HAZARDS FROM FRICTIONAL SPARKS OF

CERTAIN LIGHT ALLOYS

G. N. Badami

L IGHT metal and their alloys have some attractivefeatures e.g., lightness and strength which haveled to their widespread use in various fields. A

draN0ack which certain of these alloys sillier from andwhich is not generally known, is their propensity to givefrictional sparks. This propensity has been the causeof serious explosions in mines' and needs to be high-lighted in it forum of this nature where metallurgistshave gathered to discuss all the aspects concerning lightalloys.

The hazard arises only when the sparks are producedin it flammable gas air mixture such as occurs some-times in it mine and in other locations where anyflammable gas or ,apour may be present continuouslyfor any length of time. It is only certain light alloyswhich give rise to frictional sparks of an incendivenature and it is with these that we are concerned in thisbrief review.

Many examples may he cited of the application oflight alloys in mines but one or two will serve thepurpose to illustrate the hazard. Hand-held drillcasings weighing tip to 401b are often made of lightalloy and it is not difficult to imagine such a toolslipping from the hands of its operator and falling onto a piece of rusty steel below. Or. a light alloy partof a rotating machine may hit accidentally the rustycasing at high peripheral speeds. Even it glancingmanual blow with it hammer on an aluminium-paintedrusty steel structure can produce a dangerous spark'' 3.1.The first instance given represents an example of low-speed impact, and the others high-speed impact.Rubbing friction, at logy speeds but under heavy load-ing conditions, can also engender frictional sparks ofa dangerous nature. Articles such as lighting fittings,methanomcter and anemometer casings, sylvesters.shovels, jacks, roof bars, props and certain parts inthem, beams and channels could be and have been

made with advantage from light alloys but any or allof these, tinder suitable conditions, may prove to bethe igniting source from sparks generated by frictionfrom them. It is obvious that spark production mayoccur under widely different conditions : experimentshave accordingly been designed to simulate them in the

Dr. G. N Badami, Assistant Director, Central Mining Re-search Station, Dhanbad.

laboratory. This is important because a light alloytested in a particular way may not ignite gas but thesame alloy. when tested in a different way. may producesparks which are incendive.

The three chief methods' of testing light alloys havebeen : (1) the Drop test in which sparking by impactis caused when one material strikes another, eitherin free fall or by hamnier blows. (2) the FrictionalSmear test in which the softer light alloy leaves a smearon rusty steel or other hard surface and is subsequentlystruck a sliding or grazing blow and (3) the Rub-bin,, friction test which simulates the conditions obtain-ing when moving parts of a power-driven machine arefouled by a stationary object or surface. lure lightmetals and the softer alloys. generally s1ieaking, appearto produce incendive sparks under the Smear testconditions. while the harder alloys of light metals yielddangerous sparks under the other lest conditions. Inall the tests, a critical gas mixture known to he ignitedreadily by frictional sparks surrounds the point ofimpact or friction, and an explosion results whenincendive sparking occurs in it.

It may he thought that it would he possible to sayby mere visual examination whether or no certainsparks are dangerous. This is not ease. For instanceit is known that the discrete sparks living through aninflammable atmosphere---particularly of methane andair :arc dangerous' i"" unless they are arrested andbrought to rest. The discrete sparks from light alloysare. however, dangerous (III SLICII ;atmospheres) even inflight : the material of the spark is undergoing rapidoxidation and a higher temperature is produced9.Sparks which are condensed, compact and whichappear as a bright flash arc believed to he a cloud ofthe eroded metal burning in air and are usually stillmore dangerous. It is never safe therefore to rely onthe appearance of it spark to assume its incendivity-

it is necessary to subject the alloy to all the tests andto any others that may need to he devised to reproducethe conditions in actual use.

In the foregoing observations. rusty iron has oftenbeen mentioned in connection w\ ith the generationof the incendive frictional sparks, and there is Itreason for this. It is now generally""' agreed that it-thermite" type of reaction is involved. and an oxygen-carrier in the form of rust, red lead. ammonium

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nitrate , nitro-cellulose lacquer, etc. plays a part in theprocess. It would appear that at the moment ofimpact of a light alloy on a rusty steel surface (or whena light hammer blow is struck on the smear producedby a soft light alloy), an intimate mixture of the lightalloy and the rust (oxide) is formed instantaneouslyand begins to react by the frictional heat. The heatof combustion of this "thernite" mixture is sufficientto ignite any fine particles of the light alloy that mayhave been eroded on impact and that may be presentas a cloud round the point of impact. Alternatively.the heat produced is sufficient to raise any erodedparticle to incandescence and send it flying as acontinuously self-oxidising discrete spark into theflammable gas. An interesting theory'' (which inci-dentally would explain certain observed effects ofhumidity or moisture) attributes the ignition of fire-damp primarily to an ignition of hydrogen which isbelieved to be produced locally by decomposition ofwater or moisture associated with the rust or theatmosphere.

A comparison has been made between the energyof a single electric spark required to cause ignition ofa gas and that of the minimum energy required in africtional spark. From experiments to determine theminimum size of a burning particle of magnesiumrequired to cause ignition in a methane-air mixture,it has been established" that a particle weighing notmore than one half of a micro-gramme and havingan energy equal to only ten times that dissipated ina single electric spark is sufficient to cause an ignition.In other words, the energy is only of the order of10 millijoules or perhaps less ! The energies involvedin the tests on frictional sparking propensity are, ofcourse, very much larger.

The light metals, aluminium , magnesium, titaniumand cerium. are all capable of giving incendive sparks.Of the alloys, those based on magnesium are moredangerous than the aluminium-based ones, the sus-ceptibility to sparking increasing with magnesium con-tent. Thus a 501io probability of ignition at an energylevel of 390 ft lb has been established for cast alumi-nium alloy LMb which contains no magnesium, about12%% silicon and the balance aluminium , while forthe same probability of ignition the energy level isonly 89 ft lb for "Elektron" alloy containing between92 and 93 per cent magnesium13. Other things remain-ing the same, the susceptibility to sparking also appearsto be related to the hardness, susceptibility increasingwith hardness.

Attempts have been made" to produce alloys freefrom hazard by adding : (I ) anti-frictional materials suchas polytetrafluoro-ethylene and molybdenum disulfide,(2) metals of lower melting point such as tin and leadand (3) anti-oxidants which would act as inhibitorsof the `'thermite" type reaction. So far, such attemptshave met with little success. Efforts" to find coatingswhich will reduce the sparking hazard have metwith somewhat better success. Coatings of polythene,epoxy resin. moulded rubber or neoprene, solder,stove enamel, paint, soft sprays of zinc or lead, etc.have been tried. While the protection resulting varies

in degree from coating to coating , one must rememberthat such protection lasts only as long as the coatingis intact.

To summarise, where there is danger of flammableatmospheres the advantage accruing from the use oflight alloys has to be weighed against their propensityto generate incendive frictional sparks-which areparticularly dangerous with certain light alloys. Themetallurgist is thus faced with a challenge if lightmetals are to find unqualified use in hazardous loca-tions. To meet this challenge, he has probably to findan alloying addition which not merely confers thedesired engineering properties but also serves to elimin-ate the production of incendive sparks.

References

Hawkins Colliery, Canncck Chase 1950 and Horden Colliery1953.Fire-( 1938 ) Vol. 31, pp. 156.Thomas T.S.E.. and Iron Coal Trades Review, Vol. 143,pp. 210 (194J).

4 Kingman F. E. T., Coleman E. H. and Rojonsky Z.W., Journalof Applied Chemistry (1952) Vol. 2, pp. 449.

5 Bailey-Trans. Inst. of Min. Engg . Vol. 118, pp. 223,(1958-i959).

G Margerson , Robinson and Wilkins, S .M.R.E. Res. Rep-No. 75 (1953).

s Silver R.S. Phil Mag. Vol. 23, pp. 633 (1937).

Rae , D., S.M.R .E. Res . Rep. No. 177 (1959).

Titman, H., S.M.R,E. Res. Rep. No. 90 (Feb. 1954).10 Titman , H., Trans . Inst. of Min . Engg ., Vol. 115, pp . 535-537

(1955-1956).

Latin , A., Colliery Engg . Vol. 36, pp. 397-404 and 440-446(1959).Brenner , P. and Eversheim , P : Gluckauf. Vol. 94, pp. 180-186 (1958).Titman H. and Wynn A.H.A., S .M.R.E. Res . Rep. No. 95(1954).

DISCUSSIONS

Mr. G. S. Warrier, Alcan Asia Lid., Calcutta: Thequestion as to what fire hazards arise by the use oflight alloys in collieries which give out explosive gaseshas been raised in the paper. I believe thai thisproblem was investigated in some detail by the NationalCoal Board in the United Kingdom about 3 or 4years ago following an explosion when a hand drillhaving a magnesium alloy casing fell on a rusty steelroof support. The results of the investigation showed,I believe, that the explosion was mainly due to cer-tain aluminium magnesium alloys, and that ma3 also

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be in a measure due to the rusty steel and the reactionbetween aluminium'magnesium alloy and the steel.

The author has also referred to the common castingalloy of aluminium silicon, LM6, and has mentionedthat these alloys may also give rise to frictionalhazards. I do not think that the investigation foundthe alloy to be dangerous. Many alines in the U.K.use aluminium roof supports and other aluminiumapplications and the advantage is that aluminium dotsnot rust. Hence the answer to avoid hazards ofsparking may equally lie in the more extensive useof aluminium and less steel. I may add that alumi-nium alloys are being used to a much greater extentin Germany than in the U.K. in coal mines, includingthose having explosive gases but no ban on the useof aluminium magnesium alloys has been imposed inGermany. Hence the question as to what extent thepossibility of sparking exists cannot he said to havebeen really determined. Many metals and alloys cangive rise to sparking under certain conditions of which,I believe. moisture may be one contributing factor.I wonder whether all aspects of sparking hazards havebeen thoroughly investigated.Dr. G. N. iJadami (Author) : Mr. Warrier has opinedthat the explosions may be "in a measure" due torusty steel. We have already made it clear in ourreview that the hazard arises out of a thertnite typeof reaction and that rusty steel or any suitable oxygen-carrier is essential. Regarding his contention thatLM6 alloy has not been found to be dangerous, wemust beg to differ. The latest evidence available indi-cates that all alloys based on aluminium and havingmore than half their atoms aluminium mutt he con-

sidered hazardous. Only those alloys which containless than one-third atomic proportion of aluminiummay be looked upon as having their incendivityreduced but at such low aluminium content, the alloyhas already lost the main property for which it isbeing chosen viz. lightness.

Mr. Warrier is right in claiming that light alloyscontinue to he used in Germany. The question hereis related to the level of hazard acceptable in anyparticular industry in any particular country and theNational bodies can and do sometimes differ in theirevaluation of a hazard and practice based on theevaluation. It is, however, true to say that frictionalsparking hazard of light alloys is being graduallyrealised and there is increasing awareness of theproblem and of the need to solve it.

The role of moisture has been briefly touched uponbut it is perhaps right to say that moisture is notalways essential.

Regarding Mr. Warner's suggestion to use less steeland more aluminium. we would only like to saythat it is not a practical suggestion. We do not thinkit will ever he possible to replace steel with lightalloys entirely for the properties of hardness and duc-tility (which are so desirable from the mechanicalengineering point of view) are the very properties whichtogether increase incendivity at any particular alumi-nium content Ievel. It may also he added that researchinto development of light-weight steel sections for roofsupport bars, etc. is proceeding apace in the UnitedKingdom. The object is to replace light alloys whichhave been withdrawn from use following the manyaccidents attrihuted to them.

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