Tuesday, September 23, 2014

Ammonia, stress corrosion cracking, and 14K gold nibs

Several years ago I posed the question of whether pen cleaning solutions containing ammonia could pose a risk to gold nibs via the process known as stress corrosion cracking (SCC). The original discussion at FPN can be reviewed here (the responses on Lion & Pen contained less of substance, and in any event are now lost). The most useful scientific paper cited in that discussion was David A. Scott, "The Deterioration of Gold Alloys and Some Aspects of Their Conservation", Studies in Conservation, vol. 28, no. 4 (Nov 1983), pp. 194-203. According to Scott, gold alloys in the 9-14K range are most vulnerable to stress corrosion cracking, whereas gold of 18K purity and greater is largely invulnerable. A number of reagents that could provoke stress corrosion cracking were mentioned, including aqueous solutions of ammonia. Unfortunately, details were not provided concerning the concentration of these solutions, or the length of exposure required before embrittlement took place.

That particular online discussion ended without any firm conclusions being drawn. My concerns remained, however -- enough that I drastically cut back on my own use of ammonia solutions on gold nibs. If ammonia could embrittle gold nibs, it would be a slow, cumulative process -- very possibly taking years or even decades before becoming apparent -- so I did not find any reassurance in arguments that consisted of little more than, "I've not had any problems, so it must be OK." What worried me particularly was the large number of  vintage gold nibs with cracks in locations that just didn't make sense. Cracks where a nib is subjected to repeated bending are readily explained by metal fatigue. But what about cracks in the heel of a nib, where it is firmly sandwiched between the feed and section? Fatigue would not seem to be a concern there, so why are such cracks so common? Conventional wisdom is that the gold was made too hard, but if so, why are these nibs so often cracked only where they are wedged into the section? The answer may be that while the heel is the part of a nib least subject to the cyclic loading that causes fatigue, it is the part of a nib under the greatest constant stress. And it is that very stress that makes materials vulnerable to stress corrosion cracking.

A recent reexamination of the materials science literature available online has helped fill out this picture. The studies cited by Scott, for example, are referenced in more detail in Jennifer M. M. Dugmore and Charles D. DesForges, "Stress Corrosion in Gold Alloys", Gold Bulletin, vol. 12, no. 4 (Dec 1979), p. 141:
Much of the published work on the stress corrosion cracking [of] gold alloys is that of Graf and his co-workers and results were obtained using binary gold alloys. For example, Graf (15) found that the susceptibility of such alloys actually increases as the gold contents increase from 5 to between 15 or 20 atomic per cent, in which range maximum susceptibility is observed. The precise gold content at which the maximum susceptibility occurred depended on both the corrosive media and the stress applied to the alloy. When gold-copper alloys were exposed to a mixture of ammonia, water and oxygen the maximum susceptibility occurred at a gold content of about 15 atomic per cent . . . Graf also observed that as the gold content was increased above that at which the susceptibility to stress corrosion cracking was a maximum, the susceptibility dropped rapidly and the alloy appeared to become virtually immune. In other work (3), this author reported that ultimately the susceptibility dropped to a constant low value at higher gold contents, an effect he attributed to strong general surface attack. He also observed that when the stress was increased, not only did the susceptibility to stress corrosion cracking increase but its maximum was shifted towards higher gold contents.
It is previously explained that "9 carat alloys contain about 18 atomic per cent, and 14 carat alloys often less than 30 atomic per cent gold. In each caratage, the percentages vary according to the atomic weights and the proportions of the non-gold components present in the alloys." While this might at first glance suggest that the gold content of 14K alloys puts them comfortably above the level of maximum susceptibility to ammonia-induced stress corrosion cracking, the last sentence of the longer passage quoted above indicates that under higher stress there might be no safety margin after all.

This is at least partially confirmed by another article, W. S. Rapson, "Tarnish Resistance, Corrosion and Stress Corrosion Cracking of Gold Alloys", Gold Bulletin, vol. 29, no.2 (Jun 1996), pp 61-69, which makes specific mention of fountain pen nibs, and of ink (p. 64):
SCC may be induced not only by exposure to acids during pickling but also a result of contact with reagents such as ink, traces of hydrochloric acid in the atmosphere, perspiration, etc. It has frequently been initiated at points of stress created in annealed low carat alloys by subsequent stamping. Articles such as fountain pen nibs, rings, chains, etc, provide well-known examples.
A further reference appears on p. 66:
In early production of 14 carat fountain pen nibs, for example, Loebich (27) has stated that when ternary Au-Ag-Cu alloys were used, it was found desirable to age the fabricated nibs. In the aged condition they did not undergo stress corrosion cracking in use; whereas if heated to the point where they became homogeneous, cracking by the action of the ink became likely. When certain 14 carat quaternary Au-Ag-Cu-Zn alloys are used, however, such ageing is apparently unnecessary. This could be due to the known limiting effect of the zinc on phase separation in these alloys. The susceptibility of alloys of this type to stress corrosion is apparently considerably influenced both by their zinc contents and by heat treatment. Analogous anomalies occur in the case of the white Au-Cu-Ni-Zn alloys and these have been discussed by Graf.
I have not yet been able to consult the articles by Loebich (who worked for Degussa: Otto Loebich, "Metallkundliche Probleme bei der gewerblichen Goldverarbeitung", Zeitschrift für Metallkunde 44 (1953) p. 288-92) or Graf (L. Graf and J. Budke, "Zum Problem der Spannungskorrosion homogener Mischkristalle III: Abhangigkeit der Spannungskorrosionsempfindlichkeit von Kupfer-Gold und Silber-Gold Mischkristallen von Goldgehalt und Zusammenhang mit dem "Mischkristall-Effect"", Zeitschrift für Metallkunde 46 (1955) pp. 378-385). Nonetheless, the fact that 14K nibs did have problems with stress corrosion cracking is highly significant. While that exposure was to ink -- that is, to acids and salts -- rather than to ammonia, we now have incontrovertible evidence that under the right conditions, even very weak solutions of known problem reagents can eventually leave alloy gold fatally embrittled.

What can we conclude from this? It seems clear that the observed cracking of nibs at the heel is indeed due to stress corrosion cracking -- and on balance, it is likely that this is primarily due to long-term exposure to ink, rather than to much shorter exposures to ammonia. We cannot assume that ammonia exposure is no more damaging than  ink exposure, however, given that typical ammonia cleaning solutions are far more reactive than ordinary modern inks. And given the cumulative nature of stress corrosion, minimizing ammonia exposure is only prudent.

One precaution to be considered is to remove nibs from sections before cleaning. Stress corrosion does not take place where there is no stress, and by all indications, nibs are most stressed by being wedged into their sections -- though internal stresses cannot be entirely ignored. If nibs are removed and cleaned separately, one also has the option of using other solvents not implicated in stress corrosion at all, such as denatured alcohol.

A further question is how ammonia solutions react with gold alloys in an ultrasonic cleaner. The scrubbing action of an ultrasonic will typically remove more encrusted ink in a minute than in hours of soaking, but ultrasonics are also known to be able to erode objects by driving the cleaning solution into microscopically small surface cracks and pores. Though it would seem that ultrasonic cleaning has the potential to dramatically accelerate the process of stress corrosion, I have not been able to find any relevant studies (including with other metals and other reagents).

Two final notes: While we might worry about ammonia, chlorides are a proven danger. I have personally seen a 14K gold pen overlay spontaneously crack to pieces after a few minutes of exposure to undiluted bleach. Even in very dilute form, bleach (and even heavily chlorinated water) will rapidly embrittle gold jewelry many times thicker than a pen nib. Jewelers' websites routinely caution against wearing gold rings of 14K purity or less when using cleansers, swimming in chlorinated pools, and even handling some foodstuffs (vinegar and salt, for example). And we can also learn from the jewelers regarding another aspect of stress corrosion cracking prevention, which is heat treatment. A stress relief anneal, typically 30 minutes at 250°C, is recommended after any jewelry repair. This is consistent with Loebich, cited above -- though I wonder how many of those who currently engage in nib repair have any knowledge of this.

ADDENDUM: Worth noting is the near-total absence of heel cracks on gold dip pen nibs, which were made in much the same way as fountain pen nibs, and which in use would have been subject to very similar fatigue cycling -- but very different clamping stress, and no ink exposure.

1 comment:

Eli Barak said...

Fascinating, and answers the question with authority. Thank you.