US2480448A - Protective surface treatment of magnesium base alloys - Google Patents

Description

A 1949- M G. E. COATES 2,480,448

PROTECTIVE SURFACE TREATMENT OF MAGNESIUM BASE ALLOYS I Filed Feb. 21, 1944 2 Sheets-Sheet 1 lo 9 LL] .(0 8 I v F 7 Z 5- a 5 O E 01 I 4- 3 3 Z z z SULPHA TE CONCENTRATION, HOLES. PER L/TRE lo v 5 CORROSION RATE IN EMPIRICAL UN/TvS N c, -h. o, 0: \T 00 a Aug. 30, 1949. e. E. COATES ,4

PRO'IECTIVE SURFACE TREATMENT OF'MAGNES IUM BASE ALLOYS Filed Feb. 21, 1944 2 Sheets-Sheet 2 Fig. 3,

Nu4x=ncnw om CORROSION RATE IN EMPIRICAL UNITS I /0 20 TIME or TREATMENT IN M/NU 7E6 Patented Aug. 30, 1949 PROTECTIVE SURFACE TREATMENT OF MAGNESIUM BASE ALLOYS Geoffrey Edward Coates, Swansea,-Wales, assignor, by mesne assignments, to The Permanente Metals Corporation, a corporation of Delaware Application February 21, 1944, Serial No. 523,295 In Great Britain April 28, 1943 1 Claim. 1

This application, which is a continuation in part of pending United States application Serial No. 469,444, of Geoffrey Edward Coates, filed December 18, 1942, now abandoned, for Treatment of magnesium base alloys, is for an invention which relates to the protective surface treatment of magnesium base alloys, especially but not exclusively those alloys prepared according to specifications of the British Air Ministry Department of Technical Development hereinafter referred to as D. T. D. specifications.

The protection afforded by dichromate solutions is due to the formation of a protective film of reaction products on the surface of the metal, but simple dichromate solutions are of little practical value because both the rate and total extent of film formation are severely limited.

Amongst what may be described as the dichromate type of treatments for the corrosion protection of Mg base alloys, recourse has been made to a variety of methods in endeavors to overcome the above difficulties but no hitherto published process combines all the desired attributes of a commercial protective treatment which may be summarized as the combination of the following features or requirements:

(a) The treatment should cause the minimum dimensional change so that machined articles may be treated without disturbance to machining tolerances, for instance, less than can be reasonably measured with a thousandths of an inch micrometer.

(b) The raw materials for compounding the solution should be cheap and of innoxious character, e. g., strongly acid or alkaline solutions should be avoided and the rate of consumption of the ingredients in operation should be low.

Time of treatment should be as short as possible, i. e., preferably not more than a few minutes and less than half an hour.

(d) The process should be capable of operation at room temperature.

(6) The process should be characterized by ease and economy in operation and maintenance and the solutions should be stable.

The present invention provides good commercial protection in a short time at room temperatures where such protection can be secured even in 1 to 3 minutes in certain circumstances. On the other hand, when a higher temperature is used for speeding up the process, for instance to fit in with conjoined processes, the bath still remains stable. Heavy metal salts are avoided because they form heavy metal deposits in the coating, or precipitated or colloidal matter, or

cause spontaneous change of pH with time or form double salts with some ofthe solution constituents of insufficient solubility to permit attainment of optimum sulphate concentration. Such salts are therefore excluded from the present application, which is' limited to those. compounds of which the cations remain in stable solution in the concentration and pH ranges set out below.

The object of the present invention is to provide an improved process for the protection of Mg base alloys by the dichromate method, characterized by, namely, short-time treatment, capacity for treatment at substantially room temperature, avoidance of strongly acid solutions which, in addition to being unpleasant to use, are likely to cause undue dimensional loss and employment of economical stable solutions.

The characteristics of the solutions of this in vention are that, in the pH range specified, the cations are stable in aqueous solution, undergo no spontaneous change, are stable in the presence of dichromate ions, do not react with other constituents of the bath to prevent attainment and maintenance of the sulfate and dichromate ion concentrations specified, and do not react with magnesium or form other metal or undesirable deposits on the magnesium surface.

The new combination consists of the following essential requirements (a) to (e), namely:

(a) Dichromate ions in concentration not less than005 molar and preferably between 0.2 and 0.5 molar, which region is the optimum when the other factors are at their optima. The most convenient sources of dichromate ions to ensure that the cation simultaneously introduced shall satisfy the above requirements, are the dichromates singly or mixed of H, Li,Na, Mg, K or NH4.

(12) Sulphate ions-In concentration not less than 0.2 molar and preferably between 0.6 and 1 molar which region is the optimum when the other factors are at their optima. The most convenient sources of sulphate ions to ensure that the cation simultaneously introduced shall satisfy the above requirements, are the sulphates of H, Li, Na, Mg, K or NH4 or mixtures or combinations of any of these. The following cations are also useful in thedichromate constituents, singly or mixed, of my solutions: tetramethyl ammonium, tetraethyl ammonium, and N-methyl-pyridinium ions. Some of these sulphates have never previously beenspecified as constituents of dichromate solutions for Mg protection. sulphates such asAl sulphate, alums, nickel sulphallic, chromiumsulphate or nickel ammonium sulphate which have been specified for known processes are not suitable for the present process because they would all contribute cations undesirable in respect of the requirements listed above and are excluded. It is; believed. that the advantages of the sulfate addition in the present process may be explained as follows: The salts employed previously as activator salts contain cations injurious in my process'due'to; format-ion of precipitated or colloidal matter, and; the activator salts which contain anions other than sulfate do not produce the desiredi'results. This is because sulfate ions have the proper charge and are of the proper size to diffuse through the thin, first-formed film and enable produc tion of a satisfactorily thick filml, without. causing pitting, reducing the dichromate; or forming insoluble reaction products.

I have found that not only is the choice of the sulphate of essential importance but likewise its concentration. Figure '1' ispa. graph showing the relationship between sulphate concentration inmoles per litre and corrosion rate in empirical units employinguniform treatment times. There is an optimumconcentrationnn the region of 0.6-1 molar.. Whereas thereislittleloss inprotection with. higher concentrations, concentrations below this region becomecritical and the protection rapidly falls off so that at less than about 0.2. M the solutions are of no practical value.

(c) Ammonium tuna-Thepresence of ammonium ions improves the protective value of the film for any given time oftreatment. Theconcentration is suiiicient ifqthe ammonium salt is used for at least one of the other ingredients (a) or (b). above..

(d) Hydrogenion c01rcentmtz'on.-It has long been known that if the acidity of dichromate' solutions is increased; the rate. of protective film formation is also increased". Unfortunately the rate of. dissolution of metal. from the workpiece surface also increases and; the; permissible increase in acidity is therefore limited by dimensional tolerances; The. protective value is also affected if the acidity falls outside certain limiting ranges for each type of solution. For the solutions according; to the present invention the eifect of pH variation on the protection afiorded by the film in the case of two. particular alloys is shown in Figure 2., Thereare marked. indications of an optimum pH.-value for each alloy. Generally speaking; the optimum depends on the Alcontent of the alloy as shown in Figure 3 which shows the relationship between the hydrogen ion concentration andpercentage of Al in alloys treated according to certain solutions of the present invention. The degree of protection falls ofi rapidly on either side of the optimum values for each. alloybut. allvalues fall within the extreme pH- range 4-6;

The pH values quotedrwerezdetermined. by the glass electrode method. Other methods of pH measurement may be used but allowance must be made for variations from the glass electrode values.

(6) Bu-fier anemia-Maintenance of the pH value at the optimum or within the permissible range for a given alloy is complicated by the fact that the pH- rises as the Mg dissolves during the treatment. In practical operation it is highly desirable for reasons of economy, tobe able to treat a number of workpieces in the same batch of solution. This necessitates frequent controlled additions of acid but the frequency of such additions is greatly reduced by incorporating in the solution a weak acid and one of its salts capable of buffering the solution within the desired pH range. Acetic acid is about the best and cheapest organic acid for the bufier mixture together with its sodium or ammonium salt, although any of the other bases listed under (1)) above may be used. Other weak acids of suitable dissociation constant and salts thereof, both of suflicient solubility, may be used in place of or in addition to the acetic acid and acetate. The bufier acid must not however form an insoluble salt with Mg and must not be oxidized by the dichromate solution. The buiier capacity or resistance of the solution to change of pH is rough-- ly proportional to the, concentration of the weak acid and its salt. Occasional major adjustments in pH may be appropriately efiected by additions of sulphuric, chromic or nitric acids, ammonia, caustic soda or other suitable bases selected from the list in- (b) above. The acids and their salts which I prefer for cold solutions are single or mixed acids and single or mixed salts of the acids and preferably of the same acids within the following group, namely, acetic, propionic, butyric, valeric, malonic, succinic, glutaric, adipic, pimelic, phthalic, the cations of the salt being those of lithium, sodium, magnesium, potassium or ammonium. The acids and their salts which I prefer for hot solutions are single or mixed acids and single or mixed salts ofthese acids and preferably of the same acid, the acids being chosen so that the buffer agent is sufl'iciently soluble at the temperature-of use, from the following group, namely, caproic, oenanthic, malonic, succinic, glutaric, adipic, suberic, pimelic, benzoic and phthalic, and the cations of the salts being those of lithium, sodium, magnesium, potassium or ammonium. In cases where it is de sired to secure the maximum possible protection, the sulphate ion concentration should not exceed the optimum region indicated under (1)) above, and in such cases the major pH adjustment should be made by chromic or nitric acid additions.

The salts used for introducing the dichromate and sulfate ions, the salt of the buffer acid and the base for pH adjustments must be chosen to meet the requirements hereinabove set forth, and suitable cations have been listed above. Any species of cation can be tested by compounding a solution as described, containing it, and determining whether the solution filters without leaving appreciable residue on the filter paper, and whether it shows a change of pH on standmg.

To sum up the aforesaid items (a) to (e), the invention consists in a process for the protective surface treatment of magnesium base alloys in solutions characterized by a dichromate concentration not less than 0.05 mole per litre, a sulphate concentration not less than 0.2 mole per litre, and a hydrogen ion concentration controlled by a buiTer agent or addition within the pH range 4-6; the constituent salts being so selected that within this pH range their cations remain in stable solution and the upper limits for the dichromate and sulphate concentrations corresponding to saturation point in the particular solutions used and temperature of use.

Time of treatment to secure the best results for any particular alloy depends on the composition of the alloy. Generally speaking time of treatment must be increased with increasing Al content. In some cases there is an optimum region fromthe point of view of timelof treatment as shown by the shape of curve a in Figure 4. Curve e applies to an alloy of composition 7.5% A1, 0.4% Zn, 0.2% Mn. Curve applies to an alloy comprising 6% Al, 1% Zn,0.2% Mn. The region of maximumcurvature of curve dis about thepoint correspondin'g to, say, 9 minutes." That in curve e is about thepoint corresponding to, say, 7 minutes, although with'the alloy of curve d an increased time of treatment up to 30 minutes is not detrimentaL'curve e shows that with the alloy of that curve an-increased time oftreatment-other things being equal-reduces the protection, that is to say increases the corrosion rate in respect of the effect obtained by treatment at a lower time corresponding to the knee of the curve.

In anygiven case, the optimum value for any of the variables may be determined by means of accelerated corrosiontests representative of the particular service i-conditi'onsrthe article is required to withstand. A saline immersion or spray test applied in well-known manner' 'is generally regarded as representative of marine "atmospheric exposure and it is on'this basis that my ranges and the results ini'th'e figures have'been established.

Variation in the ionic concentrations do not cause sudden changes in .the effectiveness of the solution, and working limits-must therefore be selected having regard'to'the degree of protection required. V

Drag-out loss is minimized by using the lower concentrations in any given range, by treating in the cold, and'by suspending the workpiece above the bath for a few seconds after treatment. The dichromate concentration tends to fall with use and further additions must be made as indicated by simple analysis.: The sulphate concentration on the other hand rises when sulphuric acid instead of chromic or nitric acid is'used forth occasional major adjustments'in pI-I.

One additional advantage arising from the fact that good protection is secured by a few minutes treatment in the cold is that it may be applied as a simple swabbing process to finished parts in situ, e. g. cut edges of sheet, iwelds, repairs or even as a maintenance treatment. If, nevertheless, the solutions are heated, the time of treatment may be 'even' further reduced and for certain applications this may more than offset the disadvantages incurred by the heating. Ex-

amples are .where thetreatment is -a step in a continuous production line and tank capacity is limited by considerations of floor space or where output must be temporarily or permanently increased in an existing treatment plant.

tion loss of acetic acid b-ecomes serious, the acetic acid and acetate should be substituted in whole or in part by single or mixed acids'and single or mixed salts of these acidsand preferably of the same acid,-as disclosedabove for hot solutions.

For best results, whether the solution is used cold or hot, it is desirable to submit the work- .pieces to a cleaning pre-treatment. Solvent de- The solutions are stable onheating but if the evapora- In the latter case it may be desirable in some cases to extend slightly the protective treatment time. Alternatively, anodic polishing in accordance with the methods disclosed in British patent specifications Nos. 550,175 and 550,176 may be employed as the pre-treatment.

The following table gives a number of specific examples in accordance with which the invention may be carired into eifect:

Table I For magnesium alloys substantially Al free and containing up to 2.5% manganese in accordance with-specifications D. T. D. 118, 140A and 142, a high degree of protection is secured by cleaning by one of the methods described above and immersing for three minutes, at room temperature in a solution made up as follows:

Parts by weight Ammonium, sodium or potassium dichromate, singly or mixed 1 50-100 Ammonium sulphate (or, if desired, the equivalent amount of sodium sulphate if the ammonium salt is used for one The pH of this solution must be adjusted, e. g. with ammonium hydroxide, to between 5.2 and 5.9 but preferably to 5.5. After washing the material is foundto be coated with an adherent greenish or gold brownprotective film which also provides an excellent basis for the usual type of paint coating applied to magnesium alloys.

In cases where a good, but not necessarily the highest, degree of protection is required, the treatment timemay be reduced to one minute. Alternatively, the above concentrations may be reduced by as much as 50% and the treatment time kept at 3 minutes.

. For magnesium alloys containing Al to spec,- ifications D.'T. D. 59A, 88B, 136A, A, 281, 285, 289, 325, 348 and 350, a very high degree of protection is secured by cleaning by one of the methods described above and immersing for 8-12 minutes in the same solution but of pH adjusted to suit the A1 content as indicated in Figure 3. .In this figure the central curve (1 represents the pH values for maximum protection. A tolerance is provided at each side of this curve as shown by the curves b and c. The shaded area between the curves 5 and 0 gives the working-range for good protection. Thus, for an alloy containing 6% Al, .1% Zn and 0.2% manganese the pH range should be 4.8-5.3..but preferably 5.0-5.1. For an alloy containing 7.5% A1, 0.4% Zn and 0.2% Mn the pH should be adjusted to between 4.6 and 5.2

I but preferably between 4.8 and 4.9. After Washminutes.

7 In the following Tables II to VI the pH values may be adjusted as described above.

Table II This table may be taken to comprise the ommended range of ingredients.

Dichromate (CrzO7)--Over 0.05 molar (up to saturation point).

Sulphate (s04)-OVI 0.2 molar. (In cases where the dichromate concentration is increased, the sulphate Concentration should preferably follow suit up to saturation point.)

Bufier mixture, e. g., acetic acid and sodium acetate, total acetate concentration as acetic acid plus sodium acetate together, over 0.1 molar CH3COO-. When other weak acids and their salts are used as buffer mixtures, these concentrations must be altered having regard to the equivalent weight of the acid relative to that of acetic acid.

pH 4-6 depending on alloy composition and time of treatment.

Time of treatment-Thi depends on the composition of the alloy, the solution composition and its pH and varies between /z-3Q minutes.

Table III This table may be taken to comprise the range where the degree of protection is that corresponding to temporary protection of parts during fabrication, storage or transit, or the like,

Dichromate (CrzO'z)-Over 0.1 molar (up to saturation point).

Sulphate ($O4)-Over 0.3 molar. (In cases where the dichromate concentration is increased, the sulphate concentration should preferably follow suit up to saturation point.)

Buifer mixture, e. g. acetic acid and sodium acetate, total acetate concentration as acetic acid plus sodium acetate together, over 0.1 molar CH3COO. When other weak acids and their salts are used as buffer mixture, these concentrations must be altered having regard to the equivalent weight of the acid relative to that of acetic acid.

pH 4-6 depending on alloy composition and time of treatment.

Time of treatment-As in Table II,

Table IV This table comprises the ranges for parts requiring a good average degree of protection.

Dichromate (Cr2Ow)--0.1 to 0.6 molar.

Sulphate (SO4)-0.4 to 2.0 molar.

Buffer mixture, e. g. acetic acid and sodium acetate, total acetate concentration as acetic acid plus sodium acetate together, from 0.2 molar CHsCOO to 1.0 molar CI-I3COO. When other Weak acids and their salts are used as buffer mixtures, these concentrations must be altered having regard to the equivalent weight of the acid relative to that of acetic acid,

pH 4-6 depending on the alloy composition and time of treatment.

Time of treatment-As in Table II.

Table V This table gives the optimum composition for the protective values of the highest order.

Dichromate (C1'2O'1)0.2 to 0.5 molar.

Sulphate (SOD-0.6 to 1 molar.

Bufier mixture, e g. acetic acid and sodium acetate, total acetate concentration as acetic recacid plus sodiumacetate: together, 0.2 to 1.0 molar CHaCOO. When other weak acids and their salts are used as buffer mixtures, these concentrations must be altered having regard to the equivalent weight of the acid relative to that of acetic acid.

Ammonium ions (NH4) 0.4 to 3 molar.

pH 4.6-5.9 depending on the alloy composition and time of treatment.

Time of treatment-L430 minutes depending on the alloy composition.

Major pH adjustments in this case should preferably be made by nitric not sulphuric acid, to maintain the sulphate ion concentration at the optimum.

Table VI This represents the effect of increased temperature.

A magnesium alloy containing 7.5% A1, 0.4% Zn and 0.2% Mn is given a cleaning pro-treatment as described above and treated for ten minutes in the optimum solution of Table V containing adipic acid and sodium adipate and of pH 4.85 (measured in the cold by glass electrode) heated to (SO- C. After washing, the specimen is found to be coated with a black adherent film. When tested by a saline test in known manner, the protective value of this film is found to be of the order of twice that obtained by treatment at room temperature under the optimum conditions; instead of taking the benefit of the heat by increased protection for the same time of treatment, it may be taken by similar protection for a shorter time of treatment.

I claim:

A process for the portective surface treatment of magnesium base alloy which comprises treating the alloy with a solution consisting of water, 0.2 to 0.5 mol per liter of dichromate ion of at least one compound of the group consisting of the dichromates of lithium, potassium, sodium, magnesium, and ammonium, 0.6 to 1.0 mole per liter of sulfate ion of at least one compound chosen from the group consisting of the sulfates of lithium, potassium, sodium, magnesium, and ammonium, a bufler mixture comprising acetic acid and sodium acetate and containing 0.2 to 1.0 mole per liter of acetate ion and adapted to maintain the pH of the solution at from 4.6 to 5.9. the said dichromate and sulfate compounds being so chosen as to yield an arm- ;ntonium ion concentration of 0.4 to 3 moles per GEOFFREY EDWARD COATES.

REFERENCES CIT ED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,138,794 Nelson et al Nov. 29, 1938 2,178,977 Tcsterud Nov. '7, 1939 2,224,245 Allen Dec. 10, 1940 2,224.528 Sutton et al Dec. 10, 1940 2,288,552 Siebel et al June 30, 1942 2,352,076 Bushrod June 20, 1944 FOREIGN PATENTS Number Country Date 353,415 Great Britain July 14, 1931 558,983 Great Britain Jan. 31, 1944

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