GB2072225A

GB2072225A - Process and composition for coating metal surfaces

Info

Publication number: GB2072225A
Application number: GB8105435A
Authority: GB
Original assignee: Pyrene Chemical Services Ltd
Current assignee: Pyrene Chemical Services Ltd
Priority date: 1980-03-21
Filing date: 1981-02-20
Publication date: 1981-09-30
Also published as: ATE8277T1; PT72709B; BR8101682A; PH17558A; PT72709A; EP0036689A1; GB2072225B; EP0036689B1; ES501134A0; ES8205878A1; DE3164514D1; JPS56142872A

Abstract

Phosphate coatings can be formed on iron, aluminium or zinc surfaces by spraying at low temperature with a phosphating solution containing zinc, phosphate, manganese and an oxidising accelerator comprising sodium nitrobenzene sulphonate and chlorate. The solution may also contain nickel, a borofluoride and an alpha-hydroxy aliphatic acid e.g. tartaric acid, or a salt thereof. Nitrate may also be included as an additional oxidising accelerator. The pH of the solution is preferably in the range 3-5.

Description

1 GB 2 072 225 A 1

SPECIFICATION

Processes and compositions for coating metal surfaces There is described in British Patent Specification No. 983,924 a process in which a phosphate coating is formed on a ferrous or zinc surface by spraying with a solution at a temperature of 381C or less of zinc and manganese phosphates including an oxidising accelerator, the manganese content of the solution being between 5 and 50% by weight of the zinc content. In German Patent Application No. P2818426 (corresponding to British Patent Application No. 18402/77) the addition of borofluoride is said to be advantageous and to result in the production of satisfactory coatings on aluminium and in great improvement in the quality of coatings on ferrous surfaces.

Over the years a very large number of oxidizing accelerators have been proposed for solutions containing zinc and phosphate including chlorates, nitrates, organic nitro compounds, hydrogen peroxide, nitrites, bromates and iodates. Included amongst organic nitro compounds that have been proposed are sodium m-nitrobenzene sulphonate and dinitro benzene sulphonate. Also British Patent Specification No. 1,542,222 describes the use of an accelerator comprising chlorate and sodium nitrobenzene sulphonate in a phosphate coating process comprising zinc and phosphate.

Despite the numerous disclosures of various oxidising accelerators the preferred accelerator in British Patent Specification No. 983,924 and the said German Application is said to be nitrate, optionally with nitrite. Chlorate is also mentioned as an optional addition. However, it would have been expected that compositions containing both manganese and chlorate would have had unsatisfactory storage stability thereby necessitating preparation of the compositions only 105 at the place and time of use, thereby rendering the use of chlorate commercially much less satisfactory than compositions free of chlorate.

The processes described in British Patent Specification No. 983,924 and the said German 110

Application are commercially very important processes because they do permit satisfactory coating formation at relatively low temperatures, 3811C or less. This therefore results in reduction of the amount of energy required for the processes. 115 However it would still be very desirable to be able to accelerate the coating formation, especially at lower temperatures for instance around 301C.

We have now surprisingly found that the desired improvement in coating formation can be 120 achieved if a mixture of sodium nitrobenzene sulphonate and chlorate is used as part at least of the oxidising accelerator in processes such as those described in British Patent Specification No.

983,924 and the said German Application. Thus 125 the nitrate may be omitted entirely and replaced by this mixture or this mixture may be used in combination with nitrate.

A phosphate coating solution according to the invention comprises zinc ions, phosphate ions, manganese ions in an amount of 5 to 50% by weight zinc and oxidising accelerator comprising chlorate and a nitrobenzene sulphonate. A process according to the invention comprises forming a phosphate coating on a ferrous, zinc or aluminium surface by contacting the surface with such a solution.

All percentages in this specification are percentages by weight and are based on the total composition unless otherwise specified.

The solution that is applied to the surface generally contains 0.15 to 0. 9%, preferably 0.2 to 0.5% Zn and 0.01 to 0.18, preferably 0.02 to 0.07% Mn. The amount of manganese, based on the amount of zinc, is generally below 33%, for instance from 5 to 20% and most preferably from 9 to 13%.

The solution may contain nickel in amounts up to 0.5%, preferably 0.0 1 to 0.03%.

The amount of phosphate is generally 0.4 to 2.25%, preferably 0.5 to 1.5%.

The solution may advantageously include an aliphatic alpha hydroxy acid, generally tartaric acid, or a water soluble salt thereof. The amount of such acid or salt is generally up to 0.8%, - preferably 0.02 to 0.2%.

The solution may also include fluoride which may be introduced as a simple fluoride or, preferably as a complex fluoride, generally borofluoride. This may be introduced as, for example,.sodium borofluoride or preferably as borofluoric acid. The amount of fluoride (measured as F) is generally up to 0.4% preferably being 0.05 to 0. 1 %. If fluoride is introduced as a complex fluoride some may dissociate in the composition to, for instance, simple fluoride and boric acid.

The nitrobenzene sulphonate must be water soluble and is generally introduced as a sodium salt of a nitrobenzene sulphonate. The sulphonate may be a dinitro compound but is preferably a mono-nitro compound, generally meta nitro benzene sulphonate.

The amount of sodium nitrobenzene sulphonate or other sulphonate is generally from 0.005 to 0.2%, preferably 0.01 to 0.05%, and the amount of chlorate (measured as C103) is generally 0.1 to 0.5%, preferably 0.2 to 0. 4%.

These may be the only oxidising accelerator or other accelerators, preferably nitrate, may be included. If nitrate is included the amount is generally below 2.25%, with preferred amounts of nitrate being 0.4 to 1. 0%. The compositions are preferably free of nitrite.

It is of course necessary that the solution should be capable of forming the desired coating at the low temperature and for this purpose it is desirable to optimise its pH, total acid and free acid contents. The pH is generally at least 3, for instance 3 to 5 and preferably from 3.4 to 4. 4. The total acidity is generally above 15 and mutually 15 to 30, most preferably about 20. By total acidity we mean the number of ecs of N/1 0 NaOH necessary to neutralise 10 cc of the solution to a phenolphthalein end point. The free acidity is 2 GB 2 072 225 A 2 generally 0.5 to 1.5, preferably about 0.8 to 1.0. Free acidity means the number of ccs of N/1 0 NaOH necessary to neutralise 10 cc of the solution to a bromophenol blue end point. The ratio of total 5 acid to free acid is preferably between 20 and 25.

The described solution may be formed by dissolving the desired ingredients in water or, preferably, by dilution of concentrated chemicals which provide the various ingredients in the same relative proportions as in the described working solution. Generally two concentrates are used. One concentrate is acidic and may contain 1.5 to 9 parts zinc, 0.1 to 1.8 parts manganese, up to 5 parts nickel, 4 to 22.5 parts phosphate, up to 8 parts alpha hydroxy acid or salt, up to 4 parts fluoride. The other concentrate may contain 1 part sodium nitrobenzene sulphonate (or other sulphonate) and 2 to 5, preferably 3.5, parts sodium or other chlorate. 20 When the fluoride is introduced into the acidic concentrate as borofluoride, borofluoric acid is preferably used as this reduces or eliminates precipitation problems that may sometimes occur with sodium borofluoride. Suitable concentrates are an aqueous solution of the described components having a total solids content of at least 25%, preferably at least 40% and especially 40to65%. The working solutions of the invention can be used for forming coatings on ferrous, zinc or, especially when the solution contains borofluoride, aluminium surfaces, that is to say on surfaces of iron, zinc or aluminium or an alloy consisting predominantly of iron, zinc or aluminium. Before contacting the surface with the solution the surface should be cleaned and rinsed for instance in conventional manner.

Contact between the solution and the surfaces is normally effected by spray and is generally continued for periods of 50 to 120 seconds, preferably about 80 seconds. Contact may be at temperatures between 25 and 551C, generally around 25 to 351C. After the contact with the solution the surface may be given a chemical after-rinse in conventional manner, for instance with a dilute aqueous solution of chromium compounds, water rinsed and then dried in 110 conventional manner. The final coatings give good corrosion resistance and serve as an excellent base for paint.

During using the working solution may be replenished by use of an acidic concentrate as 115 described above and an alkaline concentrate of NaOH, NaCI03 and sodium nitrobenzene sulphonate.

As an example of the invention a solution was formed containing 0.35% Zn, 0.05% Mn, 0.02% Ni, 1. 16% PO,, 0.4% N03, 0.1 % tartrate, 0.07% borofluoride (measured as F), 0.1 % sodium nitrobenzene sulphonate and 0.35% sodium chlorate. The solution has a pH of 3.4, free acidity of 0.7 and total acidity of 23.0.

Steel panels W' x 49 were then sprayed with an alkaline cleaning solution with a total alkalinity of 26 points at a temperature of 35'C for seconds. They were then rinsed with water and sprayed with the above working solution for 80 seconds at 301C. They were then rinsed with water, rinsed with distilled water and dried by blasting with compressed air. The resultant coatings were smooth, complete, tight and refined' with crystal sizes between 5 and 20 microns.

These panels were then painted with a cathodic electroprimer (ED 3002) supplied by International- Paints and then stoved at 1821C for 20 minutes. Dry film thickness of the paint was 18 microns.

These panels were then cross-scored and subjected to salt spray testing to ASTM B 117-64 for 500 hours. At the end of the test, adhesive tape was used to determine the amount of loose paint around the score. There was no paint removal. This shows that the process is capable of giving a good coating despite the low temperature.

As a comparison, this example was repeated except that the sodium nitrobenzene sulphonate was omitted and 0.02% sodium nitrite added in its place. The final painted panels were subjected to the described salt spray test and the adhesive tape removed 15 to 25% of the paint along the score to a width of 3 mm. This shows that good results at low temperatures are due to the combination of chlorate and nitrobenzene sulphonate as accelerators.

Claims

1. A phosphate coating solution that comprises zinc ions, phosphate ions, manganese ions in an amount of 5 to 50% by weight zinc and oxidising accelerator comprising chlorate and nitrobenzene 100 sulphonate.

2. A solution according to claim 1 additionally containing fluoride ions.

3. A solution according to claim 2 in which the fluoride ions have been introduced as sodium 105 borofluoride.

4. A solution according to any preceding claim in which the nitrobenzene sulphonate is introduced into the solution as sodium meta nitro benzene sulphonate.

5. A working, solution according to claim 4 containing 0.15 to 0.9% zinc, 0.01 to 0.18% manganese, 0 to 0.5% nickel, 0.4 to 2.25% phosphate, 0 to 0. 8% aliphatic alpha hydroxy acid or water soluble salt, 0 to 0.4% fluoride, 0.03 to 0.2% sodium nitrobenzene sulphonate, 0. 1 to 0.5% chlorate measured as C103 and 0 to 2.25% nitrate.

6. A solution according to claim 5 containing 0.2 to 0.5% zinc, 0.02 to 0. 07% manganese, 0.01 120 to 0.03% nickel, 0.5 to 1.5% phosphate, 0.02 to 0. 2% aliphatic alpha hydroxy acid or water soluble salt, 0.05 to 0. 1 % fluoride, 0.05 to 0. 15% sodium nitrobenzene sulphonate and 0.2 to 0.4% chlorate.

7. A solution according to any of claims 4, 5 or 125 6 in which the amountof manganese ions is 5to 33% by weight zinc.

8. A solution according to any preceding claim in which the amount of manganese ions is

9 to i i 3 GB 2 072 225 A 3 13% by weight zinc. 9. A solution according to claim 6 or claim 7 having a pH of from 3.4 to 4.4, total acidity of about 20 and free acidity of 0. 8 to 1. 5

10. A method of forming a phosphate coating on a clean ferrous, aluminium or zinc surface comprising contacting a working solution according to any of claims 1 to 8 with the surface at a temperature of 25 to 501C.

11. A method according to claim 10 in which the contact is effected by spraying for 50 to 120 seconds at a temperature of 25 to 350C.

Printed for Her Majesty's Stationery Office by the Courier Press, Leamington Spa, 1981. Published by the Patent Office, 25 Southampton Buildings, London, WC2A lAY, from which copies may be, obtained.