If the context relates about reactions between the slag and the metal:

Please see this link: http://www.edpsciences.org/articles/metal/pdf/2001/01/ostber...
(related about the exchange between slag and steel)

En projetant avec force l’acier liquide dans ce
laitier, on obtient une sorte d’émulsion favorisant un
échange métal-laitier très intense.

In passing, it could be mentioned that the observed exchange
of metallic elements between slag and steel eventually
became known as the “Perrin effect” when dealt with in
studies of the occurrence of slag inclusions (40).

"exchange though slag" could be a good proposal.

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Note added at 11 hrs (2006-06-08 07:34:35 GMT)
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"exchange through slag" could be a good proposal.