GB1582354A - Processes for producing phosphate coatings on ferrous metal surfaces - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO PROCESSES FOR PRODUCING PHOSPHATE COATINGS ON FERROUS METAL SURFACES (71) We, UNION CARBIDE AGRICULTURAL PRODUCTS COMPANY, INC., (formerly AMCHEM PRODUCTS, INC., [formerly UCAN CORPORATION]) a Corporation organized under the Laws of the Commonwealth of Pennsylvania, United States of America, of Brookside Avenue, Ambler, Pennsylvania, 19002, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention concerns processes for producing phosphate coatings on ferrous metal surfaces. As used herein the term "ferrous metal" includes not only iron but also its alloys, especially but not exclusively steels.

Processes are well-known for producing crystalline heavy-metal phosphate coatings upon ferrous metal surfaces, mainly to ensure good bonding of subsequent paint coatings. A variety of such heavy-metal phosphate coatings have been suggested, including those based upon cadmium, calcium and manganese phosphate, but one in particular has been in general use for decades, namely zinc phosphate.

The high price of zinc phosphate would make it very desirable to substitute another, cheaper phosphate such as calcium phosphate, but that is not as simple as it seems.

As would be expected, phosphate coating of a metal surface is generally-speaking accomplished with more facility the higher the temperature, because chemical activity increases with increasing temperatures. Heavy-metal phosphates moreover have inverse solubility characteristics. that is their solubility decreases as the temperature increases - and the lower their solubility the greater the ease in securing a deposition-type coating. The coating process is thus doubly expedited when performed at higher temperatures, and consequently heavy-metal phosphatizing solutions have been operated at relatively high temperatures. The boiling point of the acid phosphate solution has indeed been a common operating temperature.Unfortunately, however, phosphatizing solutions are difficult to handle when heated to boiling or near-boiling temperatures, require large expenditures of power to keep them in the heated state nearer to boiling they operate, and tend to throw down precipitates upon the heating coils or the hotter parts of the retaining tanks. These problems of course have been well-known for years, and there has long been a desire for a heavy-metal phosphate coating process operable economically over a wide range of temperatures, thus far below the boiling point of the phosphate solution. That however has till now been stultified because the use of a lower temperature results unfortunately in a greater solubility of the heavy-metal phosphate, hence a reduction in the heavy-metal phosphate remaining available to deposit the coating, which impairs the efficiency and hence the economy of the process.

Attempts have been made to solve the problem of high solubility at low temperatures.

Some 30 years ago it was proposed in United States of America Patent No. 2,316,811 to raise the pH so much that at the chosen operating temperature in the range of from 60"F to 1290F (approx. 16"C - 54"C) a supersaturated solution is formed, at best 20% greater than the saturated concentration of the metal phosphate utilized at that temperature, thus maximizing the amount of heavy-metal phosphate available to be deposited. This procedure however raises new problems. Any supersaturated solution is inherently only semi-stable, and highly liable to break down, precipitating the phosphate in excess of saturation as a sludge. Furthermore, at the requisite very high pH of the supersaturated solution free acid (H+) is no longer present.This lack of free acid (H+) is more or less fatal to the deposition because the acid initiates the process of depositing the phosphate coating on the metal surface, and without it no coating will deposit, or anyway the coating will deposit only at an unsatisfactory rate. It is said that an oxidizing agent can be used as an initiator to generate the requisite initial free acid; and this no doubt is true as regards zinc phosphate solutions.

As regards calcium, cadmium and manganese these are disclosed as heavy-metal equivalents which (although no examples of calcium are disclosed) are presumed utilizable in the same manner - but they are not, as emerges from the subsequent German Patent No.

741,937. This attempted to employ calcium phosphate at lower than boiling temperatures, using what appear to be both supersaturated and non-supersaturated solutions - but in the end the only satisfactory adherent coating achieved utilizing calcium phosphate was at high temperature (98"C) and low pH (2.62), that is to say the already-known combination.

Earlier work concerning zinc phosphate was confirmed, since satisfactory zinc phosphate coatings were produced at low temperature and high pH - but the production of adherent calcium phosphate coatings at low temperatures was shown to be a still unsolved problem.

Much additional work has since been done to overcome various problems, for example concerning the oxidizers, see United States Patent No. 2,351,605. A partial step towards the replacement of zinc phosphate has been taken by the phosphatizing solutions known today which combine zinc and calcium phosphate. There are however no known methods for successfully producing calcium phosphate coatings at low temperatures. Till now zinc phosphates, in spite of their higher price, have thus reigned supreme and virtually unchallenged.

We have however now found that by using low concentrations of calcium phosphate, minimal amounts of oxidizing agent and relatively high pHs in such combination that calcium phosphate is always present in amounts less than its saturation concentration, a calcium phosphate coating solution can be provided which is able to form amorphous (i.e.

non-crystalline), lightweight and tightly-adherent phosphate coatings at relatively low temperatures.

Accordingly, this invention provides a process for producing a phosphate coating on a ferrous metal surface, in which the surface is treated with an aqueous phosphate coating solution at a temperature within the range of from 50"F to 120 F (10 C - approx 49"C), said solution containing calcium phosphate in a concentration within the range of from 0.01 to 1.0 moles per litre (as measured by the Ca++ cation) and at least a trace of an accelerating agent, the pH of the solution being lower than the lowest pH which would cause the solution to be saturated with calcium phosphate.

The process of this invention produces amorphous coatings which are tightly adherent and, in comparison with conventional crystalline coatings of the prior art, of very low coating weight. A further important advantage of the process is that phosphate coatings may be produced from a calcium phosphate coating solution at comparatively low temperatures of less than 1200F (approx. 49"C), thus requiring less energy and avoiding the problems discussed above associated with the use at high temperatures of phosphate coating solutions.As might be expected, however, the coating efficiency of the process increases with temperature, and is too low to be practical below 50"F (10"C). It is preferred if the temperature at which the process is performed is maintained at or above 90"F (approx. 32"C), since this offers the optimum compromise between coating efficiency and energy expenditure.

The process of this invention should of course be carried out upon ferrous metal articles, such as steel panels, which have been suitably prepared for the coating process by any of the conventional cleaning and de-greasing procedures well known in the art. The panels after cleaning and de-greasing should then be rinsed in water, before being sprayed or otherwise brought into contact with the calcium phosphate solution. The optimum period of contact will of course, depend upon the application technique employed. A competent man should be able to determine the appropriate period of contact for satisfactory coating in any given case.For general guidance it can however be noted that using a spray technique with conventional equipment a spray time of 30 seconds or more has usually been found to be most effective; and particularly good results.have been obtained using a spray time of approximately 60 seconds. Using an immersion (or "dip") technique, a longer contact time is normally required, which typically may range from 1 minute up to 20 minutes; and an immersion time of about 10 minutes has generally been found to be satisfactory.

After they have been phosphate-coated, the panels will preferably be rinsed with water, and either dried or given a final "after-rinse" to enhance their corrosion resistance still further and then dried. Any of the conventional "after-rinse" solutions may be employed, based upon hexavalent chromium or other materials suitable for this purpose, as disclosed for example in United States of America Patents No. 3,063,877 and No. 3,450,579. After drying, the panels or other ferrous articles are ready to receive a paint or lacquer coating.

The concentration of calcium phosphate in the aqueous phosphate coating solution of this invention may be chosen anywhere within the range of from 0.01 to 1.0 moles of calcium (Ca++) per litre. It may however be noted that for many large-scale industrial processes, a 0.025 to 1.0 molar solution is preferable.

It is necessary in the process of this invention for the aqueous phosphate coating solution to contain at least a trace of an accelerating agent, since without this a satisfactory coating cannot be achieved at the relatively high pH's which are preferably used in the process (see below). Any of the accelerating agents conventionally employed in the phosphatizing art may be used, including hydroxylamine salts and oxidizing agents such as alkali metal chlorates and particularly sodium chlorate, nitrobenzene sulphonate and peroxides, as well as the usually-preferred oxidising agents, namely nitrates, and especially sodium nitrate.

which give good results both in spray and dip applications.

It has surprisingly been found that a very low concentration of nitrite is sufficient in the coating solutions of this invention thus even a trace of nitrate will produce the desired results. For convenience in practice we recommend a lower limit of about 2() parts per million of nitrate (calculated as sodium nitrate). Although the coating weight increases as the concentration of nitrate is increased, so too does the iron-loss, and consequently the coating efficiency remains substantially the same. Moreover the corrosion-resistance and paint-bonding characteristics of the coating are not improved by increasing the amount of nitrate in the coating solution.It is therefore preferred that the nitrate content of the solution should not be greater than 300 ppm (calculated as sodium nitrate), since this gives satisfactory results without causing either unnecessary etching of the ferrous metal surface or pointless expenditure.

Hence when using nitrates the phosphate solution will nbrmally contain from 20 ppm to 300 ppm of nitrate (calculated as sodium nitrate). Particularly good results have been obtained when spray coating with a nitrate content of less than 100 ppm, and the preferred range is as low as from 30 ppm to 80 ppm.

When using dip techniques. the preferred accelerator again is nitrate. and particularly sodium nitrate. The preferred concentrations when using sodium nitrate in a dip technique are substantially the same as those specified above for spray techniques. Chlorates are another preferred oxidizing agent for use in dip techniques, when indeed they give considerably better results than when used in spray techniques. The preferred chlorate is sodium chlorate. which in a dip technique should preferably be used in a concentration of from 0.5Chc to 2.06sic (by weight) of sodium chlorate.

At this point it may conveniently be noted that the oxodizing agent is conventionallv added to the solution before the pH-adjusting step discussed below. but this order of addition is not essential. The oxidizing agent may in fact be added to the solution at any time before treatment of the metal commences.

Supersaturation of calcium phosphate tends to make the solution less stable than is acceptable. and thus it is necessary to ensure that calcium phosphate is not present in the solution in amounts greater than or equal to its saturation concentration. Since the saturation concentration of calcium phosphate in the solution depends on the pH and temperature of the solution - it increases as the temperature rises and/or the pH lowers that may be ensured by causing the pH of the solution to be lower than the lowest pH which would cause the solution to be saturated with calcium phosphate. Thus it will be appreciated that the upper limit of pH is determined by the chosen temperature and calcium phosphate concentration of the solution.

It has been found that the coating efficiency (coating weight/iron loss) of the solution falls of markedly at pHs below 3.0 making the process increasingly impractical. since too much iron is removed from the surface. It is therefore much preferred that the pH of the solution should not be less than 3. which of course necessarily requires that the temperature and calcium phosphate concentration must be chosen so that the upper limit of pH is 3 or greater.

In general it may be said that coating efficiency increases with increasing pH. so that normally the pH should be adjusted to be as close as possible to the upper limit set bv the temperature and calcium phosphate concentration. Using the industrially-preferred 0.025 to 1.0 molar solutions. the pH of the solution will usually be in the range of 3.4 to 4.0, and preferably 3.7-3.8. which gives particularly excellent results as regards the corrosion resistances and paint-bonding characteristics of the coating. and at a low rate of iron-loss.

In practising the invention. it is generally most convenient to use a standard concentrate containing calcium phosphate. which can most advantageously be made up from calcium carbonate and phosphoric acid - and to dilute that concentrate to some chosen concentration for use in the process. and having also selected the intended operating temperature then. either before or after adding the oxidizing agent. to adjust the pH to that appropriate to the chosen temperature by adding a pH-adjusting agent.

Satisfactory pH-adjusting agents include any alkaline material which raises the pH but does not interfere with the coating operation. Examples of suitable pH-adjusting agents include calcium carbonate, sodium hydroxide, potassium hydroxide, ammonium hydroxide, sodium carbonate and potassium carbonate. Calcium carbonate is, however, preferred.

To illustrate what has just been said, a typical example showing the performance of the invention is as follows. A concentrate is first prepared which contains: Concentrate Calcium carbonate (98.5% pure) .... 9.28% Phosphoric acid (as a 75% solution) . * ..... 33.09% Water to make 100.00% [Note. Above, as elsewhere in this specification, "percent" means percent by weight unless otherwise stated].

From this concentrate, the coating solution is prepared by diluting each litre of the concentrate with sufficient water to make 0.025 molar (as measured by ca++) solution.

Having selected an intended operating temperature of 1000F (38"C), based on this temperature and the concentration of calcium phosphate present, calcium carbonate is added to the solution until pH of 3.7 to 3.8 is reached. 10 Grams of sodium nitrite (sufficient to make 250 ppm) are also added. The solution is then heated to 1000F (38"C), and the process is then run at that temperature.

If however the chosen temperature had been say 77"F (25"C), the otherwise identical solution could have been adjusted up to a pH of 4.2, but no higher.

From the above it will be seen that a preferred solution for use in the process of this invention may for general convenience be prepared first as a concentrate containing substantially 9% of calcium carbonate and substantially 30% of phosphoric acid.

Concentrates should in general be prepared at a lower pH than the solution to be used, because a lower pH ensures a better shelf-life for the concentrate. Thus the concentrate utilized above was prepared so that it contains approximately three phosphate (P043-) anions for every calcium (Ca++) cation. This ratio has been found to ensure good shelf-life, with little or no deterioration of the concentrate over an extended period of time.

In general, it may be said that the phosphate coating solution will be derived from a concentrate comprising from 5% to 20% of calcium carbonate and from 10 to 60% of phosphoric acid.

A particularly preferred coating process of this invention consists of the following steps: (A) an aqueous solution is prepared which consists of from 1.0 to 5.0 grams/litre calcium carbonate, from 3 to 15 grams/litre phosphoric acid, and from 0.05 to 0.30 grams/litre sodium nitrite; (B) the resultant solution is heated to a temperature within the range of from 90"F to 1200F (approx. 32"C - approx 49"C); and (C) the heated solution is applied to the ferrous metal surface for such a time as to deposit a coating of calcium phosphate thereon to a coating thickness corresponding to a coating weight of less than 50 milligrams per square foot, the pH of the solution being lower than the lowest pH which would cause the solution when heated to be saturated with calcium carbonate.

Having made up a solution containing the desired amounts of calcium phosphate and oxidising agent, chosen an operating temperature between 50"F and 120ole (10 C - approx.

49"C), and (if necessary) adjusted the pH of the solution with one of the suggested pH-adjusters to above 3.0 (but not so high as to cause the solution to become saturated with calcium phosphate). ferrous metal articles can then be coated with the coating solution by conventional methods such as by dipping or spraying.

The process of this invention has many advantages. Chief among them is no doubt the possibility of substituting the less expensive calcium phosphate coating for the previously conventional zinc phosphate coating in a process where low temperatures may be employed. As compared with conventional low-temperature processes the process of this invention also achieves a further economy in that it is no longer necessary to utilize a supersaturated solution. whose inherent instability and concomitant wasteful sludge problems are thereby avoided.

The superior properties of the calcium phosphate coatings produced by the process of this invention are also surprising. since such excellent results in terms of corrosion-resistance and paint-bonding characteristics were not to be expected with such a relatively small coating weight, typically less than 50 mg/ft2. and frequently within the range of 10 to 40 mg/ft2. This coating weight is to be contrasted with conventional phosphating processes which form heavier coatings, typically of the order of 200 to 300 mg/ft2 in the case of zinc phosphate, 20 to 100 mg/ft2 in the case of iron phosphate, and 1000 to 5000 mg/ft2 in the case of manganese phosphate.

It will naturally be understood that the invention includes ferrous metal articles whenever calcium phosphate coatings have been produced thereon by the process herein described.

In order that the invention may be well understood it will now be disclosed in more detail, though only by way of illustration, with reference to the following Examples.

Five sets of steel panels (each approximately 4" x 12") were cleaned, rinsed and then coated, according to the process of this invention, by spraying them with a coating solution containing 0.025 moles/litre of calcium phosphate (calculated as Ca++) and various amounts of sodium nitrite at various pHs. The temperature of the coating solution was 100"F (approximately 38"C.) and the spray time was 60 seconds.

The physical properties of the test panels after preparation and before testing are summarized in Table I, and there represented as a single number - but it should be appreciated that this number is generally an average of two or three test runs.

TABLE I Preparation and Properties of Panels 1 2 3 4 5 6 Total Acid NaNO, pH Coating Wt. Iron Loss Effic PANEL (points (ppm) (mg/ft") (mg/ft2) iency A 8.8 217 3.30 30.3 93.9 0.32 B 8.8 229 3.65 34.5 72.9 0.47 C 8.4 236 3.80 27.3 66.6 0.41 D 8.7 279 4.02 27.0 40.2 0.67 E 8.6 248 4.20 26.7 39.6 0.67 In Table I: Column 1 indicates total acid points, which is measured as the number of millilitres of 0.1 molar sodium hydroxide required to neutralize a 10 ml. bath sample to a phenolphthalein end-point. Total acid points are used to indicate the phosphoric acid concentration.

Column 2 shows the amounts of sodium nitrite accelerator utilized, measured in parts per million.

Column 3 indicates the various pH units utilized.

Column 4 indicates the weight of the coating deposited on the panel, measured in milligrams per square foot.

Column 5 indicates the weight of the iron lost from the panel as a result of the process of this invention, measured in milligrams per square foot.

Column 6 indicates the coating efficiency. which is the ratio of coating weight to iron lost.

The five panels thus coated were utilized in tests as outlined in Table II, which sets out data concerning the corrosion resistance of the panels, as measured in a salt-water spray test and in a water-immersion test, in comparison with similar data concerning steel panels coated instead with zinc phosphate coatings and with iron phosphate coatings as standards for comparison. The values shown as single number again also each represents an average of two or more panels.

Preparation of Zinc-Phosphate Coated Panel A conventional zinc phosphate coating solution was prepared by diluting the following concentrate: Constituent Amount (by weight) Zinc Oxide 12.51% 75% Phosphoric Acid 58.14% Nickelous Oxide 1.12% Sodium chlorate 3.85% Water to 100% with water to give a coating solution concentration of 1% (by volume), which is about 0.025 molar zinc. The coating solution was applied to a set of steel panels identical to those used above, to give a coating weight of approximately 250 mg/ft2.

Preparation of Iron-Phosphate Coated Panel A conventional phosphate coating solution for preparing iron phospate coatings on ferrous metals was prepared by diluting a concentrate consisting of: Constituent Amount (by weight) 75% Phosphoric Acid 27.94% Soda Ash (Sodium Carbonate) 8.40% Sodium Chlorate 11.66% Water to 100% with water to a concentration of 3.3coo (by volume), which is aboult 0.1 molar in phosphate.

The coating solution was applied to a set of steel panels identical to those used above, to give a coating weight of approximately 40 mg/ft2.

The five sets of test panels whose properties and specifications are listed in Table I together with the two sets of standard panels were subjected to the following tests under the following described test conditions.

The phosphate-coated panels were painted with various commonly-used test paint primers plus top-coat; and some of the thus coated and painted panels were scribed through the coating layers to bare metal. All the panels were then subjected either to a salt-spray test or to a water-immersion test. The results are summarized in Table II below: TABLE II Corrosion Rating of Panels Paint System 1 1 1 1 1 + TC 2 2 Test Used SS SS SS SS SS SS WS Length (hrs.) 96 168 240 336 336 240 240 Scribed panels No No No No Yes Yes Yes Panel A 10 9.7 8.3 7.2 10 10 10 Panel B 10 10 10 8.0 9.7 10 10 Panel C 10 10 9.0 9.0 9.9 10 10 Panel D 10 9.8 9.2 8.7 9.9 9.9 10 Panel E 10 9.8 9.2 8.7 9.9 9.9 10 Iron Phosphate 10 8.7 7.7 7.7 9.0 9.0 10 Zinc Phosphate 10 8.0 8.0 8.0 8.3 10 10 In Table.II:: In the first row, Paint System 1 and Paint System 2 refer to paint systems employed by the automobile industry as standards to test the efficacy of proposed new phosphate or phosphate-type coatings.

System 1, currently employed by General Motors, consists of a PPG water-based paint applied electrophoretically as a primer coat.

System 1 + TC utilized the same primer coat followed by an E.I. duPont de Nemours spray surfacer and spray top-coat for a total of three coats of organic finish.

System 2, currently used by Ford Motor Company, utilizes a solvent-based first and second primer coat followed by an internally-developed Ford Motor Company top coat.

In the "Test Used" second row, "SS" refers to a standard salt-water spray test, as described in detail in American Society of Testing Materials Bulletin No. ASTM-B 117; and "WS" likewise refers to a standard water-immersion test, as described in American Society of Testing Materials Bulletin No. ASTM-D 870.

Length refers to the duration of the exposure, measured in hours. "Scribed" indicates whether the panel was scribed or not; and when the panel has been scribed, one evaluates the degree to which corrosion has extended, from the exposed metal of the scribe mark outwardly into the painted area.

The panels are rated by visual examination on a scale ranging from 1 to 10, where 10 represents the best results and 1 represents the worst results.

As can readily be seen from Table II, the calcium phosphate coated panels produced in accordance with this invention show corrosion resistance results which overall are equal to and sometimes perhaps even superior to the conventional phosphate-coated panels under the same test conditions.

WHAT WE CLAIM IS: 1. A process for producing a phosphate coating on a ferrous metal surface, in which the surface is treated with an aqueous phosphate coating solution at a temperature within the range of from 50"F to 1200F, said solution containing calcium phosphate in a concentration within the range of from 0.01 to 1.0 moles per litre (as measured by the Ca+ cation) and at least a trace of an accelerating agent, the pH of the solution being lower than the lowest pH which would cause the solution to be saturated with calcium phosphate.

2. A process as claimed in claim I. in which the surface is treated at a temperature in the range of from 90"F to 12() F.

3. A process as claimed in claim l or claim 2, in which the calcium phosphate is present

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (19)

**WARNING** start of CLMS field may overlap end of DESC **. The phosphate-coated panels were painted with various commonly-used test paint primers plus top-coat; and some of the thus coated and painted panels were scribed through the coating layers to bare metal. All the panels were then subjected either to a salt-spray test or to a water-immersion test. The results are summarized in Table II below: TABLE II Corrosion Rating of Panels Paint System 1 1 1 1 1 + TC 2 2 Test Used SS SS SS SS SS SS WS Length (hrs.) 96 168 240 336 336 240 240 Scribed panels No No No No Yes Yes Yes Panel A 10 9.7 8.3 7.2 10 10 10 Panel B 10 10 10 8.0 9.7 10 10 Panel C 10 10 9.0 9.0 9.9 10 10 Panel D 10 9.8 9.2 8.7 9.9 9.9 10 Panel E 10 9.8 9.2 8.7 9.9 9.9 10 Iron Phosphate 10 8.7 7.7 7.7 9.0 9.0 10 Zinc Phosphate 10 8.0 8.0 8.0 8.3 10 10 In Table.II:: In the first row, Paint System 1 and Paint System 2 refer to paint systems employed by the automobile industry as standards to test the efficacy of proposed new phosphate or phosphate-type coatings. System 1, currently employed by General Motors, consists of a PPG water-based paint applied electrophoretically as a primer coat. System 1 + TC utilized the same primer coat followed by an E.I. duPont de Nemours spray surfacer and spray top-coat for a total of three coats of organic finish. System 2, currently used by Ford Motor Company, utilizes a solvent-based first and second primer coat followed by an internally-developed Ford Motor Company top coat. In the "Test Used" second row, "SS" refers to a standard salt-water spray test, as described in detail in American Society of Testing Materials Bulletin No. ASTM-B 117; and "WS" likewise refers to a standard water-immersion test, as described in American Society of Testing Materials Bulletin No. ASTM-D 870. Length refers to the duration of the exposure, measured in hours. "Scribed" indicates whether the panel was scribed or not; and when the panel has been scribed, one evaluates the degree to which corrosion has extended, from the exposed metal of the scribe mark outwardly into the painted area. The panels are rated by visual examination on a scale ranging from 1 to 10, where 10 represents the best results and 1 represents the worst results. As can readily be seen from Table II, the calcium phosphate coated panels produced in accordance with this invention show corrosion resistance results which overall are equal to and sometimes perhaps even superior to the conventional phosphate-coated panels under the same test conditions. WHAT WE CLAIM IS:

1. A process for producing a phosphate coating on a ferrous metal surface, in which the surface is treated with an aqueous phosphate coating solution at a temperature within the range of from 50"F to 1200F, said solution containing calcium phosphate in a concentration within the range of from 0.01 to 1.0 moles per litre (as measured by the Ca+ cation) and at least a trace of an accelerating agent, the pH of the solution being lower than the lowest pH which would cause the solution to be saturated with calcium phosphate.

2. A process as claimed in claim I. in which the surface is treated at a temperature in the range of from 90"F to 12() F.

3. A process as claimed in claim l or claim 2, in which the calcium phosphate is present

in a concentration of from 0.025 to 1;0 moles per litre.

4. A process as claimed in any one of the preceding claims, in which the accelerating agent is an oxidising agent consisting of one or a mixture of more than one alkali metal nitrite and/or alkali metal chlorate.

5. A process as claimed in claim 4, in which the oxidising agent is nitrite in a concentration within the range of from 20 ppm to 300 ppm (calculated as sodium nitrite).

6. A process as claimed in claim 6, in which the nitrite concentration is less than 100 ppm.

7. A process as claimed in claim 6, in which the nitrite concentration is within the range of 30 ppm to 80 ppm.

8. A process as claimed in any of the preceding claims, in which the pH of the solution is not less than 3.0.

9. A process as claimed in claim 8, in which the pH of the solution is as close as possible to the lowest pH which would cause the solution to be saturated with calcium phosphate.

10. A process as claimed in claim 3, or any claim dependent thereon, in which the pH of the solution is within the range of from 3.4 to 4.0.

11. A process as claimed in claim 10, in which the pH of the solution is within the range of from 3.7 to 3.8.

12. A process as claimed in any of the preceding claims, in which the phosphate coating solution is derived from a concentrate comprising: from 5% to 20% calcium carbonate (by weight) and from 10% to 60% phosphoric acid (by weight).

13. A process as claimed in claim 12, in which the phosphate coating solution is derived from a concentrate in which calcium phosphate is present in a ratio of substantially 1 calcium cation for every 3 phosphate anions.

14. A process as claimed in claim 12 or claim 13, in which the concentrate contains (by weight) substantially 9% of calcium carbonate and substantially 30% of phosphoric acid.

15. A process as claimed in any of the preceding claims, in which the calcium phosphate coating is applied to a thickness corresponding to less than 50 milligrams per square foot.

16. A process for producing a phosphate coating on a ferrous metal surface, as claimed in any of the preceding claims, in which: (A) an aqueous solution is prepared with consists of from 1.0 to 5.0 grams/litre calcium carbonate, from 3 to 15 grams/litre phosphoric acid, and from 0.05 to 0.30 grams/litre sodium nitrite.

(B) the resultant solution is heated to a temperature within the range of from 90"F to 1200F; and (C) the heated solution is applied to the ferrous metal surface for such a time as to deposit a coating of calcium phosphate thereon to a coating thickness corresponding to a coating weight of less than 50 milligrams per square foot, the pH of the solution being lower than the lowest pH which would cause the solution when heated to be saturated with calcium carbonate.

17. Processes for producing an amorphous, light-weight, tightly-adherent phosphate coating on a ferrous metal surface as claimed in any of the preceding claims and substantially as herein described.

18. Processes for producing an amorphous, light-weight, tightly-adherent calcium phosphate coating on a ferrous metal surface substantially as herein described with reference to the Examples.

19. Ferrous metal articles whenever amorphous, light-weight, tightly adherent calcium phosphate coatings have been produced thereon by the processes claimed in any of the preceding claims.