<PAGE>   1
                                                                   EXHIBIT 10(m)
                                                                  Execution Copy


                               FOURTH AMENDMENT TO
                                CREDIT AGREEMENT
                               -------------------



         THIS FOURTH AMENDMENT ("Amendment") dated as of March 15, 1999, by and
among the borrowers listed on Schedule 1 (collectively "Companies") and Comerica
Bank, a Michigan banking corporation ("Bank").

                                    RECITALS:

         A.    Companies and Bank entered into a Credit Agreement dated as of
June 13, 1996, which was amended by a First Amendment dated December 5, 1996, a
Second Amendment dated March 31, 1997, and a Third Amendment dated April 22,
1998 (as amended, "Agreement").

         B.    Companies and Bank desire to amend the Agreement and the
Revolving Credit Note (as defined in the Agreement) as hereinafter set forth.

         NOW, THEREFORE, the parties agree as follows:

         1.    The definition of Revolving Credit Maturity Date set forth in
Section 1 of the Agreement is amended to read in its entirety as follows:

               "Revolving Credit Maturity Date' shall mean May 1, 2001." 

         2.    Section 2.A is amended to read in its entirety as follows:

               "2.A.1 THE INDEBTEDNESS: Equipment Line of Credit"

               2.A.1 Bank may lend to Companies at any time and from time to
         time from the date hereof until the Equipment Line Maturity Date, sums
         not to exceed Two Million Dollars ($2,000,000) in aggregate principal
         amount. Each of the borrowings hereunder shall be evidenced by an
         Equipment Note. Bank shall not be obligated to make any advance under
         this Section 2.A.

               2.A.2 The indebtedness represented by each Equipment Note shall
         be payable in equal monthly principal installments equal to the amount
         necessary to amortize the original amount of the Equipment Note over a
         five year term commencing on the first day of the first month after
         such loan is made and on the first day of each month thereafter until
         the maturity date thereof, when the entire unpaid balance of principal
         and interest thereon shall be due and payable. The maturity date for
         each Equipment Note shall be the date which is five (5) years after the
         date thereof. In addition to the above required payments on principal,
         Company 


<PAGE>   2

         agrees to pay interest on the unpaid principal balance of each
         Equipment Note from time to time outstanding at a per annum rate equal
         to three quarters of one percent (3/4%) above the Prime Rate, provided,
         however, upon the occurrence of any Event of Default hereunder,
         interest shall be payable at a per annum rate of three and three
         quarters percent (3 3/4%) above the Prime Rate. Interest payments shall
         be made monthly, commencing on the first day of the first month
         following the advance under the applicable Equipment Note and on the
         first day of each month thereafter. Interest shall be computed on a
         daily basis using a year of 360 days for the actual number of days
         elapsed, and in such computation effect shall be given to any change in
         the interest rate resulting from a change in the Prime Rate on the date
         of such change in the Prime Rate.

               2.A.3 Bank shall not make advances under this Section 2.A unless
         Companies shall have first filed with Bank a request for draw in form
         acceptable to Bank executed by an authorized officer of Companies. Each
         such request for an advance shall be submitted to Bank not less than
         ten (10) days prior to the requested date of disbursement of the
         advance.

               2.A.4 Bank will approve requests for draws upon presentation by
         Company of such documents, instruments or opinions, in form and
         substance satisfactory to the Bank, as the Bank may require.

               2.A.5 Companies may prepay such Equipment Note in whole or in
         part without penalty. Any prepayments shall be applied to principal
         payments due under an Equipment Note in the inverse order of their
         maturities.

               2.A.6 Proceeds of each Equipment Note shall be used solely to
         finance the acquisition of machinery and equipment which is acceptable
         to Bank.

               2.A.7 The Companies shall pay to the Bank an equipment facility
         commitment fee quarterly in arrears commencing July 1, 1999, and within
         three Business Days after the last day of each calendar quarter
         thereafter. The fee shall be an amount equal to the average daily
         balance of the unborrowed portion of the equipment facility for the
         quarterly period then ending, multiplied by one half of one percent
         (1/2%). The fee shall be computed on the basis of a year of three
         hundred sixty (360) days and assessed for the actual number of days
         elapsed. Upon request of Companies, Bank shall provide to Companies the
         detail of Bank's computation of the fee. Whenever any payment of the
         fee shall be due on a day which is not a Business Day, the date for
         payment thereof shall be extended to the next Business Day.

               2.A.8 The aggregate amount of advances available under this
         Section 2.A shall not exceed $2,000,000. Each advance shall be in an
         amount not less than $600,000."


                                       2
<PAGE>   3


         3.    The first three sentences of Section 4.1 of the Agreement are
amended to read in their entirety as follows:

               "The Revolving Credit Notes and the Advances under Section 2
         hereof shall bear interest from the date thereof on the unpaid
         principal balance thereof from time to time outstanding, at a rate per
         annum equal to one quarter of one percent (1/4%) plus the Prime Rate.
         Interest shall be payable monthly on the first Business Day of each
         calendar month, commencing on the first Business Day of the calendar
         month during which such Advance is made, and at maturity.
         Notwithstanding the foregoing, from and after the occurrence of any
         Event of Default and during the continuation thereof, the Advances
         shall bear interest, payable on demand, at a rate per annum equal to
         three and one quarter percent (3 1/4%) above the Prime Rate."

         4.    Sections 9.1, 9.2, 9.3 and 9.4 are amended to read in their
entireties as follows:

               "9.1 Leverage Ratio. Beginning December 31, 1998, permit the
         Consolidated Leverage Ratio at any time to be more than 2.5 to 1.0."

               9.2 Cash Flow Coverage Ratio. Permit the Consolidated Cash Flow
         Coverage Ratio at any time to be less than the amounts specified below
         for the determination date specified below:


December 31, 1998                                           .25 to 1.0 
March 31, 1999                                              .30 to 1.0 
June 30, 1999                                               .30 to 1.0
September 30, 1999                                          .30 to 1.0
December 31, 1999 and as of the last day of each fiscal
   quarter thereafter                                       .50 to 1.0


               9.3 Current Ratio. Beginning December 31, 1998, permit the ratio
         of Consolidated Current Assets to Consolidated Current Liabilities at
         any time to be less than 1.1 to 1.0.

               9.4 Consolidated Tangible Net Worth. Permit Consolidated Tangible
         Net Worth at any time to be less than the following amounts during the
         periods specified below:

December 31, 1998                                           $16,500,000
January 1, 1999 through September 30, 1999                  $17,000,000
October 1, 1999 through December 30, 2000                   $17,500,000
December 31, 2000 and thereafter                            $18,000,000"


         5.    The definition of "Measuring Period" is amended to read in its
entirety as follows:


                                       3
<PAGE>   4


               "Measuring Period' shall mean for the determination date referred
         to below the applicable period shown opposite such determination date:

<TABLE>
<CAPTION>
         Determination Date                  Measuring Period
         ------------------                  ----------------

<S>                                          <C>
         March 31, 1999                      January 1, 1999 through March 31, 1999 
         June 30, 1999                       January 1, 1999 through June 30, 1999 
         September 30, 1999                  January 1, 1999 through September 30, 1999 
         December 31, 1999                   The four preceding fiscal quarters ending on such 
         and the last day of each            determination date 
         fiscal quarter thereafter"
</TABLE>

         6.    Companies hereby represent and warrant that, after giving effect 
to the amendments contained herein, (a) execution, delivery and performance of
this Amendment and any other documents and instruments required under this
Amendment or the Agreement are within each Company's corporate powers, have been
duly authorized, are not in contravention of law or the terms of any Company's
Articles of Incorporation or Bylaws, and do not require the consent or approval
of any governmental body, agency, or authority; and this Amendment and any other
documents and instruments required under this Amendment or the Agreement, will
be valid and binding in accordance with their terms; (b) the continuing
representations and warranties of each Company set forth in Sections 7.1 through
7.15 of the Agreement are true and correct on and as of the date hereof with the
same force and effect as made on and as of the date hereof; (c) the continuing
representations and warranties of each Company set forth in Section 7.16 of the
Agreement are true and correct as of the date hereof with respect to the most
recent financial statements furnished to the Bank by Companies in accordance
with Section 10.1 of the Agreement; and (d) no Event of Default (as defined in
the Agreement) or condition or event which, with the giving of notice or the
running of time, or both, would constitute an Event of Default under the
Agreement, has occurred and is continuing as of the date hereof.

         7.    Except as expressly provided herein, all of the terms and 
conditions of the Agreement remain unchanged and in full force and effect.

         8.    This Amendment shall be effective as of the date first above 
written and the payment by Companies to Bank of a non-refundable amendment fee
in the amount of $10,000.


                                       4
<PAGE>   5


IN WITNESS the due execution hereof as of the day and year first above written.


COMERICA BANK                              DETREX CORPORATION



By:         [SIG]                          By: GERALD J. ISRAEL
   ------------------------------             ------------------------------
                                                  Gerald J. Israel

Its: Assistant Vice President              Its:   Vice President-Finance and
                                                  Chief Financial Officer


                                           THE ELCO CORPORATION

                                           By: GERALD J. ISRAEL
                                              ------------------------------
                                                  Gerald J. Israel

                                           Its:   Treasurer


                                           HARVEL PLASTICS, INC.


                                           By: GERALD J. ISRAEL
                                              ------------------------------
                                                  Gerald J. Israel

                                           Its:   Director


                                           SEIBERT-OXIDERMO, INC.


                                           By: GERALD J. ISRAEL
                                              ------------------------------
                                                  Gerald J. Israel


                                           Its:   Treasurer


                                       5
<PAGE>   6

                                   SCHEDULE 1
                                   ----------



Detrex Corporation

The Elco Corporation

Harvel Plastics, Inc.

Seibert-Oxidermo, Inc.



                                       6