1-
. .
ZINC PHOSPHATE SPH ERULlT IC
ZINC PHOSPHATE IRON & CHROMIUM PHOSPHATES
I I Il 1 11111 I
1
I IRON
GRAIN BOUNDARY
CHROMIUM COMPLEXED POLYVINYL BUTYRAL POLYVINYL BUTYRAL
Figure 1.
Structure of wash primer film on iron surface
JEROME KRUGER' and
M. C. BLOOM
Naval Research laboratory, Washington 25, D. C.
Mechanism of Wash Primer Action
DURING
World War 11 Whiting and Wangner (77) empirically developed compositions known as wash primers. One of these compositions, which they prepared by adding a dilute alcoholic solution of phosphoric acid to an alcoholic dispersion of zinc tetroxychromate in poly(viny1 butyral), yielded a primer having an unusually high order of adhesion to a variety of metals. Furthermore, paint films adhered well to it and the system so produced exhibited remarkable protection of the metal against corrosion. This wash primer, usually designated as WP-1, had the following composition : Base Grind Parts by Wt. Vinylite resin XYHL [poly(vinyl butyral) la 7.2 Zinc tetroxychromatea 6.9 1.1 Talcb Isopropyl alcohol, 99 70, or ethyl alcohol, 9570 48.7 Butyl alcohol 16.1 80.0%
Present address, National Bureau of Standards, Washington, D. C.
1 354
Acid Diluent Phosphoric acid, 85 70 Water Isopropyl alcohol, 9970, or ethyl alcohol, 9570
3.6 3.2 13.2 20.070
a PVB and ZTC are used in this report to designate, respectively, poly(viny1 butyral) and zinc tetroxychromate. *Talc was omitted in all cases in this study on assumption that it is merely inert filler.
The introduction of the treatment of metal surfaces by this wash primer, one of the most commonly used, is considered one of the most important steps in corrosion prevention in recent years. I t is being applied increasingly by the Navy for the treatment of bare metal surfaces prior to painting to inhibit corrosion and increase paint adhesion. Hence, an understanding of the mechanism by which this treatment operates is worthy of a considerable effort. Various authors (77, 73, 7 4 , speculating on the manner in which this wash primer acts to provide its protective and adhesive
INDUSTRIAL AND ENGINEERING CHEMISTRY
action, postulate two steps: the formation of an inorganic film by a reaction of some of the constituents of the wash primer with the metal, and the subsequent deposition of an organic film on top of this inorganic film. Thus far, no experimental evidence has been offered to support this view. Indeed, no fundamental study has been made of the metal-wash primer interface in spite of the fact that a knowledge of the nature of this interface would seem to bt. a prerequisite to an understanding of the mechanism of wash primer operation. The work described in this paper is an initial attempt to investigate the strmture of this wash primer film on an iron surface, its mechanism of formation, and its mode of action in retarding corrosion and increasing paint adhesion. The results illustrated in Figure 1 indicate that the wash primer treatment on iron surfaces generates a protective oxide film, supplies chromate for repair of any damage to such a film, supplies a zinc phosphate film analogous to the films produced in phosphate treatments, and
superimposes on these a poly(viny1 butyral) film which acts as a good mechanical protection and as a bond for subsequent paint films.
Table 1. Comparison of X-Ray Diffraction Patterns of Zinc Tetroxychromate Prepared from Different Proportions of Zinc Oxide to Chromium Trioxide (6.25 grams of chromium trioxide used for all ZTC preparations)
Experimental Methods and Results
Because of the highly complex system that wash primer comprises, it was necessary to use many different experimental techniques and tools in an attempt to understand its mode of action on an iron surface. Below are listed these techniques, the manner in which they were employed, and the results obtained. X-Ray Diffraction Studies. T o determine the nature of the solid crystalline phases present in wash primer, x-ray diffraction studies were carried out both before and after the constituents were mixed. Most of these experiments were made employing the Debye-Scherrer technique using ,CrKcr radiation with a vanadium filter. X-ray diffraction patterns were obtained from both commercial ZTC (zinc tetroxychromate) and ZTC prepared in this laboratory according to the procedure outlined by Leisy (7). The method consisted of stirring 25 grams of finely divided zinc oxide into 250 ml. of water to form a suspension and adding to this 6.25 grams of chromium trioxide dissolved in 7.5 ml. of water. With continued stirring a t room temperature, this mixture stiffened considerably in about 9 minutes. I t was filtered, washed with water, and dried in an oven a t approximately looo C. When zinc oxide and chromium trioxide were combined in proportions calculated to yield the compound proposed by Leisy to be 5ZnO.CrOa.4Ha0, the pattern obtained did not differ essentially from those of ZTC preparations made in the same manner but containing 26, 30, or 35 grams of zinc oxide for each 6.25 grams of chromium trioxide. I n Table I are listed the d values for ZTC prepared using these four proportions of zinc oxide to chromium trioxide. Also listed are the d values obtained for zinc oxide. The ZTC patterns are all essentially the same, but they all contain the lines from zinc oxide. Thus, Z T C made up as Leisy’s patent requires contains free zinc oxide and a species that yields a pattern different from that of the only zinc chromate listed in the A.S.T.M. card file of x-ray diffraction data. Further x-ray work on the characterization of zinc chromates is needed. An x-ray diffraction pattern was obtained from wash primer after its mixed components were allowed to stand 0.5 hour. The pattern obtained contained the lines of the patterns listed in the A.S.T.M. card file of x-ray diffraction data, Nos. 2346 and 2373, for tertiary zinc phosphate tetrahydrate. The lines for ZTC also appeared.
VS M W S 25
Relative intend, A.
sity
7.60 4.23 3.73 3.34 2.86 2.73 2.65 2.60 2.52 2.22 2.04 1.94 1.85 1.78 1.64 1.59
S W M W S MS MS S
.
.
I
1.50
...
... 1.39 1.37 1.35
.... 1.23 1.18
.... .... .... ....
....
.... ....
vs W vw
= = = =
Very strong Medium Weak Strong
MW = Moderately weak VW = Very weak MS = Moderately strong
Zinc Oxide, Grams 26 30 RelaRelative tive intenintend, A. d, A. sity sity 7.61 4.26 3.70 3.25 2.87 2.78 2.65 2.60 2.51 2.24
vs W M W S MS S
vs vs
W
...
...
W W
1.94 1.85
MW W
...
...
M S
1.64 1.59 1.55 1.49
MS
vw ... S ... ... S M
W
..
vw vw .. .. .. ..
.. .. ..
...
... 1.39 1.37 1.35
....
1.24 1.18
.... ....
.... .... .
.
I
.
,,.. ....
S W
S ... ... S
M W
..
W
vw .. ..
.. .. ..
..
..
7.58 4.41 3 .72 3.36 2.87 2.78 2.65 2.61 2.52 2.22 2.04 1.98 1.85 1.78 1.64 1.59 1.56 1.49
S W MS MW M MS M
vs S
W VW
25
Relative intend, A.
7.60 4.29 3.73 3.34 2.87 2.76 2.65 2.56 2.50 2.23
...
.... .... ....
A transmission x-ray diffraction pattern was made of the clear green film obtained after allowing all the solid constituents of the wash primer to settle out for 3 days. The pattern obtained (Figure 2) gives no evidence of the existence of any crystalline phases incorporated in the film. An experiment was carried out in which all the components of wash primer except PVB [poly(vinyl butyral)] were mixed in the proportions given previously. The green solution resulting after these constituents were mixed was filtered to remove the solids present in the mixture. After the solution was allowed to stand a few minutes with the addition of more ethyl alcohol to induce further precipitation, a green precipitate was observed. This solid was filtered off and dried. Examination by x-ray diffraction indicated that the material was amorphous. Chemical analysis of
...
...
...
...
2.81
S
2.61 2.46
MS
... ...
...
...
...
vs
... ...
...
M MS W M
M
...
... ... ...
W
1.61
MS
....
....
...
...
..
....
... ...
... ...
... ...
MS MW MW
....
vs vw
...
1.91
1.39 1.37 1.35
..,.
S MS MS MS
sity
MW
... ...
1.24 1.18
MW W
d, A.
1.94 1.85
... ....
S
vw
tive inten-
vw MW vw
1.64 1.59 1.56 1.49 1.42 1.42 1.39 1.37 1.31
...
sity
Pure Zinc Oxide Rela-
W W
1.24 1.18
.. .. .. ..
.... .... ....
..
.... .... ....
..
....
vw
vw
M
...
...
S
1.47
MS
W W S M
... ...
vw ..
VW
vw .. .. .. .. ..
.. ..
1.40 1.37 1.35 1.30 1.24 1.18 1.@9 1.04 1.02 0.98 0.95 0.94 0.91
...
...
W M W W W W W W W W W W W
this green solid revealed the presence of 25% zinc, 7% chromium, 56% phosphate, and 12% water. Infrared Spectroscopic Studies. T o gain some information on the fate of the polymer in the wash primer mixture, some infrared spectroscopic studies were made. The procedure consisted of obtaining spectra from films of PVB, using the technique described by Beachell (2). This involved dissolving 1 to 2 grams of PVB in 15 to 20 mi. of a solvent, and then painting a clean microscope slide with this solution. The rate of evaporation of the solvent influenced the clearness of the film. Satisfactory films could be obtained if n-propyl or n-butyl alcohol or tetrachloroethane was used, but ethyl alcohol, acetone, and a number of other solvents either yielded powdery films or did not dissolve PVB. Once the solvent had completely evaporated, the film that had formed on the microscope VOL. 48, NO. 8
AUGUST 1956
1355
Figure 2. X-ray diffraction pattern of green film obtained by allowing all solid constituents of wash primer to settle out
01
'
3
I
I
I
I
I
I
I
I
I
5
6
7
a
t
4
9
IO
II
12
13
WAVE LENGTH, MICRONS Figure 3.
slide was soaked in water for 10 to 15 minutes, stripped off, dried over anhydrous magnesium perchlorate, and inserted in the holder of a Perkin-Elmer infrared spectrophotometer. Films were cast from different solvents and different sources of PVB. Thus, Vinylite XYHL (Bakelite) and Butvar B90 (Monsanto) were dissolved in npropyl alcohol and n-butyl alcohol, respectively. As Figure 3 shows, these films exhibited no differences. Accordingly, films of PVB with ZTC. with chromic acid, and with phosphoric acid were prepared. The films of PVB with phosphoric acid could not be stripped off using the described technique because they apparently dissolved when exposed to water to remove them from the microscope slide. The spectra of the films cast from these mixtures contained the same absorption peaks as the spectrum of a pure PVB film (Figure 4). The wash primer films also exhibited the same spectra if the solid materials in the mixture were allowed to settle and a film was cast of the supernatant liquid, as Figure 5 shows. Films cast with the solids incorporated in them were too opaque to infrared radiation to yield satisfactory spectra. Special mention must be made of the experiments with PVB and chromic acid. Films incorporating chromium trioxide in the PVB could not be produced under all circumstances. Four alcohols were used as diluents for the preparation of the films: ethyl, isopropyl, n-propyl, and n-butyl. Using ethyl alcohol and casting rhe film 5 to 10 minutes after mixing the constituents produced a powdery film unsuitable for infrared studies. With isopropyl alcohol as a diluent no film could be cast even if the attempt was made shortly after the chromium trioxide was added to the PVB, for the solution turned progressively darker and began to form a gel. Films could be formed from n-propyl alcohol but only if the solutions had stood for less than 0.5 hour, as these solutions also formed gels on longer
1 356
Infrared spectra of Monsanto and Bakelite PVB
--
Monsanto, n-butyl alcohol
__------- Bakelite, n-propyl alcohol
WAVE LENGTH, MICRONS Figure 4.
Infrared spectra of PVB, alone and in mixtures
PVB (film from n-propyl alcohol)
---_----- PVB + ZTC (film from tetrachloroethane)
-._.-.
PVB
+ chromium trioxide + water (film from n-butyl alcohol);
\LJ
cast 3 days after mixing
; J
I
I
I
I
I
'4
5
6
7
8
9
I
I
I
IO
II
12
WAVE LENGTH, MICRONS Figure 5. Infrared spectra of films cast from WP-1, from which solids have been removed, and containing varying amounts of phosphoric acid PVB (film from n-butyl alcohol) Other spectra: base grind (in n-butyl alcohol 3 days after mixing Theoretical amount of acid
-.-._.
+ troce_-_-._-_of ethyl alcohol) + acid mix o-butyl alcohol); Excess amount of acid mix
standing. With n-butyl alcohol films could be cast from solutions that stood
INDUSTRIAL AND ENGINEERING CHEMISTRY
(in
-. .-.
, Trace of acid mix
as long as 3 days. I KFigure ~ 6 is shotvn a comparison betwern the film obtained
I 13
w
i
using ethyl alcohol and the one obtained using n-butyl alcohol. Paramagnetic Resonance Studies. Since it has been shown by Singer (75) that it is possible to show the existence of trichelated chromium(II1) compounds by the use of a paramagnetic resonance technique, a few experiments using this technique were carried out to determine if such compounds are formed by the interaction of the PVB and chromium(111) ions in the wash primer mixture. The paramagnetic resonance absorption experiments were carried out on a powder made from ground wash primer film. The measurements were made on the 1.2-cm. wave-length microwave apparatus described by Singer. The absorption peak obtained for the wash primer was essentially the same as that obtained from a chromic phosphate precipitated from a solution of chromium(II1) ions in phosphoric acid by the addition of ethyl alcohol. The absorption peak for the chromic phosphate is typical of most hexacoordinated chromium(II1) compounds. When measurements were carried out on wash primer from which all solid material was filtered after 3 days' standing in order to remove chromic phosphate, very little chromium(II1) could be detected in the polymeric substance that resulted after solidification of the filtrate. However, the small amount of chromium(II1) remaining yielded essentially the same absorption peak as the chromic phosphate. I n no instances were absorption peaks obtained similar to those measured by Singer for a number of trichelated chromium(II1) compounds. Microscopic Studies of Wash Primer -Treated Surfaces. The object of the
a.
Ethyl alcohol diluent
Figure 6.
experiments using the metallurgical microscope was to study the interaction of wash primer with an iron surface. I n this procedure both polycrystalline electrolytic iron and single crystalline Armco iron slices were polished with 0 through 0000 grade emery paper, washed in running tap water, ethyl alcohol, diethyl ether, ethyl alcohol again, and finally tap water. Then the slices were chemically polished using a MirroFe bath (MacDermid Inc., Waterbury, Conn.) for 45 seconds, washed in boiling distilled water, and dried in a stream of hot air. After this preparation of the surfaces to be studied, the specimens were introduced into the apparatus shown in Figure 7. I n this apparatus, which was also used in the electron diffraction and microscopy experiments described below, it was possible to anneal the metal specimens in hydrogen and then to immerse them in wash primer while controlling the nature of the surrounding atmosphere. The metal specimens were annealed in a stream of purified hydrogen a t 750' C. for 12 hours. During the annealing process the wash primer mixture was frozen by placing a container of liquid nitrogen around A . After the annealing process, the system was evacuated, the specimens were brought to room temperature, and hydrogen was admitted to the system. The wash primer mixture was allowed to thaw and reach room temperature; the chambers containing the metal specimens and the wash primer were sealed off from the vacuum and gas inlet lines a t X and Y, and the apparatus was placed in the position shown in Figure 6,b. This procedure allowed the wash primer to
b.
n-Butyl alcohol diluent
Films obtained from incorporating chromic acid in PVB, 15 X
run out of its chamber and come into contact with the metal specimens without contacting lubricated joints or stopcocks. After 15 minutes the specimen chamber was cracked open, the specimens were removed, and the wash primer was allowed to dry as a thin film on the specimen surface. After drying for 1 hour, the wash primer-coated specimens were immersed in ethyl alcohol for a number of days. This treatment, along with light rubbing with absorbent cotton, removed the wash primer film. The specimens were then examined in a Bausch & Lomb Metallograph. Experiments using this refined procedure as well as a cruder procedure, which involved simply painting wash primer on the metal surface, produced the following significant results: The metal surface is attacked by the wash primer. Figures 8, 9, and 10 show the appearance of three single crystal surfaces of different orientations after the removal of wash primer films using the technique described above. These photographs indicate that the degree of attack varies with the orientation of the surface exposed to the wash primer. The (111) face shows the least attack, the (110) face the most. Further, if the amount of the anisotropic solid on each of the three crystal faces can be used as a n indication, the solid showed the greatest tendency to adhere to the (110) face, next to the face about 12' off the (100) face, and least to the (111) face. Figure 11 shows a polycrystalline iron surface after the removal of a wash primer film. I n this case it can be seen that the anisotropic crystalline material [presumably the Zn3(P04)2.4HzO found in the x-ray studies] adheres very tightly to the grain boundaries of the iron when a wash primer film is removed from the iron surface. Figure 12 shows another single crystal surface, illuminated with polarized light, containing remnants of a wash primer film. In this photograph spherulites can be readily observed. They appear under the microscope as small white circular particles each containing a dark cross. A spherulite consists of a large number of crystals radiating as fibers in all directions from a center, a particular direction of each crystal lying consistently along the fiber axis. The possibility exists that these spherulites may be PVB rather than Zn3(P04)2.4HzO, but it is remote since microscopic examination of thin PVB films reveals no spherulite formation or any optical anisotropy, Zinc phosphate, however, does form spherulites according to Wieler (78) and Morse, Warren, and Donnay (70) and is optically anisotropic. Electron Microscopy a n d Diffraction Studies, Electron microscopy and diffraction were employed as more sensitive means of examining the metal-wash VOL. 48, NO. 8
AUGUST 1956
1 357
i- 1/I
TO VACUUM
/ HZ
patterns were made using the RCA EhIU-2 electron microscope with an extended field lens. With this apparatus an electron diffraction pattern can be obtained from a small area of about 1 square micron which is under study bv the electron microscope. Figure 14 shows that arcing exists in the transmittance pattern of the stripped film. This indicates that the FesO4 or ?-Fez03 has preferred orientation in the stripped film. This agrees with the results obtained by Mayne and Pryor ( 9 ) for yFez03 films formed on iron by chromate solutions.
A
INLE
MIXTURE FROZEN MWN I N LlPUlD N2 SPECIMENS (a) APPARATUS I N ANNEALING POSITION
(b) APEARATUS IN POSITION FOR
CONTACTING SPECIMENS WITH WASH PRIMER MIXTURE Figure 7.
Apparatus for depositing films of wash primer on metal surfaces
primer interface. Both reflection and transmittance techniques were used. The metal surfaces were prepared as described in the preceding section, except that the surfaces were etched with 1y0 nital (nitric acid-ethyl alcohol) rather than polished with MirroFe, because the rougher surface produced by etching was more suitable for electron diffraction studies. Also, instead of treating the annealed metal surfaces with the ordinary wash primer mixture, a very dilute mixture was used. The dilute mixture was made from PVB. ZTC, 85% phosphoric acid, and water in the proportions listed previously, but it contained approximately 70 times as much alcohol, the alcohol used being nbutyl alcohol. This dilution was necessary to obtain a film thin enough for electron diffraction studies. To obtain
Figure 8.
transmittance patterns and micrographs it was necessary to strip these wash primer films from the surfaces on which they were formed. This was accomplished using the technique described by Phelps. Gulbransen, and Hickman ( 7 4 , who removed thin films from metal surfaces by the anodic dissolution of the metal beneath the film, carrying out the process in an atmosphere of hydrogen. The reflection patterns obtained from thin wash primer films on both polycrystalline and single crystal iron surfaces revealed the presence of randomly oriented ferriferrous oxide (FesOl) or yferric oxide (y-FezOa), the patterns for these two oxides being indistinguishable (Figure 13). The transmittance patterns obtained from stripped films also gave some indication of FesO4 or y-FeaOs. These
( 1 1 1) face
Figure 9.
(1 10) face
Appearance of iron crystal face after removal of wash primer film.
1358
INDUSTRIAL AND ENGINEERING CHEMISTRY
Discussion From the results just described an attempt will now be made to form a rudimentary picture of the structure of the wash primer film and the mechanism of its formation on an iron surface. In order to do this it would be best to examine first the processes that take place when the base grind and the acid diluent are mixed together before application to the metal surface. As soon as all of the constituents of wash primer are mixed, the zinc tetroxychromate begins to dissolve. introducing chromate and zinc ions into the solution. This dissolution process results in a slow neutralization of the phosphoric acid by the zinc tetroxychromate. That this process is relatively slow can be seen from the work of Muller ( I I ) , who found that the acid value of wash primer (milligrams of potassium hydroxide to neutralize 1 gram of wash primer) declines from approximately 9 to 4 in a period of 5 days. As the acidity of the mixture is reduced, tertiarv zinc phosphate tetrahydrate is precipitated as indicated by the x-ray diffraction studies. Some secondary zinc phosphate niav form, but it is not enough to give linrs in the
Figure 10. Face about 12" off (100) orientation Polarized light illumination, 500X
Figure 1 1 . Appearance of polycrystalline iron surface after removal of wash primer film Polarized light illumination, 500 X
Figure 12. Iron single crystal surface, after removal of wash primer film, showing spherulitic zinc phosphate Polarized light illumination, 500 X
Figure 13. Reflection electron diffraction pattern of thin film o$ wash primer on iron surface
powder patterns taken. While this process is going on, the chromate ion oxidizes the alcohol of the wash primer mixture, forming the green chromium(111) ions. Judging from the length of time required for the wash primer mixture to become markedly green, it would be safe to assume that this also is a relatively slow process. As the acidity of the mixture is further reduced by more neutralization of the zinc oxide and the quantity of chromium(II1) increases, some of the chromium(II1) ions may be precipitated as a phosphate. I n addition to this possibility, the chromium(II1) ions created in wash primer may form a complex with the poly(viny1 butyral), as the results of Eirich and coworkers (4) tend to show. The paramagnetic resonance experiments described here indicate that any complex which may be formed is probably not of the trichelated types studied by Singer. Other than chromium complex formation and the production of small amounts of poly(viny1 alcohol) by hydrolysis, as revealed by the work of Barnes and Allen (7)) the infrared experiments revealed that no drastic structural changes in the PVB occur even in the presence of chromic acid. Larson (6) mentions that the action of the chromic acid on the resin results in its tanning or insolubilization. I t is probable that this tanning results from cross linkage in the resin. If so, the cross linkage has apparently not proceeded far enough to show up in the infrared spectra obtained in these experiments. As shown in the work on preparing films for the infrared studies, it was noted that the quality of the PVB film is markedly influenced by the nature of the alcohol used as a diluent. The rate of evaporation is undoubtedly a factor; the slowly evaporating n-butyl alcohol produces more satisfactory films than the rapidly evaporating ethyl alcohol. However, more work needs to be done to gain a real understanding of the influence of the alcohol used on the texture of the
Figure 14. Transmittance electron diffraction pattern of thin film of wash primer stripped from iron surface
PVB film formed and on the life of wash primer formulations. The processes that occur when the wash primer mixture is applied to a n iron surface must also be examined. These processes a t the metal surface may be similar to those occurring during phosphating; the latter are essentially electrochemical in nature. Figures 8, 9, and 10, showing different degrees of attack and different amounts of zinc phosphate tetrahydrate adhering to the iron surface depending on the crystallographic orientation (and thus the solution potential) of the surface, serve to emphasize this. Thus, when wash primer is applied to an iron surface containing a thin air-formed oxide film, a reaction takes place a t the anodic areas, frequently the grain boundaries. This reaction can be represented by FeO + Fe++
+ 26-
At the cathodic areas the reaction that occurs is probably 2H+
+ ?.e-+
2H
or CrOa--
+ 8H+ + 3 e -
-+
4H20
+ Cr+'+
These effective cathodic areas to which appreciable current flows from a given anode must be close to the anodic areas because of the high resistance of the wash primer mixture. As a result of the cathodic reaction, the p H of the solution surrounding the cathodic regions is raised and tertiary zinc phosphate tetrahydrate precipitates in those regions. Figure 11, showing zinc phosphate tetrahydrate a t or near the grain boundaries, would support this picture. While this deposition process is going on near the cathodic regions, a secondary reaction 3Fef+
+ CrOd-- + 8 H f - + 3Fe+++
+ Cr+'+
f 4H20
is taking place around the anodic areas. Thus, the pH is also raised in these areas and ferrous ions, ferric ions, and zinc ions may be expected to be deposited as phosphates here. I n those areas which are not in close proximity to the anodes of the surface and to which, hence, appreciable current cannot flow, phosphate deposition does not take place. Instead, y-FeaO3 or Fer04 is present, as shown by the electron diffraction experiments. They are present either before the metal is coated with wash primer, or the ?-Fez03 is formed in the presence of the strongly oxidizing chromate ion, as Mayne and Pryor ( 9 ) have shown. As the p H of the wash primer solution is continually being raised by the neutralization processes a t the metal surface and in the solution itself, chromium phosphate may precipitate as the chromium(II1) ion concentration increases. VOL. 48, NO. 8
AUGUST 1956
1359
zinc phosphate tetrahydrate, which probably starts to form in the wash primer mixture even before its application to the metal, will be settling out. Along the metal surface the spherulitic zinc phosphate tetrahydrate (Figure 12) will be found. Finally, any chromium complex of the PVB and PVB itself will be deposited onto the inorganic reaction products on the metal surface. A schematic representation of the situation that exists on an iron surface containing wash primer, as just described, is shown in Figure 1. I t is now necessary to examine the nature of the bonding between the various phases in the wash primer film. Since the ferrous ions which are formed at the anodes and the zinc ions which are present in solution have nearly equal ionic radii, the formation of a phosphate phase containing both of these ions seems likely. Actually, such a phosphate does exist in nature in the form of the monoclinic mineral phosphophyllite, (Fe, Zn)s(P04)2.4H20. The possibility exists that some solid phase of this type containing both iron and zinc is formed and would attach itself to the iron surface. This phase would probably be composed entirely of a ferrous phosphate at the iron surface, but upon moving away from the metal surface it would become increasingly poorer in ferrous ion content and richer in zinc ion. Associated with this mixed zinc-ferrous phosphate would be ferric phosphate formed from the ferric ion produced by the chromic acid oxidation of the ferrous ions. That a ferric phosphate thus formed would be strongly bonded to the underlying phosphate appears likely because the ferric phosphate, FeP04. 2H20 (the mineral metastrengite), has a crystal geometry close to that of phosphophyllite (76) and occurs with it in nature. Any chromic phosphate that may be formed may also be attached to the ferric phosphate, because chromic ion has an ionic radius whose value is close to that of ferric ion. Hence, mixed chromic-ferric phosphate crystals may be formed. Finally, the bonding of the polymer to the inorganic substrate may be effected by the fitting of the chromium of a complex into the lattice of the mixed ferric-chromic phosphate. These are only plausible hypotheses suggested by the available data. Their validity must be tested by further work. The question now arises: Why does this particular arrangement of substances (if it exists as described) impart the corrosion-resistant qualities that wash primer gives to metal surfaces? The great advantage of the wash primer system is that it depends on more than one means of preventing corrosion. First, it produces a y-FezOa and a zinc phosphate film exactly similar to the kind produced in various phosphatizing treatments. These phosphate films are
1 360
essentially composed of Zns(P04) 2.4Hz0 in the case of zinc phosphate treatments. The fact that a protective zinc phosphate film can be produced by treating the metal surface with wash primer is not surprising. because according to Burbank ( 3 )such a film can be formed using a gellike medium at room temperature rather than the conventional hot bath used in phosphatizing treatments. However, as pointed out by Machu (8). phosphate treatments of this sort are not always completely satisfactory because they exhibit porosity. He suggests treatment of this film with chromates or chromic acid. Wash primer provides just this. The function of the chromate is to patch up any holes in the phosphate film by oxidizing the exposed metal to r-FezOa (9). Chromates are, however, sometimes dangerous to use as inhibitors if they are not present in sufficient quantity (5). Wash primer takes care of this by providing a steady stream of chromate ions from the very slowly dissolving unreacted ZTC which is held in place in the polymeric film matrix. Finally, the tanned polymeric film acts as a good mechanical protection for the metal surface and serves as a place for paint anchorage.
aluminum or perhaps to improve the wash primer for iron by substituting other compounds for the ones currently used. Another important problem to be attacked is the effect of the aging of wash primer on its interaction with metal surfaces and on the structure of the films deposited, and the influence that the type of alcohol has on this process. Acknowledgment
The authors are greatly indebted to the following persons for their aid which made possible the use of many techniques in this study: D. P. Eftax, for developing the technique for casting and casting the PVB and wash primer films for the infrared studies; R. E. Kagerise, for obtaining and helping to interpret the infrared spectra; W. C. Sadler and F. W. von Batchelder, for the x-ray diffraction patterns; L. S. Singer, for the paramagnetic resonance measurements; A. R. Donaldson, for the metallography; and J. B. Burbank, for her assistance with the electron diffraction studies.
literature Cited Future Work
A numbrr of questions need to be resolved. First, more information is needed on the crystal growth relationships of the zinc, iron, and chromium phosphates. Studies should be undertaken on systems of this sort on the macro scale as well as on the ultramicro scale, which exists at the wash primer-iron interface. More electron diffraction studies should also be undertaken to study the nature of the inorganic deposits in the cathodic and anodic areas of the iron surface-for example. the changes in composition and orientation that occur as the area at and then near the grain boundaries is in turn explored. The fact that the successful phosphating processes for iron and steel surfaces are those containing Z n + + or M n + + ions, which are close to Fe"+ in ionic radius, suggests that another aspect that should be looked into is the effect of the ionic radii of the ions making up a protective film. Does the fact that Zn++ and Fe++ and F e f + - and Cr++" have similar radii in the present wash primer system have any importance? Use of ions such as C o A T or Mn++, which have ionic radii close to Z n + + and Fe++, in place of these latter may shed some light on the crystal growth processes in wash primer. With this understanding it would be possible to predict, on the basis of the size of the ions needed, what ions should be incorporated into a more effective wash primer for magnesium or
INDUSTRIAL AND ENGINEERING CHEMISTRY
(1) Barnes, R A., Allen, E. A., private communication, 1954. ( 2 ) Beachell, H. C., Fortis, P., Hucks, J . , J . Polymer Sci. 7, 353 (1951). (3) Burbank, J. B., Naval Research Laboratory, Report 3519 (1949). (4) Eirich, F., private communication, 1954. ( 5 ) Evans, U. R., "Metallic Corrosion Passivity and Protection" 2nd ed., p. 541, Edward Arnold, London. 1946. ( 6 ) Larson, V. L., 0 8 6 . Dig. Federation Paznt & Vurnish Production Clubs 26,
837 (1954). (7) Leisy, R. W,, U . S. Patent 2,251,846 ( A ~ r i29. l 1940). (8) M\/l.acLu, W., Arch. Metallkunde 3, 335 (1949). ( 9 ) Irlayne, J. E. O., Pryor, M. J.: J . Chem. Sac. 1949, 1831. (IO) Morse, H. W., Warren, C. H., Donnav. J. D. H.. A m . J . Sci. 23, 421 (1932). (11) Muller, G., Paint Munzif. 24, 311 (1954). (12) Phelps, R. T., Gulbransen, E. A>.\ Hickman, J. W., Ind. Eng. Chem., Anal. Ed. 18, 391 (1946). (13) Rosenbloom, H., IND. ENG. CHEM. 45, 2561 (1953). (14) Schiefle, B. F. H., Werkstoffe 21. Korrosion 4, 208 (1953). (15) Singer. L. S., J . Chem. Ph.ys. 23, 379 /rnccj
\I'/JJ,.
(16) Strunz, H., Xaturwissenschajten 34, 531 (1942). (17) Whiting? L. R., Wangner? P. F. (to Secretary of Navy, USA), U. S. Patent 2.525.107 (Oct. 10, 1950). (18) Wieler, A.,'Kolioid-Z. 70, 79 (1935). RECEIVED for review November 14, 1955 ACCEPTED March 27, 1956 Presented in part, Division of Paint, Plastics and Printing Ink Chemistry, 128th Meeting, ACS, Minneapolis, Minn., September 1955.
