**4. Conclusions**

Cationic lignin-based polymer (Lignin-DMC) was synthesized by copolymizing DMC with kraft lignin. The prepared lignin-DMC was an e ffective corrosion inhibitor in the 1 mol/L HCl solution. The corrosion inhibition e ffect was best at 75 mg/<sup>L</sup> at room temperature, reaching a state at 87.65%. Meanwhile, the results revealed that the proper dosage may be used as an inhibitor. However, a superfluous dosage had a negative e ffect on the anticorrosion performance. Additionally, the corrosion inhibitor-lignin-DMC was a mixed corrosion inhibitor. The adsorption mode of lignin-DMC was the Langmuir adsorption isotherm. The mechanism of adsorption was found to be a physical and chemical model in nature. The adsorption model indicated that the corrosion inhibitor was an adsorption type corrosion inhibitor

*Materials* **2019**, *12*, 1776

**Supplementary Materials:** The following are available online at http://www.mdpi.com/1996-1944/12/11/1776/s1, Figure S1. Schematic diagram of a lignin-DMC synthesis device, Figure S2. (a) The reaction mechanism and (b) The sketch of the lignin grafting reaction, Figure S3. FT-IR spectra of kraft lignin and lignin-DMC polymer, Figure S4. SEM micrographs of the Fe surface without 1.0 mol/L HCl (a), Fe surface after submersion in 1.0 mol/L HCl (b), 1.0 mol/L HCl with the addition of 75 mg/<sup>L</sup> lignin-DMC (c), and (d) Fe immersed in 1.0 mol/L HCl + 100 mg/<sup>L</sup> Lignin-DMC solutions, Table S1. Molecular weight and charge density of lignin, and lignin-DMC polymer, Table S2. Elemental analysis of lignin and lignin-DMC polymer.

**Author Contributions:** S.W., X.Z., F.K. conceived and designed the experiments; C.G., X.D. performed the experiments, C.G., K.L. wrote the main manuscript.

**Funding:** This research was funded by the National Key R&D Program of China (Grant No. 2017YFB0308000), the National Natural Science Foundation of China (Grant No. 31570566, 31500489, 31800499), the Key Research and Development Program of Shandong Province (Grant No. 2017GSF17130) and the Foundation of Guangxi Key Laboratory of Clean Pulp & Papermaking and Pollution Control of China (Grant No. KF201717).

**Acknowledgments:** We acknowledge support from the National Key R&D Program of China, the National Natural Science Foundation of China, the Key Research and Development Program of Shandong Province and the Foundation of Guangxi Key Laboratory of Clean Pulp & Papermaking and Pollution Control of China (KF201717).

**Conflicts of Interest:** The authors declare no conflict of interest.
