The reaction of aluminum with hydrogen and other substances

Alu­minum is an ac­tive met­al, which is sta­ble in air, and quick­ly ox­i­dizes at a nor­mal tem­per­a­ture, gain­ing a dense film of ox­ide, which pro­tects the met­al from fur­ther dam­age.

In­ter­ac­tion of alu­minum with oth­er sub­stances

In or­di­nary con­di­tions, wa­ter does not af­fect this met­al even in a boil­ing state. If the pro­tec­tive ox­ide film is re­moved from the sur­face of the alu­minum, the met­al en­ters into a vig­or­ous in­ter­ac­tion with the wa­ter va­por in the air, turn­ing into a crumbly mass of alu­minum hy­drox­ide with re­lease of hy­dro­gen and heat. The equa­tion of the re­ac­tion is:

2Al + 6H₂O = 2Al(OH)₃ + 3H₂

If the pro­tec­tive ox­ide film is re­moved from alu­minum, the met­al en­ters into a vig­or­ous in­ter­ac­tion with oxy­gen. The alu­minum pow­der burns, form­ing ox­ide. The equa­tion of the re­ac­tion is:

4Al + 3O₂ = 2Al₂O₃

This met­al also in­ter­acts ac­tive­ly with many acids. In a re­ac­tion with hy­drochlo­ric acid, re­lease of hy­dro­gen is ob­served. The equa­tion of the re­ac­tion is:

2Al + 6HCl = 2Al­Cl₃ + 3H₂

In or­di­nary con­di­tions, the con­cen­trat­ed ni­tric acid does not in­ter­act with alu­minum, as ni­tric acid, as a strong ox­i­diz­er, makes the ox­ide film even stronger. For this rea­son, ni­tric acid is stored and trans­port­ed in alu­minum con­tain­ers.

Alu­minum at an or­di­nary tem­per­a­ture is pas­sive to di­lut­ed ni­tric and con­cen­trat­ed sul­fu­ric acid. In hot sul­fu­ric acid, the met­al dis­solves, and the equa­tion of the re­ac­tion is:

2Al + 4H₂­SO4 = Al₂(SO4)₃ + S + 4H₂O

In­ter­ac­tion of alu­minum with non-met­als

It re­acts with halo­gens, sul­fur, ni­tro­gen, car­bon and all non-met­als. For the re­ac­tion, heat is re­quired, af­ter which in­ter­ac­tion takes place with the re­lease of a large amount of heat.

In­ter­ac­tion of alu­minum with hy­dro­gen

Alu­minum does not di­rect­ly re­act with hy­dro­gen, al­though there is a sol­id poly­mer­ic com­pound, alane, in which so-called three-cen­ter bonds ex­ist. At a tem­per­a­ture of over 100 de­grees Cel­sius, alane ir­re­versibly breaks down into sim­ple sub­stances. Alu­minum hy­dride re­acts vig­or­ous­ly with wa­ter. Alu­minum does not re­act with hy­dro­gen di­rect­ly — the met­al forms com­pounds, through the loss of elec­trons, which are re­ceived by oth­er el­e­ments. The hy­dro­gen atom does not re­ceive elec­trons, which met­als give to form com­pounds — hy­dro­gen atoms can only be “forced” to ac­cept elec­trons with the for­ma­tion of sol­id ion­ic com­pounds (hy­drides) by very re­ac­tive met­als (potas­si­um, sodi­um, mag­ne­sium, cal­ci­um). The di­rect syn­the­sis of alu­minum hy­dride from hy­dro­gen and alu­minum re­quires enor­mous pres­sure of around 2 bil­lion at­mos­pheres and a tem­per­a­ture of over 800 K. Here you’ll find out about chem­i­cal prop­er­ties of oth­er met­als.

We should note that hy­dro­gen is the only gas which sig­nif­i­cant­ly dis­solves in alu­minum and its al­loys. The sol­u­bil­i­ty of hy­dro­gen changes in pro­por­tion to the mag­ni­tude of tem­per­a­ture and the square root of pres­sure. The sol­u­bil­i­ty of hy­dro­gen in liq­uid alu­minum is sig­nif­i­cant­ly high­er than in sol­id alu­minum; this prop­er­ty changes in­signif­i­cant­ly de­pend­ing on the chem­i­cal com­po­si­tion of al­loys.

Alu­minum and its hy­dro­gen poros­i­ty

The for­ma­tion in alu­minum of hy­dro­gen bub­bles de­pends di­rect­ly on the speed of cool­ing and hard­en­ing, and also the pres­ence of cen­ters of for­ma­tion for the re­lease of hy­dro­gen — ox­ides trapped in­side the al­loy. For alu­minum to be­come por­ous, the con­tent of dis­solved hy­dro­gen must sig­nif­i­cant­ly ex­ceed the sol­u­bil­i­ty of hy­dro­gen in sol­id alu­minum. If there are no cen­ters of for­ma­tion for the re­lease of hy­dro­gen, a rel­a­tive­ly high con­cen­tra­tion of the sub­stance is re­quired.

The lo­ca­tion of hy­dro­gen in hard­ened alu­minum de­pends on the lev­el of its con­tent in liq­uid alu­minum and the con­di­tions in which the hard­en­ing took place. As hy­dro­gen poros­i­ty is the re­sult of mech­a­nisms of for­ma­tion and growth, con­trolled by dif­fu­sion, such pro­cess­es as a re­duc­tion in the con­cen­tra­tion of hy­dro­gen and an in­crease in the speed of hard­en­ing sup­press the for­ma­tion and growth of pores. For this rea­son, alu­minum that is cast us­ing the split mold method is more li­able to de­fects con­nect­ed with hy­dro­gen than alu­minum that is cast un­der pres­sure.

There are dif­fer­ent sources for hy­dro­gen to en­ter alu­minum.

Melt­ing stock — scrap met­al, in­gots, foundry re­turns, ox­ides, sand and lu­bri­cants used in me­chan­i­cal pro­cess­ing. These pol­lu­tants are po­ten­tial sources of hy­dro­gen formed in the chem­i­cal break­down of wa­ter va­por or the re­duc­tion of or­gan­ic sub­stances.

Melt­ing tools — scrap­ers, pikes and spades are a source of hy­dro­gen. Ox­ides and the re­mains of flux­es on tools ab­sorb mois­ture from the sur­round­ing air. Fur­nace bricks, dis­tri­bu­tion chan­nels, la­dles for se­lect­ing sam­ples, lime chutes and ce­ment so­lu­tions are po­ten­tial sources of hy­dro­gen.

The stove at­mos­phere — if the melt­ing fur­nace runs on oil or nat­u­ral gas, there may be in­com­plete com­bus­tion of fuel with for­ma­tion of free hy­dro­gen.

Flux­es are hy­dro­scop­ic salts ca­pa­ble of ab­sorb­ing wa­ter in­stant­ly. For this rea­son, a moist flux in­evitably adds hy­dro­gen to an al­loy, which is formed in the chem­i­cal break­down of wa­ter.

Foundry molds — in the process of fill­ing a foundry mold, liq­uid alu­minum flows tur­bu­lent­ly and cap­tures air in­side it. If the air does not es­cape from the mold be­fore the alu­minum starts to hard­en, alu­minum will en­ter the met­al.