Method for darkening a color-selection electrode

A method for making a dark, adherent coating on exposed metal surfaces of a color-selection electrode assembly of a CRT includes the steps of applying a first solution of aqueous phosphoric acid to the metal surfaces of the assembly; contacting the metal surfaces with a second solution of selenium dioxide and methanol; rinsing the assembly; and then, applying a sealing solution to the surfaces of the assembly.

This invention relates to a method for darkening a metal part for use 
within a cathode-ray tube (CRT) and, more particularly, but not 
exclusively, to a method for making a dark, adherent coating on a 
color-selection electrode assembly of a CRT. 
BACKGROUND OF THE INVENTION 
It is a common practice to blacken the surfaces of the color-selection 
electrode and its support frame, which together comprises the 
color-selection electrode assembly, in order to increase the radiative 
characteristics and to reduce the reflectivity thereof. The term 
color-selection electrode includes not only the conventional shadow mask 
and tension mask but also similar structures, adjacent to the luminescent 
screen of the CRT, which intercept at least a portion of an electron beam 
directed toward the screen to assure excitation of the proper 
color-emissive phosphor. 
One method of blackening the surfaces has been to fire the color-selection 
electrode, the frame, or the completed assembly in an oxidizing atmosphere 
to yield a black oxide of iron. However, the quality of such oxide 
coatings vary considerably. Also, iron oxide has a tendency to flake off 
the surfaces and to decompose in vacuum when it is heated and bombarded 
with electrons, as is the case during operation of the color cathode-ray 
tube. Additionally, such a conventional process is unsatisfactory for 
darkening color selection electrodes made from materials having a high 
nickel content. Also, the elevated temperature required during the firing 
process tends to non-uniformly affect the tension of some of the elements 
of the tension mask, rendering some of the masks unacceptable for their 
intended purpose. A need therefore exists for a process that does not 
require elevated temperatures, is inexpensive to perform, is applicable to 
both shadow masks and tension masks, and provides uniform results on steel 
and low expansion nickel-iron alloys. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a method for making a dark, 
adherent coating on exposed metal surfaces of an element of a cathode-ray 
tube comprises the steps of applying a first solution of aqueous 
phosphoric acid to the metal surfaces; contacting the metal surfaces with 
a second solution of selenium dioxide and methanol; rinsing the surfaces; 
and then, applying a sealing solution to the surfaces.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 shows a color CRT 10 having a glass envelope 11 comprising a 
rectangular faceplate panel 12 and a tubular neck 14 connected by a 
rectangular funnel 15. The funnel 15 has an internal conductive coating 
(not shown) that contacts an anode button 16 and extends into the neck 14. 
The panel 12 comprises a viewing faceplate or substrate 18 and a 
peripheral flange or sidewall 20, which is sealed to the funnel 15 by a 
glass frit 21. A three color phosphor screen 22 is carried on the inner 
surface of the faceplate 18. The screen 22, shown in FIG. 2, is a line 
screen which includes a multiplicity of screen elements comprised of 
red-emitting, green-emitting and blue-emitting phosphor stripes R, G and 
B, respectively, arranged in color groups or picture elements of three 
stripes or triads, in a cyclic order. The stripes extend in a direction 
which is generally normal to the plane in which the electron beams are 
generated. In the normal viewing position of the embodiment, the phosphor 
stripes extend in the vertical direction. Preferably, at least a portion 
of the phosphor stripes overlap a relatively thin, light-absorptive matrix 
23, as is known in the art. Alternatively, the screen can be a dot screen. 
A thin conductive layer 24, preferably of aluminum, overlies the screen 22 
and provides a means for applying a uniform potential to the screen, as 
well as for reflecting light, emitted from the phosphor elements, through 
the faceplate 18. The screen 22 and the overlying aluminum layer 24 
comprise a screen assembly. 
A multi-apertured color-selection electrode 25 is secured to a frame 26 to 
form a color-selection electrode assembly which is removably mounted, by 
conventional means, in predetermined spaced relation to the screen 
assembly. The color-selection electrode 25 may be a shadow mask, a tension 
mask, or other similar structure known in the art. The color selection 
electrode may be made of steel or a low expansion nickel-iron alloy. A 
dark, adherent coating 27, shown in FIG. 2, is formed on the exposed metal 
surfaces of the color-selection electrode assembly by the novel method of 
the present invention. 
An electron gun 28, shown schematically by the dashed lines in FIG. 1, is 
centrally mounted within the neck 14, to generate and direct three 
electron beams 29 along convergent paths, through the apertures in the 
mask 25, to the screen 22. The gun 28 may be, for example, a bi-potential 
electron gun of the type described in U.S. Pat. No. 4,620,133, issued to 
Morrell et al., on Oct. 28, 1986, or any other suitable gun. 
The tube 10 is designed to be used with an external magnetic deflection 
yoke, such as yoke 30, located in the region of the funnel-to-neck 
junction. When activated, the yoke 30 subjects the three beams 29 to 
magnetic fields which cause the beams to scan horizontally and vertically, 
in a rectangular raster, over the screen 22. The initial plane of 
deflection (at zero deflection) is shown by the line P-P in FIG. 1, at 
about the middle of the yoke 30. For simplicity, the actual curvatures of 
the deflection beam paths in the deflection zone are not shown. 
An example of the novel method for forming a dark, adherent coating on the 
metal surfaces of a color-selection electrode assembly, for example on a 
shadow mask-frame assembly, will now be described. The metal surfaces must 
be clean and free of oil and rust. The surfaces, initially, are cleaned by 
vapor degreasing in a suitable hydrocarbon solvent, such as FREON. A first 
solution of aqueous phosphoric acid having a concentration within the 
range of about 1.0 to 2.0 percent, by weight, 1.5 wt. % being preferred, 
then is applied to the metal surfaces to remove, and further inhibit, 
surface rust. Next, the surfaces of the color-selection assembly are 
contacted, e.g., by dipping, with a second solution consisting essentially 
of selenium dioxide and methanol. The concentration of selenium dioxide is 
within the range of about 0.01 to 8.0 percent, by weight, 2.0 wt % being 
preferred. The degree of darkening of the metal surfaces is controlled by 
varying the concentration, and/or the time of contact between the metal 
surfaces and the second solution. Typically, the metal is immersed in the 
preferred concentration of the second solution for about 15 seconds to 
uniformly contact the exposed metal surfaces of the mask assembly and to 
form the dark, adherent coating 27 on the exposed surfaces thereof. A 
suitable second solution is Birchwood Casey's Perma Blue, available from 
Birchwood Laboratory, Inc., Eden Prairie, MN. The mask assembly is rinsed 
in water to remove therefrom any residual second solution that did not 
react with the mask assembly A sealing solution of aqueous tannic acid 
having a concentration within the range of about 10.0 to 15.0 percent, by 
weight, and, preferably about 12.0 wt %, is applied, e.g. by dipping, to 
the surfaces of the mask assembly. The adherences of the dark coating 
formed by the novel process is superior to coatings formed by prior 
processes as evidenced by the resistance of the novel coating to cracking 
and flaking-off when the coated metal is subjected to a bend test, such as 
that described in U.S. Pat. No. 4,819,489, issued on Apr. 11, 1989 to 
Nelson et al., and incorporated by reference herein for the purpose of 
disclosure.