Abstract
Carbon steels are the most widely used structural metals because of their suitable mechanical properties, weldability, and low cost; however, they readily corrode in marine environments. Stainless steels (SS) are often utilized in their place when there are prolonged corrosion concerns, though the additions of Cr and Ni make SS alloys many times more expensive than carbon steels and cost prohibitive for larger components. Therefore, coatings are desirable for protecting carbon steel components used in marine energy applications and SS overlay claddings are a choice coating method because of their strong bond to the carbon steel substrate. During cladding, SS feedstock, either wire or powder, is repeatedly melted by a heat source over the carbon steel substrate, which then metallurgically bonds the dissimilar metals, forming a strong and corrosion resistant composite, as shown in Figure 1 (a).
Figure 1: (a) Illustration of laser-wire-DED cladding process, (b) Optical micrograph of a two-layer laser-wire-DED cladding in cross-section
Cladding processes that use a high energy heat source (e.g., laser, electron beam) rather than a plasma arc are desirable for creating claddings with minimal distortion, a small heat-affected zone (HAZ), and thin layer(s). In particular, laser-wire-directed energy deposition (DED), an additive manufacturing (3D printing) based process is an ideal cladding fabrication technique because it uses spooled welding wire, which is substantially lower cost compared to powder. Yet with the newness of the technology, optimized processing parameters are key to maximizing the claddings’ material properties and effectiveness as a corrosion resistant barrier.
In this work, we fabricate austenitic 309L SS claddings on low carbon steel using laser-wire-DED and examine their corrosion behavior in marine environments. Cladding samples are tested in both 3.5 wt.% NaCl immersion and 5 wt.% NaCl salt spray conditions to compare corrosion behavior for different exposures in marine environments. In the immersion test, a two-layer cladding is compared to a single-layer one, and in both tests the two-layer cladding corrosion performance is benchmarked using wrought SS and carbon steel.
In the immersion test, electrochemical methods, including open circuit potential (OCP), electrochemical impedance spectroscopy (EIS), linear polarization resistance (LPR), and cyclic polarization (CP) are used to measure corrosion behavior. The single layer cladding exhibits relatively poor performance due to the localized corrosion of partially mixed zones (PMZs), which form in the first layer during the dissimilar metal cladding process, as evident in Figure 1 (b). The two-layer cladding is outstandingly corrosion resistant, and its second layer protects from corrosion of PMZs. The low corrosion rates and high pitting resistance is attributed to the cladding’s high alloying (Cr, Ni). In the salt spray test, mass loss is used to measure corrosion rate following the ASTM G1 specification for exposures up to 60 days. Microscopic and diffraction-based methods are used to examine the corrosion product species and surface morphologies after exposure. The results of this work will aid the adoption of SS overlay claddings for use in marine environments, including offshore wave and wind energy applications.