SECOND SUPPLEMENTAL  INDENTURE,  dated as of October 16, 1998,
to the  Indenture,  dated as of September 20, 1996,  as  heretofore  amended and
supplemented (the "Indenture"),  between Commonwealth Industries, Inc. (formerly
Commonwealth Aluminum Corporation), a Delaware corporation (the "Company"), each
of the Subsidiary  Guarantors (as defined  therein) and Harris Trust and Savings
Bank, as Trustee (the "Trustee").

                  RECITALS:

                  The Indenture has heretofore been amended and  supplemented by
a First Supplemental Indenture, dated as of November 12, 1996. Subsequent to the
date of the  First  Supplemental  Indenture  and prior to the date  hereof,  the
former  Commonwealth  Aluminum  Corporation  changed  its  name to  Commonwealth
Industries,  Inc., the former Commonwealth Industries,  Inc. changed its name to
CI Holdings, Inc. and thereafter was merged into Commonwealth Industries,  Inc.,
the  former  Commonwealth   Aluminum   Lewisport,   Inc.  changed  its  name  to
Commonwealth  Aluminum Corporation and then to Commonwealth  Aluminum Lewisport,
Inc.,  and  the  former  Barmet  Aluminum   Corporation   changed  its  name  to
Commonwealth Aluminum Concast, Inc.

                  The  Company   has  duly   organized   Commonwealth   Aluminum
Corporation,  a Delaware  corporation,  as a  Restricted  Subsidiary,  and it is
proposed  that this  corporation  (the "new  subsidiary")  become an  additional
Subsidiary Guarantor, as permitted by Section 901(7) of the Indenture.

                  The Company,  each of the  Subsidiary  Guarantors  and the new
subsidiary  have  been  authorized  by  Board  Resolutions  to enter  into  this
supplemental indenture.

                  NOW, THEREFORE, THIS SUPPLEMENTAL INDENTURE
WITNESSETH:

                  For  and in  consideration  of  the  premises  it is  mutually
agreed,  for  the  equal  and  proportionate  benefit  of  all  Holders  of  the
Securities, as follows:

                                   ARTICLE ONE

                                   Definitions

                  For all purposes of this  supplemental  indenture,  unless the
context otherwise  requires,  the terms used herein shall have the same meanings
as in the Indenture.



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                                   ARTICLE TWO

                              Subsidiary Guarantors

                  The new  subsidiary  is  hereby  subjected  to the  provisions
(including the  representations and warranties) of the Indenture as a Subsidiary
Guarantor, all as contemplated by Section 1303 of the Indenture.

                  IN WITNESS  WHEREOF,  the  parties  hereto  have  caused  this
supplemental indenture to be duly executed, and their respective corporate seals
to be  hereunto  affixed  and  attested,  all as of the day and year first above
written.


                      COMMONWEALTH INDUSTRIES, INC.
                      COMMONWEALTH ALUMINUM CORPORATION
                      COMMONWEALTH ALUMINUM CONCAST,INC.
                      COMMONWEALTH ALUMINUM LEWISPORT,INC.
                      COMMONWEALTH ALUMINUM SALES CORPORATION
                      ALFLEX CORPORATION


                      By ______________________
                        Mark V. Kaminski, President


Attest:


----------------------
Secretary



                         HARRIS TRUST AND SAVINGS BANK,
                                   as Trustee


                         By ___________________________


Attest:


-------------------


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<PAGE>




COMMONWEALTH OF KENTUCKY)
                            ss.:

COUNTY OF JEFFERSON)


                  On the __ day of October, 1998, before me personally came Mark
V. Kaminski,  to me known,  who, being by me duly sworn, did depose and say that
he is President of each of Commonwealth Industries,  Inc., Commonwealth Aluminum
Corporation,   Commonwealth  Aluminum  Concast,   Inc.,   Commonwealth  Aluminum
Lewisport, Inc., Commonwealth Aluminum Sales Corporation and Alflex Corporation,
corporations  described in and which executed the foregoing instrument;  that he
knows the seal of said  corporations;  that the seals affixed to said instrument
are such corporate  seals;  that they were so affixed by authority of the Boards
of Directors of said  corporations,  and that he signed his name thereto by like
authority.


                                            ------------------------




STATE OF ILLINOIS)
                     ss.:

COUNTY OF COOK   )


                  On the ____ day of __________, 1998, before me personally came
____________,  to me known, who, being by me duly sworn, did depose and say that
she is a  _____________________  of Harris  Trust and Savings  Bank,  one of the
corporations described in and which executed the foregoing instrument;  that she
knows the seal of said corporation;  that the seal affixed to said instrument is
such  corporate  seal;  that it was so  affixed  by  authority  of the  Board of
Directors  of said  corporation,  and that she signed  her name  thereto by like
authority.


                                            -------------------------




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