CS242433B1 - Silicic acid coating for surface protection of steel and cast iron and method of forming such a coating - Google Patents

Abstract

Pickling refers to a protective coating formed on the surface of steel and cast iron by the action of an aqueous solution containing HjO+ ions, components converting Fe2+ and Fe >+ ions into insoluble compounds, silicic acid sol and substances reducing surface tension, which protects surfaces against rusting due to the action of the atmosphere and at the same time improves the adhesion of coatings applied to these surfaces and their resistance to undercutting. The coating contains from 0.1 to 10 g.m-2 of silicon dioxide, from 0.01 to 1 g.ms2 of phosphorus pentoxide and up to 1 g.m“2 of iron.

Description

The present invention relates to a coating based on silicic acid for the surface protection of steel and cast iron against atmospheric action and to a method for its formation.

Ensuring atmospheric corrosion resistance of mechanically or chemically treated steel and cast iron surfaces is a major technical problem in many manufacturing sectors. It is very difficult to maintain the metal-clean surface obtained by machining, blasting or grinding during further manufacturing operations so that the final product does not have to be rust-free.

Various methods are known to provide the necessary protection of the steel and cast iron surface against rust formation. These include temporary protection using petroleum or wax preservatives, use of contact or firing inhibitors, protection using conversion coatings, especially phosphate coatings, and finally shop primers. The choice of any of these methods results from the aggressiveness of atmospheric conditions during further processing, storage and transport, from the efficiency requirements and from the need to remove the protective coating before further manufacturing operations, or to use it as part of the final protection system. In this respect, the methods used to protect steel and cast iron surfaces against rust have hitherto been disadvantageous. While preservative agents are highly effective in terms of protective function, they require thorough cleaning of the parts with solvents or hot aqueous degreasing agents prior to further manufacturing operations in order to ensure the handling of the parts, not to interfere with welding and especially the required surface cleanliness prior to painting. Deconservation is technically and time consuming

242 433 operation, which creates difficulties in the production of fire prevention, work hygiene and waste disposal. The use of inhibitors is limited to smaller parts. Conversion coatings can be either hot or cold. A typical example of a thermally produced conversion coating is phosphating, which takes place on special production dip or spray lines. The process is multi-stage and its negative features are considerable - space and investment requirements, high energy and water consumption and the need to dispose of waste products * Protective efficiency of phosphate coatings is good, especially when stored in confined spaces. Another alternative is modern cold phosphating processes, which form very thin coatings with an organic binder component. Compared to conventional phosphate coatings, they have lower protective efficacy, but their use is considerably easier to manufacture. In some cases, phosphate coatings make subsequent manufacturing operations, such as welding, more difficult.

Workshop primers based on organic binder base cause technological and hygienic welding difficulties, especially if they contain pigments that release toxic fumes at welding temperatures. Inorganic workshop bases, for example the so-called zinc silicate, are demanding in terms of raw material base.

The disadvantages of the prior art in this field are largely eliminated by the silica-based coating for surface protection of steel and cast iron against the atmosphere of the invention. It consists of 0.1 to 10 gm < ^{2 >} of silica, 0.01 to 1 gm < ^{2 >} of phosphorus pentoxide and up to 1 gm < ^{2 > of} iron. The present invention also provides a method of forming such a coating. The coating according to the invention is formed by contacting a wettable coating of steel or cast iron with an aqueous medium containing H ² O ⁺ ions and components converting Fe ²⁺ and Fe ³⁺ ions into insoluble compounds such as PO ² ions. Fe (CN) g? Fe (CN) ³ ? chelating agents based on multifunctional phenols, hydroxy derivatives of nitrogenous hetero3

242 433 cyclic compounds and naturally modified tannins, as well as silicic acid and surface tension reducing agents, by spraying, pouring or immersion for at least 1 s at 5 to 30 ° C, after which the resulting coating is allowed to dry freely in the atmosphere from 0.15 to 12 hours,

The coating according to the invention is formed wholly or predominantly from inorganic constituents, has good protective efficacy, is compatible with subsequent manufacturing processes, in particular fusion welding, and is a good basis for the production of organic coatings. It is produced at normal temperature and does not require rinsing with water.

The essence of the coating according to the invention is the formation of a layer by chemical reactions between the metal surface and the aqueous medium and, at the same time, a silica gel by coagulation of the sol in the resulting layer. The environment contains components to disrupt the passivity of the steel or cast iron surface, components capable of reacting 2+ 3+ with Fe and Fe ions to form insoluble, strongly adhering reaction products, a silicic acid sol, optionally surface modified with other function reaction components, and wettability enhancers.

H 2 O ions introduced into the environment, for example by orthophosphoric acid, are known to be the most disruptive components of the passivity of the steel, causing the known reaction of active iron dissolution.

Fe + 2 H ₃ 0 ⁺ -> Fe ²⁺ + 2 HgO + Hg

The necessary activity ions H _Q 0 is defined for the purpose of pH = 2.5. In the presence of dissolved air oxygen in the acid reaction system, oxidation occurs

Fe ²⁺ + 4 H30 ⁺ + 02 -> 4 Fe ³⁺ + 6 HgO

2+ 3+

Preferably, the Fe and Fe ions are converted to insoluble compounds by PC ions, Fe (CN) g, Fe (CN) g, or chelating agents based on multifunctional phenols, N-hydroxy derivatives, such as heterocyclic compounds and the like. appears to be natural or modified tannin.

2+ 3+

It follows from reactions (1) and (2) that the formation of Fe and Fe ions takes place under conditions of the reaction consumption of ions, thus increasing the pH at the reaction interface. The increase in pH promotes the formation of insoluble crystalline and amorphous compounds which, under the given conditions, may, in addition to the secondary precipitation of Fe and Fe ions by the respective anions, also be formed by a direct anodic reaction, e.g.

Fe + PO4 “-> FeP0 ₄ + 3e

If a silica sol is present in the system, which is metastable at a pH <2.5, and an increase in pH results in exceeding the stability limit at the reaction interface, a silica gel is formed in the adjacent layer. It acts as a binder in the coating. Gelation is conditioned by the formation of a three-dimensional siloxane binding network from relatively small sol particles of –10 to 100 µm, high reactivity of silanol groups allows t-type bonds to form in the gel structure

- Si - O - Fe - O - Si -, (4)

They also ensure a stronger binding of the gel to both the substrate and the co-emerging reaction products. The negative charge of the silica sol sol, which is formed at pH > 2, and which leads to coagulation with the co-occurring, positive charge colloidal particles, in particular with iron hydroxides, contributes to the bond strength and thus to the favorable coating properties. If PO 2 ions are present in the reaction medium, the bond strength of the complex bond between the silanol hydrogen and phosphoric acid is enhanced.

The invention and its effects are explained in more detail below by way of example of a specific embodiment thereof.

Example 1 242 433

The clean-stained steel surface was immersed in an aqueous solution prepared by dissolving 10 wt. parts of natural tannin in 100 wt. parts by weight of hydrosol of silica with a content of 30% of silica; parts of orthophosphoric acid by adding 10 wt. parts of butyl alcohol and diluting with 60 wt. water at 15 ° C. After approx. 30 min. drying in air resulted in a red-violet coating that was abrasion-resistant, and after a further 12 hours airborne coating was water-resistant. Anal Zou -2-2, it was found that the coating comprises _about 7 gm Si0, 0.07 gm 0.03 gm Fe and P ^ Og. In the corrosion resistance tests it was found that the coating prevented the formation of rust for at least 8 hpd, in the salt fog test according to CSN '03 8132, 2 cycles in test according to CSN 03 8130 and 8 cycles in test according to CSN 03 8823. Compatibility tests with oil, nitrocellulose, epoxy and acrylate coatings have shown that all coatings have excellent adhesion and do not increase the tendency for osmotic degradation. Also, the corrosion resistance of the coatings was superior to that applied to the untreated surface.

Example 2

An aqueous solution containing 100 wt.% Was applied to the cast iron surface by blast cleaning. % of silicic acid hydrosol containing 30% silicon dioxide, wt. parts by weight of potassium hexacyanoferrate, 2 wt. parts of orthophosphoric acid, 10 wt. parts by weight of a mixture of isopropyl alcohol and ethylene glycol monoethyl ether in a volume ratio of 1: 1 and 60 wt. of water at a temperature of 8 ° C. On the surface of cast iron a deep blue coating formed, which was after 20 min. resistant to wiping and after 12 hours

water-resistant. The basis weight of the coating was 8.1 g.m, the corrosion resistance determined in the same manner as in Example 1 was comparable to the corrosion resistance of the coating obtained according to Example 1, and the coatings produced on this coating also showed satisfactory properties.

- 6 Example 3 242 433

An aqueous solution containing 100 parts by weight of silica sol containing 30% of silica, 15 parts by weight of 1,2,3 benzentriol, 1.8 parts by weight of a silica sol containing 30% silica was applied to the degreased, chipped surface roughness R _and 2.5 µm. parts by weight of orthophosphoric acid, 10 wt. parts by weight of a mixture of ethylene glycol with isopropyl alcohol in a ratio of 1: 3 by volume; parts of water. The solution was applied after degreasing the treated surface with organic solvents. After 15 minutes, a red-violet coating was formed, which, after 10 days of storage under the shelter, remained supple and prevented rust, while simultaneously exposed uncoated surfaces were coated with more than 10% of the surface. The coating has proven particularly useful for the in-process protection of machined surfaces.

Example 4

Solution containing 100 wt. parts by weight of silica sol with a silica content of 30%, 2 wt. parts by weight of 8-hydroxyquinoline, 8 wt. parts of technical mixture of dihydroxybenzenes - pyrocatechol, resorcinol and hydroquinol - 2 wt. parts of orthophosphoric acid, 10 wt. parts by weight of a mixture of ethylene glycol monoethyl ether with glycerol in a volume ratio of 2: 5 and 60 wt. parts of water were applied by pouring onto the degreased surface of a cold rolled sheet. After the excess solution had run off, a brown layer was formed which, after 30 minutes of drying in the air, allowed the sheets to be stacked without breaking the integrity of the layer. During the storage tests, a good protective function of the layer was proved and, unlike oil preservation, manual handling of the sheets was easier and safer.

The invention is applicable in engineering operations, in particular in the automotive industry.

Claims (2)

OBJECT OF THE INVENTION 242 433 A silica-based coating for the surface protection of steel and cast iron against atmospheric action, characterized in that it comprises from 0.1 to 10 gm of silica-2 -2, from 0.01 to 1 gm of phosphorus pentoxide, and up to 1 gm of iron. The second method of forming a coating according to claim 1, characterized in that the hydrophilic surface of steel or cast iron is brought into contact with an aqueous medium containing ions H ⁺ _q 0, folders, PRE2 + ** misleading 3+ ions Fe and Fe-insoluble compounds, for example 3- 4 - 3 - are, for example, PO 4 ions, Fe (CN) _g , Fe (CN) g, chelating agents based on multifunctional phenols, hydroxides of nitrogen heterocyclic compounds and naturally modified tannins, silicas and sninater substances _The surface coating is applied by spraying, pouring or immersion for at least 1 s at a temperature of from 5 to 30 ° C, after which the resulting coating is allowed to dry freely in the atmosphere for from 0.15 to 12 hours.