening is reached. Upon removal from the bath, the items are in air dried and then rinsed to remove residual salt.

Claims (11)

P A T E N T A N S P R Ü C H E 1. Blackening agents for articles containing iron, characterized by 32 - 55 percent by weight alkali metal hydroxide, 1 - 8 percent by weight alkali metal nitrite, 0.5 - 6 percent by weight alkali metal nitrate, 0.05 - 10 percent by weight iron separating agent and water to supplement to 100%. 2. blackening agent according to claim 1, characterized in that the iron separator in highly alkaline solutions at temperatures above 134 oC is stable, iron introduced into the solution can precipitate and out of the group is selected, the alcohol amines, sorbitol and derivatives, metal gluconates and derivatives, Metal nitriloacetates and derivatives and combinations of these agents. 3. blackening agent according to claim 1 or 2, characterized in that that the separating agent is selected from the group consisting of triethanolamine, sodium gluconate, Sorbitol, monoethanolamine, diethanolamine, sodium nitriloacetate, sodium heptagluconate and combinations of these agents. 4. blackening agent according to claim 1 or 2, characterized in that that the alkali metal salts are sodium salts. 5. blackening agent according to claim 4, characterized by 34-38 % Sodium hydroxide, 4.5 - 6% sodium nitrite, 2.2 - 4.0% sodium nitrate and 1 - 10 % Iron separating agent. 6. blackening agent according to claim 5, characterized in that the iron removal agent is one of the group consisting of sodium gluconate, sodium heptagluconate and triethanolamine and that its proportion is 0.05-2%. 7. S blackening agent according to claim 2, characterized in that the proportion of iron separating agent is 0.05-3.35 percent by weight, and that the Iron separating agent is selected from the group consisting of sodium gluconate, sodium heptagluconate, Has triethanolamine and combinations of these agents. 8. blackening agent according to claim 2, characterized in that the proportion of Zisenabscheidmittel is 0.35-0.70 percent by weight, and that the iron separating agent is selected from the group consisting of sodium gluconate, sodium heptagluconate, Has triethanolamine and combinations of these agents. 9. blackening agent according to claim 2, characterized in that the proportion of iron separating agent is 0.70-10.0 percent by weight, and that the iron separating agent is selected from the group consisting of triethanolamine, sodium gluconate, Sorbitol, monoethanolamine, diethanolamine, sodium nitriloacetate, sodium heptagluconate and combinations of these agents. 10. Process for extending the useful life of a blackening bath, that has lost its ability to blacken ferrous metal objects, and which is essentially dissolved from sodium hydroxide, sodium nitrite and sodium nitrate consists in water, characterized in that the bath up to 10 percent by weight an iron separating agent is added which is selected from the group which Sodium gluconate, sorbitol, monoethanolamine, diethanolamine, sodium nitriloacetate, Has sodium heptagluconate and triethanolamine. 11. The method in particular according to claim 10, characterized in that that up to 10 percent by weight of an iron separating agent is added that consists of selected by the group is, the sodium gluconate, sodium heptagluconate and triethanolamine.