Exhibit 5.1

JONES DAY

NORTH POINT • 901 LAKESIDE AVENUE • CLEVELAND, OHIO 44114.1190

TELEPHONE: +1.216.586.3939 • FACSIMILE: +1.216.579.0212

September 11, 2013

Harsco Corporation

350 Poplar Church Road

Camp Hill, Pennsylvania 17011

Re: Registration Statement on Form S-3 filed by Harsco Corporation

Ladies and Gentlemen:

We have acted as counsel for Harsco Corporation, a Delaware corporation (the “Company”), in connection with the authorization of the possible issuance and sale from time to time, on a delayed basis, by the Company of an indeterminate aggregate initial offering price or amount of: (i) debt securities of the Company (the “Debt Securities”), in one or more series; (ii) shares of preferred stock, $1.25 par value per share, of the Company (the “Preferred Stock”), in one or more series, certain of which may be convertible into or exchangeable for Common Stock (as defined below) or different series of Preferred Stock; (iii) depositary shares representing fractional interests in shares of Preferred Stock (the “Depositary Shares”); (iv) shares of common stock, $1.25 par value per share, of the Company (the “Common Stock”) and the associated stock purchase rights of the Company (the “Rights”); (v) warrants to purchase Debt Securities, Preferred Stock, Depositary Shares or Common Stock (the “Warrants”); and (vi) units consisting of one or more of the securities described in clauses (i) through (v) above in any combination (the “Units”), in each case as contemplated by the Company’s Registration Statement on Form S-3 to which this opinion is filed as an exhibit (as the same may be amended from time to time, the “Registration Statement”). The Debt Securities, the Preferred Stock, the Depositary Shares, the Common Stock (and the associated Rights), the Warrants and the Units are collectively referred to herein as the “Securities.” The Securities may be offered and sold from time to time pursuant to Rule 415 under the Securities Act of 1933 (the “Securities Act”). The Debt Securities are to be issued under an indenture, dated as of September 20, 2010 (the “Indenture”), between the Company and Wells Fargo Bank, National Association, as trustee (the “Trustee”).

In connection with the opinions expressed herein, we have examined such documents, records and matters of law as we have deemed relevant or necessary for purposes of this opinion. Based on the foregoing, and subject to the further assumptions, qualifications and limitations set forth herein, we are of the opinion that:

1. The Debt Securities, upon receipt by the Company of such lawful consideration therefor as the Company’s Board of Directors (or an authorized committee thereof) may determine, will constitute valid and binding obligations of the Company.

ALKHOBAR    •    ATLANTA    •    BEIJING    •    BOSTON    •    BRUSSELS    •    CHICAGO    •    CLEVELAND    •    COLUMBUS    •    DALLAS    •    DUBAI

DÜSSELDORF    •    FRANKFURT    •    HONG KONG    •    HOUSTON    •    IRVINE    •    JEDDAH    •    LONDON    •    LOS ANGELES    •    MADRID

MEXICO CITY    •    MILAN    •    MOSCOW    •    MUNICH    •    NEW YORK    •    PARIS    •    PITTSBURGH    •    RIYADH    •    SAN DIEGO

SAN FRANCISCO    •    SÃO PAULO    •    SHANGHAI    •    SILICON VALLEY    •    SINGAPORE    •    SYDNEY    •    TAIPEI    •    TOKYO    •    WASHINGTON

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September 11, 2013

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2. The shares of Preferred Stock, upon receipt by the Company of such lawful consideration therefor as the Company’s Board of Directors (or an authorized committee thereof) may determine, will be validly issued, fully paid and nonassessable.

3. The Depositary Shares, upon receipt by the Company of such lawful consideration therefor as the Company’s Board of Directors (or an authorized committee thereof) may determine, will be validly issued, and the depositary receipts representing the Depositary Shares will entitle the holders thereof to the rights specified therein and in the deposit agreement pursuant to which they are issued.

4. The shares of Common Stock, upon receipt by the Company of such lawful consideration therefor as the Company’s Board of Directors (or an authorized committee thereof) may determine, will be validly issued, fully paid and nonassessable.

5. When issued in accordance with the Rights Agreement, dated September 25, 2007 (as it may be amended from time to time, the “Rights Agreement”), by and between the Company and Computershare Investor Services (as successor to Mellon Investor Services LLC), as rights agent, the Rights will constitute valid and binding obligations of the Company.

6. The Warrants, upon receipt by the Company of such lawful consideration therefor as the Company’s Board of Directors (or an authorized committee thereof) may determine, will constitute valid and binding obligations of the Company.

7. The Units, upon receipt by the Company of such lawful consideration therefor as the Company’s Board of Directors (or an authorized committee thereof) may determine, will constitute valid and binding obligations of the Company.

In rendering the foregoing opinions, we have assumed that: (i) the Registration Statement, and any amendments thereto, will have become effective (and will remain effective at the time of issuance of any Securities thereunder); (ii) a prospectus supplement describing each class or series of Securities offered pursuant to the Registration Statement, to the extent required by applicable law and relevant rules and regulations of the Securities and Exchange Commission (the “Commission”), will be timely filed with the Commission; (iii) the definitive terms of each class or series of Securities will have been established in accordance with the authorizing resolutions adopted by the Company’s Board of Directors (or an authorized committee thereof), the Company’s Restated Certificate of Incorporation (the “Certificate of Incorporation”) and applicable law; (iv) the Company will issue and deliver the Securities in the manner contemplated by the Registration Statement and any Securities that consist of shares of capital stock will have been authorized and reserved for issuance, in each case within the limits of the then remaining authorized but unissued and unreserved amounts of such capital stock; (v) the resolutions authorizing the Company to issue, offer and sell the Securities will have

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been adopted by the Company’s Board of Directors (or an authorized committee thereof) and will be in full force and effect at all times at which the Securities are offered or sold by the Company; (vi) all Securities will be issued in compliance with applicable federal and state securities laws; and (vii) each of the Indenture and the Rights Agreement, and any Deposit Agreement, Warrant Agreement or Unit Agreement (each as defined below), will be governed by and construed in accordance with the laws of the State of New York and will constitute a valid and binding obligation of each party thereto other than the Company.

With respect to any Securities consisting of any series of Debt Securities, we have further assumed that: (i) any Debt Securities will have been issued pursuant to the Indenture; (ii) all terms of such Debt Securities not provided for in the Indenture will have been established in accordance with the provisions of the Indenture and reflected in appropriate documentation approved by us and, if applicable, executed and delivered by the Company and the Trustee; and (iii) such Debt Securities will be executed, authenticated, issued and delivered in accordance with the provisions of the Indenture.

With respect to any Securities consisting of Preferred Stock, unless such shares being issued are shares of Series A Junior Participating Cumulative Preferred Stock, par value $1.25 per share, we have further assumed that the Company will issue and deliver the shares of the Preferred Stock being issued and delivered after the filing with the Secretary of State of the State of Delaware of a certificate of amendment to the Certificate of Incorporation, approved by us, establishing the designations, preferences and rights of the class or series of the Preferred Stock being issued and delivered.

With respect to any Securities consisting of Depositary Shares, we have further assumed that the Depositary Shares will be: (i) issued and delivered after authorization, execution and delivery of a deposit agreement, approved by us, relating to the Depositary Shares to be entered into between the Company and an entity (the “Depositary”) selected by the Company to act as depository (the “Deposit Agreement”); and (ii) issued after the Company deposits with the Depositary shares of the Preferred Stock to be represented by such Depositary Shares that are authorized, validly issued and fully paid as contemplated by the Registration Statement and the Deposit Agreement.

In rendering the opinion set forth in paragraph 5 above, we have also assumed that (i) the Company’s Board of Directors has acted and will act in accordance with its fiduciary duties with respect to the authorization, execution, delivery and administration of the Rights Agreement and the issuance and administration of the Rights, and (ii) the Rights Agreement constitutes a valid and binding obligation of each party thereto other than the Company. It should be understood that (x) the Rights, by their terms, are subject under certain circumstances to becoming void in the hands of certain holders or purported transferees, (y) our opinion addresses the Rights

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and the Rights Agreement in their entirety and does not address the validity or binding effect of any particular provision of the Rights or the Rights Agreement, and (z) the effect, if any, that the invalidity of any particular provision of the Rights Agreement or the Rights might have on any other provision, or the entirety, of the Rights Agreement or the Rights is not settled under applicable law and could be affected by the facts and circumstances existing at the time of any adjudication of the issue. It should also be understood that our opinion does not address the substance or consequences of any determination that a court of competent jurisdiction may make regarding whether the Company’s Board of Directors would be required to redeem or terminate, or take other action with respect to, the Rights Agreement or the Rights at some future time based on the facts and circumstances existing at that time.

With respect to any Securities consisting of Warrants, we have further assumed that: (i) a Warrant agreement, approved by us, relating to the Warrants (the “Warrant Agreement”) to be entered into between the Company and an entity selected by the Company to act as the warrant agent (the “Warrant Agent”) will have been authorized, executed and delivered by the Company and the Warrant Agent; and (ii) the Warrants will be authorized, executed and delivered by the Company and the Warrant Agent in accordance with the provisions of the Warrant Agreement.

With respect to any Securities consisting of Units, we have further assumed that each component of such Unit will be authorized, validly issued, fully paid and nonassessable (to the extent applicable) and will constitute a valid and binding obligation of the Company as contemplated by the Registration Statement and the applicable Unit agreement (the “Unit Agreement”), if any.

The opinions expressed herein are limited by bankruptcy, insolvency, reorganization, fraudulent transfer and fraudulent conveyance, voidable preference, moratorium or other similar laws and related regulations and judicial doctrines from time to time in effect relating to or affecting creditors’ rights generally, and by general equitable principles and public policy considerations, whether such principles and considerations are considered in a proceeding at law or at equity.

As to facts material to the opinions and assumptions expressed herein, we have relied upon oral or written statements and representations of officers and other representatives of the Company and others. The opinions expressed herein are limited to the General Corporation Law of the State of Delaware and the laws of the State of New York, in each case as currently in effect, and we express no opinion as to the effect of the laws of any other jurisdiction.

We hereby consent to the filing of this opinion as Exhibit 5.1 to the Registration Statement and to the reference to Jones Day under the caption “Legal Matters” in the prospectus constituting a part of the Registration Statement. In giving such consent, we do

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not thereby admit that we are included in the category of persons whose consent is required under Section 7 of the Securities Act or the rules and regulations of the Commission promulgated thereunder.

Very truly yours,

/s/ Jones Day