**1. Introduction**

Maraging steels are developed to combine superior high strength (yield strength commercially ranging from 1030 to 3450 MPa) and fracture toughness [1]. The small amount of carbon makes maraging steels different from regular steels; that is, they are reinforced by intermetallic compounds derived from supersaturated martensite during the age hardening of the malleable low-carbon iron (Fe)–nickel (Ni) lath martensitic matrix [2].

Recently, super-high-strength maraging steels have been broadly utilized in applications including the manufacture of airplanes, missiles, rocket motor cases, and gas turbines [3–6]. Maraging steels are normally low-carbon, Fe–Ni-based alloys with substantial amounts of cobalt (Co) and molybdenum (Mo) along with a small percentage of titanium (Ti) and aluminum (Al). However, the material can be altered by changing its composition according to the application [7,8]. Due to the low carbon content, maraging steels mostly exhibit a high degree of machinability [9]. Manufacture of such steels has been performed by adding or completely removing certain alloying elements to create maraging steel with good mechanical and corrosion-resistance properties.

Previous studies report that the durability of Fe–18% Ni matrix alloyed by Ti significantly decreases at higher strength levels due to the formation of low-temperature, Ti-rich, metastable NiTi (B2) or Ni3Ti during the aging process [10–12]. Similarly, higher Ti levels can decrease durability via grain-boundary precipitation of the TiC or TiCN, except in cases where the carbon content is kept low and thermomechanical processing is profoundly controlled. Therefore, this suggests that recently created maraging steels likely have low Ti

**Citation:** Seikh, A.H.; Halfa, H.; Soliman, M.S. Effect of Molybdenum Content on the Corrosion and Microstructure of Low-Ni, Co-Free Maraging Steels. *Metals* **2021**, *11*, 852. https://doi.org/10.3390/met11060852

Academic Editors: Santiago Fajardo and Renato Altobelli Antunes

Received: 11 April 2021 Accepted: 16 May 2021 Published: 21 May 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

content. Mo is another alloying component that is valuable for both strength and durability. Previous studies showed that Mo-rich zones precipitate during the aging of maraging steel and play an exceptionally critical role in maintaining the strength of steel [13–16]. The role of Mo in these processes has been evaluated in ranges of 3–5% based on its inhibition of embrittlement due to intergranular isolation of contaminants in grain boundaries and as a potential precipitation hardener [17].

The corrosion and corrosion resistance of maraging steels in sulfuric acid solution have been examined previously [18–21], revealing their elevated corrosion resistance in acidic solution relative to tempered martensite alloy steels. In the present study, we incorporated varied contents of Mo into four maraging steels using an electroslag refining (ESR) process in order to evaluate its effect on the corrosion-resistance and microstructure properties of the resulting steels. These four new low-Ni, Co-free maraging steels containing 0, 2.9, 4.6, and 9.8 wt% Mo were investigated to determine their corrosion behavior in1MH2SO4 and 3.5% NaCl solution using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques.

#### **2. Materials and Methods**
