I looked up the policy of journals in the fields of physics and chemistry, and their policy regarding reviewers does not directly address your question. Most ethical guidelines say the same thing: manuscripts sent for review are confidential. For example, quoting from the APS Guidelines for Professional Conduct:
Privileged information or ideas that are obtained through peer review must be kept confidential and not used for competitive gain.
From the more loquacious ACS Ethical Guidelines:
A reviewer should treat a manuscript sent for review as a confidential document. It should neither be shown to nor discussed with others except, in special cases, to persons from whom specific advice may be sought; in that event, the identities of those consulted should be disclosed to the editor.
This clearly covers the case of revealing the part (or any part of it) before it was published, as well as nonpublished parts of the paper (i.e., the published version is of course public, but anything else is still confidential).
However it seems to me that, narrowly read, there is material that this rule does not cover. For example, the text written by the reviewer is not indicated to be confidential (except insofar as it reveals part of the authors’ confidential material), and it seems clear that you retain the right to publish it. (Whether it's a good thing to actually do is another matter. Except in extreme situations, I would advise against it.)
So, once a paper is published, are you allowed to reveal that you were a referee? I think so. Is it a good thing? I don’t think it hurts anyone.
PS: I interpreted your “after the decision has been made” as meaning “after the paper is published”. In the interval of time between editorial decision and publication, it is clear that this information is confidential.