Material Agreement · New Debt / Obligation

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 OR 15(d) of The Securities Exchange Act of 1934 ​ Date of Report (Date of earliest event reported) July 17, 2025 NL Industries, Inc. (Exact name of registrant as specified in its charter) ​ ​ ​ New Jersey 1-640 13-5267260 (State or other jurisdiction of incorporation) (Commission File Number) (IRS Employer Identification No.) ​ ​ ​ ​ ​ ​ 5430 LBJ Freeway , Suite 1700 , Dallas , Texas 75240-2620 (Address of principal executive offices) (Zip Code) ​ Registrant’s telephone number, including area code ( 972 ) 233-1700 ​ ​ ​ ​ ​ ​ ​ (Former name or former address, if changed since last report.) ​ ​ Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions (see General Instruction A.2): ​ ​ ☐ Written communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425) ​ ​ ☐ Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR 240.14a-12) ​ ​ ☐ Pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act (17 CFR 240.14d-2(b)) ​ ​ ☐ Pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act (17 CFR 240.13e-4(c)) ​ Securities registered pursuant to Section 12(b) of the Act: ​ ​ ​ ​ ​ ​ ​ Title of each class ​ Trading Symbol(s) ​ Name of each exchange on which registered Common stock ​ NL ​ New York Stock Exchange ​ Indicate by check mark whether the registrant is an emerging growth company as defined in Rule 405 of the Securities Act of 1933 (17 CFR §230.405) or Rule 12b-2 of the Securities Exchange Act of 1934 (17 CFR §240.12b-2). Emerging growth company   ☐ If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13(a) of the Exchange Act.   ☐ ​ ​ ​ ​ Item 1.01 Entry into a Material Definitive Agreement. ​ Item 2.03 Creation of a Direct Financial Obligation or an Obligation under an Off-Balance Sheet Arrangement of a Registrant.   The information disclosed by Kronos Worldwide, Inc., a Delaware corporation and an affiliate of the registrant (“Kronos Worldwide”), under Items 1.01 and 2.03 of the Current Report on Form 8-K dated July 17, 2025 that Kronos Worldwide (File No. 1-31763) filed with the U.S. Securities and Exchange Commission on July 17, 2025 is hereby incorporated herein by reference. ​ Item 9.01 Financial Statements and Exhibits. ​ ​ ​ ​ ​ ​ (d) Exhibits ​ ​ ​ ​ ​ ​ ​ Item No. ​ Description ​ ​ ​ ​ ​ 10.1 ​ Fourth Amendment to Credit Agreement dated July 17, 2025 among Kronos Worldwide, Inc., Kronos Louisiana, Inc., Kronos (US), Inc., Kronos Canada, Inc., Kronos Europe NV, Kronos Titan GmbH, Wells Fargo Bank, National Association, as administrative agent, and the lenders a party thereto – incorporated by reference to Exhibit 10.1 to the Current Report on Form 8-K dated July 17, 2025 and filed by Kronos Worldwide, Inc. (File No. 1-31763) on July 17, 2025 ​ ​ ​ ​ ​ 104 ​ Cover Page Interactive Data File (embedded within the Inline XBRL document) ​ ​ ​ ​ ​ ​ ​ SIGNATURE ​ ​ Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized. ​ ​ ​ ​ NL INDUSTRIES, INC. ​ (Registrant) ​ ​ ​ ​ ​ ​ Date: July 17, 2025 By: /s/Amy A. Samford                                     ​ Amy A. Samford Executive Vice President and     Chief Financial Officer ​ ​ ​ ​