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For several years past the author has been using a certain well-known brand of steel writing pen with great satisfaction. The demand made upon the pens is not severe, they being used only for signatures, corrections and such occasional writing, though being moist with ink for several hours at a time. Under such light and easy service the life of the pens, was always at least a week, and frequently several weeks elapsed before they reached that scratchy stage demanding replacement.
This past winter, on attempting to purchase a fresh supply of this old, reliable brand, it was found that the local stationer did not have the plain steel ones in stock, but could furnish the same style and brand gold-plated. The cost of this plated pen was about one-half more than the plain steel ones.
Thinking that the manufacturer had improved the life of the pen by this plating, a box of the new style was purchased. In use under practically the same service that the plain steel ones had been subjected to, the plated variety lasted rarely more than two days. The corrosion of the points and between the nibs was excessive, one pen frequently not writing more than five hundred words before requiring replacement.
The life of the pens being so very short, I determined to make an investigation of the cause. The results of this investigation are here produced.
Microscopic and chemical tests showed that the pens were probably first copper-plated and then flashed with gold. The total thickness of the yellow metal coating was 0.006 mm., and the pens carried approximately 0.25 milligram of gold each. The plating was excellently done and was absolutely continuous as far as could be ascertained by chemical test.
The pens could be thrown into sulphuric acid from an electroplating kit and left there for some time without loss of weight. I therefore came to the conclusion that the corrosion was electrolytic in its nature, and formulated the following hypothesis to account for it: The friction of the pen point on paper rapidly wears off the plating on the point, thus exposing the steel. An electrolytic couple, Steel—Ink—Gold, is set up and corrosion of the steel takes place with great rapidity.
Some idea of the electrochemical nature of this couple may be gained from the following: A pen was stripped of its plating by an 18-hour immersion in strong potassium cyanide solution.
This stripped pen and a plated pen were then connected in series with a high resistance voltmeter and the two pens dipped into various kinds of ink, contained in a small cell about the size of a thimble. A potentiometer was not available at the time the experiments were made, so that I had to content myself with measurements made on closed circuits of 300 to 500 ohms’ resistance.
When the two pens were dipped into “Barnes’ National Jet Black Ink,” the voltmeter showed a reading of 0.045 at once, dropping in three minutes to 0.020 volt, and in five minutes to 0.015 volt, at which point it remained constant for some time. When taken out and redipped, the whole series repeated itself. Vibration of the pens, while immersed, gave a continuous reading of about 0.045 volt. The resistivity of the ink was approximately 112 ohms per cm. cube. Index of corrosion, 0.00040.
In “Stafford’s Universal Writing Fluid,” the series was 0.08, 0.02 and 0.008 volt; vibrating the pens maintained a constant
electromotive force of 0.022 volt. The resistivity of this ink was 68 ohms per cm. cube. Index of corrosion, 0.00033. In a rather old and faded brand of this ink, the reading under vibration of the pens was 0.016 volt. No measure of the resistivity of this faded ink was made.
In “Waterman’s Ideal Ink,” the series was 0.088 and 0.040 volt. A steady electromotive force of 0.120 volt could be maintained by vibrating the pens in the ink. The resistivity of this ink was 68 ohms per cm. cube. With a freshly stripped pen against a fresh gold-plated one, a similar result of 0.12 volt was measured. Index of corrosion, 0.00176.
These experiments seem to corroborate my original hypothesis as to the cause of the excessive corrosion of the pens. When in use, the continued dipping of them into ink and the vibration occasioned as they rub over the paper maintains a short-circuited couple with the maximum electromotive force noted, and causes consequent corrosion of the steel point as the anode of this couple.
The “index of corrosion” as given above for the various writing fluids is the product of the maximum electromotive force found for the vibrated pens multiplied by the conductivity of the various inks. This index assists in choosing the least corrosive ink in case the plated pens have to be used.
While the corrosion of the plated pens is not a matter of great commercial importance, yet there is to be gained from the experience a certain wholesome lesson on the application of electroplating to the protection of less noble metals: Do not plate with a nobler metal a material that is likely to be subjected to wear on one spot and which spot is to be moistened with liquids. For, as soon as the plating is worn through on that spot, accelerated corrosion will take place, and in the end the resulting corrosion will be far worse than without the plating.
Electrometallurgical Laboratory.
Lehigh University,
March 1, 1911.
