[ New Material Developments ]
[ New Material Developments ]
is a hydrated chromium-rich oxide or oxyhydroxide in the form of Cr 2 O 3 · Cr ( OH ) 3 and iron oxyhydroxide species with a thickness of about 2 nm , with temperature having an impact on composition and thickness . The surface oxide has no fixed composition and structure ; there is a gradual change in composition across the thickness and lateral space , and it keeps changing in composition and structure over the degradation time . In varying oxidation states , other elements , such as molybdenum , silicon , and nitrogen , can also participate in oxide or hydroxide formation , depending on the alloying elements and their concentration . Previous projects using calculation of phase diagram ( CalPhaD ) -based thermochemical calculations have shown that nickel cannot form the native passive film of 25Cr-7Ni super duplex stainless steel , and the composition is a spinel-mixed type of oxide , primarily FeCr 2 O 4 . These calculations agree with our synchrotron X-ray analyses . However , bivalent nickel oxide / oxyhydroxide can be formed when the steel is anodically polarized . This means that nickel is not a central contributor to stainless steel ’ s passive film . However , it has been demonstrated that the enrichment of nickel beneath the oxide , known as the surface alloy layer , provides a barrier to the dissolution kinetics , resulting in increased corrosion resistance . Typically , stainless steel requires a chromium concentration of at least 10.5 % by weight that must be homogeneously dispersed within the iron alloy . This threshold is due to the percolation network of chromium atoms forming a continuous bond with oxygen along the entire surface . Chromium is a more active element than iron and can create a highly dense and capacitive oxide . Although the dielectric constant of iron oxides is higher than that of chromium oxide , charge transport through chromium oxide is far more sluggish due to more effective compactness and lower defect density . Therefore , the corrosion resistance of stainless steel increases with the chromium concentration . However , most engineering steel used in the industry is protected against corrosion by extrinsic means , such as coatings , inhibitors or cathodic protection . This is because cost factors limit the use of highly-alloyed stainless steel .
Test program The aim of this project was to understand why Hybrid Steel , with a concentration of 5 % Cr , exhibits corrosion resistance remarkably similar to 14 % Cr-containing stainless steel . This comparison aimed to shed light on the underlying mechanisms through two primary experimental approaches : ( 1 ) analyzing the passivation behavior in sulfuric acid solutions , including the impact of anodic polarization on passivity disruption , and ( 2 ) examining the corrosion dynamics in near-neutral chloride-containing solutions to confirm the presence of spontaneous passivity and its vulnerability to pitting . Subsequently , the structure and composition of the surface oxide layer were scrutinized using Hard X-ray Photoelectron Spectroscopy ( HAXPES ) to identify the factors contributing to Hybrid Steel ' s exceptional corrosion resistance . This work was further augmented by thermochemical modeling , enhancing our understanding of the phenomenological occurrence in materials science history . The tests were carried out on Hybrid Steel 55 in both the as-rolled condition , with a hardness of 35 HRC and tempered to increase the hardness to 55 HRC .
Test results – the beneficial effect of aluminum The electrochemical measurements showed that the Hybrid Steel has similar corrosion behavior to grade 420 martensitic stainless steel in deaerated sulfuric acid and near-neutral sodium chloride solutions . Hybrid Steel exhibits spontaneous passivity , remarkably low anodic current densities in the microamperes range , and resistance to pitting corrosion , indicative of characteristics akin to stainless steel . Nevertheless , Hybrid Steel , with merely 5 % Cr , challenges this notion , suggesting that chromium oxide ' s role in its passivity cannot be the sole explanation . Indeed , while the 10.5 % Cr threshold holds for pure iron-chromium alloy systems , Hybrid Steel ’ s composition includes additional elements , such as aluminum and molybdenum , that also play significant roles in enhancing its passive behavior . The CalPhaD-thermochemical computation analyses , augmented by the HAXPES results , showed that other
Hybrid Steel offers properties of tool steel , maraging steel , and stainless steel , combined with the production economy of engineering steel .
alloying elements participate in the surface oxides , supporting chromium oxide in corrosion protection . This indicates that the surface oxide is predominantly composed of Fe 2 O 3 · FeCr 2 O 4 · NiO · AI 2 O 3 , which changes structure , fraction and existence over electrochemical polarization . Furthermore , the thermochemical modelling shows that when Cr ( VI ) species form upon transpassive breakdown of Cr 2 O 3 , the interfacial pH is reduced , which causes some dissolution of the remaining passive surface oxides . Therefore , Hybrid Steel could repassivate despite the loss of Cr 2 O 3 . In addition , the computational calculations and HAXPES analyses have shown that Al , Ni and Mo oxides exist beyond the polarization at electrochemical potentials above the transpassive dissolution potential of chromia . The clear conclusion is that oxides other than chromium oxide contribute to the passivation of Hybrid Steel . It exhibits superior extended passivation capabilities , outshining traditional stainless steel ' s corrosion resistance , which primarily relies on chromium oxide for protection . In this context , aluminum oxide is highly beneficial . Aluminum oxide forms a dense , potent , sluggish oxide to electron transfer reactions . Hybrid Steel exhibits passive behavior in both heat-treated conditions , as apparent from its polarization behavior in acid and chloride solutions . It is worth highlighting the role of nickel in the surface oxide which also supports the passivity of Hybrid Steel . Nickel usually does not form an oxide due to
22 Stainless Steel World March 2024 www . stainless-steel-world . net