X. CHEN et al.: EFFECTS OF A Cu ADDITION ON THE CORROSION RESISTANCE OF LOW-ALLOY ... 381–388 EFFECTS OF A Cu ADDITION ON THE CORROSION RESISTANCE OF LOW-ALLOY PRE-HARDENED PLASTIC MOLD STEEL VPLIV DODATKA BAKRA NA KOROZIJSKO ODPORNOST MALOLEGIRANEGA ORODNEGA JEKLA ZA BRIZGANJE PLASTIKE Xuan Chen, Jiayuan Li, Zhi Li, Junwan Li, Xiaochun Wu * School of Materials Science and Engineering, Shanghai University, Shanghai 200444, China Prejem rokopisa – received: 2022-04-04; sprejem za objavo – accepted for publication: 2022-06-09 doi:10.17222/mit.2022.465 The influence of Cu on the corrosion performance of low-alloy Cu-bearing pre-hardened plastic mold steel was investigated and compared with non-Cu-bearing mold steel using transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), salt-spray tests and electrochemical impedance spectroscopy (EIS). The results reveal that the formed rust layer is composed of -FeOOH, -FeOOH, -FeOOH, Fe3O4 and large amounts of amorphous compounds. The cor- rosion product in the early stages of corrosion is loose and porous but becomes denser and more tightly bound to the steel matrix in the later stages, and the corrosion rate of Cu-bearing steel decreases with the increasing corrosion time. With a further Cu ad- dition, the free-corrosion potential of Cu-bearing steel increases and, consequently, the electrochemical impedance, the capaci- tive single semicircular arc on the impedance spectra and the charge transfer resistance are also significantly enhanced. There- fore, Cu effectively enhances the corrosion resistance of pre-hardened mold steel, prolonging its service life and ensuring a high quality and shape fidelity of molded plastic products. Keywords: copper, plastic mold steel, corrosion resistance, electrochemical analysis, salt spray test Avtorji opisujejo vpliv dodatka bakra na korozijske lastnosti toplotno obdelanega malolegiranega orodnega jekla za oblikovanje plastike. Primerjajo jih s podobnim nelegiranim jeklom (brez dodatka bakra). Za karakterizacijo preiskovanih materialov so uporabili presevno in vrsti~no elektronsko mikroskopijo (TEM in SEM), rentgensko difrakcijo (XRD), test z razpr{evanjem slanice in elektrokemijsko impedan~no spektroskopijo (EIS). Rezultati analiz so pokazali nastanek korozijske plasti na povr{ini jekla, ki je bila sestavljena iz -FeOOH, -FeOOH, -FeOOH, Fe3O4 in velike koli~ine amorfnih spojin. V za~etni fazi korozije je bil korozijski produkt porozen in puhel. V kasnej{ih fazah je postal gostej{i, bolj tesno povezan s kovinsko osnovo jekla in hitrost korozije z bakrom legiranega jekla se je s ~asom trajanja korozije zmanj{evala. Dodatek bakra je pove~al prosti korozijski potencial jekla in posledi~no so se tudi izbolj{ale elektrokemijska impedanca, enojni kapacitivni polkro`ni oblok na impedan~nem spektru in upornost prenosa naboja. Na osnovi izvedene raziskave so ugotovili, da legiranje pobolj{anega orodnega jekla z bakrom u~inkovito izbolj{a njegovo korozijsko odpornost in podalj{a njegovo dobo trajanja oz. `ivljenjsko dobo orodja, kar posledi~no tudi zagotavlja visoko kakovost in natan~nost oblikovanja izdelkov iz plastike. Klju~ne besede: baker, orodno jeklo za brizganje plastike, korozijska odpornost, elektrokemijska analiza, test z razpr{evanjem slanice 1 INTRODUCTION Molds composed of pre-hardened steel are critical components in the manufacture of a wide range of prod- ucts, and increasing the tool life of pre-hardened mold steel to improve production efficiency, reduce costs and maintain the quality of molded products remains a great challenge. 1,2 Pre-hardened mold steels, such as those be- longing to the medium-carbon low-alloy steel family AISI P20 and its derivatives DIN 1.2738 (German grade) and 718 (Swedish grade), have been widely used in in- dustry. 3,4 The lifespan of plastic mold steel is determined by material deterioration such as wear, cracking and cor- rosion, with corrosion being the main factor. 5–7 There- fore, corrosion management is critical. Corrosion perfor- mance is governed mainly by the environment and the material properties, and corrosion resistance can be im- proved by alloying. For instance, Cr and Cu are well- known alloying elements 8,9 that protect steel against cor- rosion induced by weather and acid exposure, respec- tively. Most studies on the corrosion behavior of low-alloy steels focused on corrosion in a single medium, either the atmosphere or an aqueous solution. It has been sug- gested that a dense rust layer composed of aggregated fine rust particles serves as a good barrier against mass transfer and that alloying elements can contribute to the formation of fine particle rusts. Cu inhibits the crystalli- zation and growth of rust particles, whereas Cr enhances the formation of uniform amorphous ferric oxyhydro- xide, thus promoting the development of a compact rust layer. 10 Moreover, Cu can reduce the conductivity of the rust layer, thus decreasing the rate of electrochemical iron dissolution. 11 Choi et al. 12 revealed that Cr and Cu Materiali in tehnologije / Materials and technology 56 (2022) 4, 381–388 381 UDK 669.15-194.2:669.3 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 56(4)381(2022) *Corresponding author's e-mail: wuxiaochun@shu.edu.cn (Xiaochun Wu) compounds promote the formation of a protective rust layer on steel in aqueous conditions as they do under at- mospheric conditions. However, a Cu addition to plastic mold steel and its effect on the corrosion resistance have not been widely studied. This study clarifies the influence of added Cu as an acid-resistant element on the corrosion of low-alloy pre-hardened plastic mold steel. Pre-hardened plastic mold steels, with and without Cu, were characterized by scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), X-ray dif- fraction (XRD), electrochemical impedance spectros- copy (EIS) and accelerated corrosion via salt-spray tests; the results were analyzed and compared to determine the mechanism for corrosion resistance enhancement by Cu. 2 EXPERIMENTAL PART 2.1 Materials The chemical compositions of SDP1 and SDP1Cu steel are listed in Table 1. Steel samples of (15 × 15 × 40) mm in size were cut from a round rod and heated at 950 °C for2hs othat copper could be solid dissolved into the steel matrix. A bainite structure was obtained with slow cooling at 0.1 °C/s followed by tem- pering at 500 °C for 2 h. Table 1: Chemical compositions (w/%) of SDP1 and SDP1Cu steels Material C Si Mn Cr Ni Mo Cu Al Fe SDP1Cu 0.20 0.60 1.55 1.12 0.60 0.22 1.16 0.11 Bal SDP1 0.21 0.65 1.60 1.08 0.50 0.20 – – Bal 2.2 Corrosion test EIS measurements were carried out using a standard electrochemical workstation (Gamry Reference 600) with a three-electrode cell containing a saturated calomel electrode (SCE) as the reference electrode, a platinum foil as the counter electrode and the SDP1 and SDP1Cu steel samples as the working electrodes. All the experi- ments were performed in a 0.5 w/% NaCl solution, which was prepared by dissolving analytically pure NaCl in deionized water at room temperature ((25 ± 2) °C). In addition, before EIS and potentiodynamic polarization measurements, the open-circuit potential (OCP) of each sample was measured for 45 min until a stable potential was reached. The EIS tests were performed with an am- plitude perturbation of ± 10 mV and a frequency range of 10 5 Hz to 10 –2 Hz over the OCP. The potentiodynamic polarization measurements were conducted at a scanning rate of 0.5 mV/s (30 mV/min), from –100 mV below the OCP to 300 mV with respect to the SCE. In order to get reliable results, at least three specimens of each alloy with different nitrogen contents were used, and the re- sults were fitted and analyzed using the measurement in- strument software. Corrosion properties were studied through salt-spray tests. SDP1 and SDP1Cu steel were cut into (15 × 15 × 2) mm test samples. The salt-spray test was performed accord- ing to Chinese standard GB/T 10125-2012 in an artificial atmosphere with the following conditions: the NaCl con- centration of the sprayed solution was 50 g/L (5.0 % NaCl), the temperature was 35 °C, the pH range of the solution was 6.5–7.2 and the samples were placed in a testing chamber with the unprotected side facing up- wards at an angle of 15° from the vertical. The salt-spray experiments were conducted for (48, 96 and 192) h. 2.3 Microstructure observation The microstructures of the samples were observed using an optical microscope, a Carl Zeiss SUPRA 40 field emission SEM, and a JEM-2100F TEM. For SEM observation, the samples were mechanically ground with sandpaper, polished, and then etched ina4%nital solu- tion. For TEM observation, 500-m thick discs were first mechanically thinned to approximately 50-m thick foils and then electropolished by a twin-jet electro polisher in a solution of 10 % volume fraction of perchloric acid and 90 % volume fraction of ethanol. After the salt-spray test, an 18 kW D/MAX X-ray diffractometer (Japan Rigaku Corporation) was used to continuously scan the samples with a Cu-K ray. The XRD acceleration volt- age was 40 kV, the scanning speed was 4 °/min, and the scanning-angle range was 20°–70°. 2.4 Direct mass-loss measurements Prior to the salt-spray exposure, the specimens for mass-loss measurements were rinsed with acetone, air-dried and weighed using a microbalance. After each exposure, the specimens were descaled via immersion in a solution of 500 mL HCl + 3.5 g hexamethylenetetra- mine and then successively rinsed in distilled water and acetone. The specimens were then air-dried and re-weighed in order to calculate the mass loss resulting from corrosion. The mass-loss rate, V S , of the material was calculated according to Equation (1): V mm St s = − ⋅ 01 (1) where m 0 is the mass in g before the test, m 1 is the mass in g after the test, S is the surface area of the specimen in m 2 and t is the corrosion test duration in h. 3 RESULTS 3.1 Microstructure Figures 1a and 1b show SEM morphologies of the SDP1Cu and SDP1 steel microstructures. The SEM morphologies show the typical lath bainite and the clas- sic granular bainite. Each lath bainite grain is composed of several lath bundles with different orientations. Some granular islands of the secondary phase are distributed in X. CHEN et al.: EFFECTS OF A Cu ADDITION ON THE CORROSION RESISTANCE OF LOW-ALLOY ... 382 Materiali in tehnologije / Materials and technology 56 (2022) 4, 381–388 the equiaxed bainitic ferrite matrix. The sizes of the is- lands are unequal, and most of them are in the 1–10 μm range. Several previous studies identified the islands as martensite-austenite (M-A) constituents. 13 There are M-A islands both inside the grains and at the grain boun- daries. The M-A islands at the grain boundaries decom- pose preferentially, and adjacent decomposed structures are connected. Precipitated interfaces nucleate and grow preferentially at the grain boundaries, enriching the grain boundaries. 14 Figure 2 shows a bright field image, a dark field im- age and diffraction pattern of SDP1Cu steel. A continu- ous arrangement of rod-like carbide is observed. The car- bide was determined to be M 23 C 6 -type carbide based on the diffraction pattern of the crystal axis band [112].As the copper-rich precipitated phase was very fine, an HRTEM analysis of the SDP1Cu steel samples was per- formed. As can be seen from Figure 3, the precipitated phase is composed of short rods of 10–20 nm in length. This is the copper-rich precipitated phase. The diffrac- tion pattern after Fourier transform confirms a face-cen- tered cubic structure, with a maximum length of the long axis of about 20 nm. X. CHEN et al.: EFFECTS OF A Cu ADDITION ON THE CORROSION RESISTANCE OF LOW-ALLOY ... Materiali in tehnologije / Materials and technology 56 (2022) 4, 381–388 383 Figure 3: HRTEM of SDP1Cu steel: a) microstructure, b) diffraction pattern of rod-like structures Figure 1: SEM scanning images after heat treatment: a) SDP1Cu steel, b) SDP1 steel Figure 2: TEM image of SDP1Cu steel: a) bright field image, b) dark field image and diffraction pattern 3.2 Electrochemical measurements Figure 4a shows the open-circuit potential versus time curves of the steels tested in a 0.5 % NaCl solution. Generally, the smaller the absolute value of the open-cir- cuit potential after stabilization, the smaller is the self-corrosion potential of the material in the current cor- rosion environment, indicating better corrosion resis- tance. The two types of steel showed the same behavior, that is, a negative shift of the open-circuit potential in the initial stage is followed by a gradual stabilization. There- fore, both SDP1Cu and SDP1 exhibited activation be- havior in the 0.5 % NaCl aqueous environment. Figure 4b shows Nyquist plots of the electrochemi- cal impedance of the two experimental steels. The capac- itive reactance arc indicates the resistance to electro- chemical reactions at the metal interface and the double-layer capacitance of the material cross-section, while the size of the arc reflects the charge transfer resis- tance. The electrochemical-impedance spectra of the two experimental steels exhibit single capacitance-arc char- acteristics, indicating that the addition of Cu does not change the main characteristics of the impedance spec- tra. An equivalent circuit diagram can be obtained through fitting, as shown in Figure 5. The Nyquist dia- gram shows that the arc resistance diameter of the Cu-containing experimental steel increases significantly, indicating that it is more difficult to transfer charge at the electrode solid–liquid interface during the corrosion pro- cess, decreasing the corrosion rate. The corrosion resis- tance of SDP1Cu is higher than that of SDP1, confirm- ing that the addition of Cu enhances corrosion resistance. Figure 4c shows the polarization curves of the tested steels. It can be observed that the cathodic polarization curves of the two steels have similar shapes, but the cor- rosion potential is shifted positively after adding Cu. The moving point polarization curve of the Cu-containing steel moves upward as a whole, and the electrode poten- tial increases. This is because the layer of corrosion products covers the surface of the matrix, meaning that Cu ions are enriched on the surface of the sample, in- creasing the potential. When the corrosion current den- sity is the same, the more positive the corrosion poten- tial, the better is the corrosion resistance of the material. The increase in the corrosion potential indicates that the standard voltage difference between the cathode and an- ode of the electrochemical reaction decreases without polarization, so Cu has a tendency to thermodynamically reduce electrochemical corrosion. The Tafel extrapolation method was used to fit the electrochemical data, and the corrosion rate was calcu- lated using the self-corrosion current density according to Equation (2): CR iM zF =×× 36 10 7 . (2) where CR is the corrosion rate in g/(m 2 ·h), i is the self-corrosion current density in A/cm 2 obtained from X. CHEN et al.: EFFECTS OF A Cu ADDITION ON THE CORROSION RESISTANCE OF LOW-ALLOY ... 384 Materiali in tehnologije / Materials and technology 56 (2022) 4, 381–388 Figure 5: Equivalent circuit adopted to fit the EIS data Figure 4: Electrochemical experiment results: a) open-circuit poten- tial curves, b) Nyquist impedance diagrams, c) potential polarization curves the polarization curve, M is the molar mass of iron in g/mol, z is the chemical valence of the material, and F is Faraday’s constant in C/mol. The results are summa- rized in Table 2. The SDP1Cu steel shows the highest electrode potential and the best surface film stability owing to the addition of Cu and Ni, which increase the electrode potential. 15 Table 2: Fitting data for the potential polarization curve Material E corr (V/SCE) I corr (×10 –4 (A/cm 2 )) CR (g/(m 2 ·h)) SDP1Cu –0.433 0.124 0.086 SDP1 –0.491 1.458 1.012 3.3 Salt spray test Figure 6 shows the surface micromorphology of SDP1Cu and SDP1 steels after corrosion for different durations in a salt-spray environment. As can be ob- served, there are still small areas not covered by the rust layer after 48 h of accelerated corrosion. When the cor- rosion duration was increased to 192 h, the rust layer completely covered the surface of the sample. The formed rust has two layers. The outer rust layer is yel- lowish brown, brittle in texture and easily falls off from the sample surface. The inner rust layer is black, tightly combined with the matrix and not easily dislodged. Figure 7 shows the corrosion-rate statistics for SDP1Cu and SDP1 steel. When the salt-spray test dura- tion is (48, 96 and 192) h, the corrosion rate of SDP1Cu steel is (1.60, 1.19, and 0.987) g/(m 2 ·h), respectively, while the corrosion rate of SDP1 steel is (1.58, 1.35, and 1.21) g/(m 2 ·h), respectively. With the increase in the cor- rosion time, the corrosion rates of both types of steel gradually decrease, indicating that the rust layer is hin- dering the elemental diffusion process between the ma- trix and the external corrosion solution, thus reducing the corrosion rate of the materials. At the same time, during salt-spray accelerated corrosion, the corrosion rates of the two steels are similar at the initial stage of corrosion, indicating that although Cu can improve the electrode potential of the matrix, it does not effectively reduce the initial corrosion rate in a harsh environment. The corro- sion rate of SDP1Cu steel decreases much faster than X. CHEN et al.: EFFECTS OF A Cu ADDITION ON THE CORROSION RESISTANCE OF LOW-ALLOY ... Materiali in tehnologije / Materials and technology 56 (2022) 4, 381–388 385 Figure 7: Corrosion-rate statistics Figure 6: Macroscopic morphology after accelerated corrosion in the salt-spray environment: a) SDP1Cu – 48 h, b) SDP1Cu – 96 h, c) SDP1Cu – 192 h, d) SDP1 – 48 h, e) SDP1 –96 h, f) SDP1 – 192 h X. CHEN et al.: EFFECTS OF A Cu ADDITION ON THE CORROSION RESISTANCE OF LOW-ALLOY ... 386 Materiali in tehnologije / Materials and technology 56 (2022) 4, 381–388 Figure 10: Changes in the rust-layer composition and relative content of each product with the salt-spray time: a) SDP1Cu steel, b) SDP1 steel Figure 9: Line-scan composition analysis of the rust layer formed after salt-spray tests: a) SDP1Cu, b) SDP1 Figure 8: SEM images of the rust layer cross-sections: a) SDP1Cu, b) SDP1 that of SDP1 steel. This indicates that in the salt-spray corrosion environment, Cu has no obvious effect in the early stage, but can effectively improve the density and integrity of the rust layer after the rust layer is produced so as to effectively reduce the corrosion rate. Samples were selected for cold mounting and the rust layer was observed. SEM images of the experimental steels after the salt-spray test for 192 h are shown in Fig- ure 8. The thickness of the rust layer of the Cu-contain- ing steel sample is about 20 μm, which is significantly lower than that of the non-Cu-containing steel sample (about 28 μm). Figure 9 shows energy dispersive X-ray analysis re- sults of the SDP1 and SDP1Cu rust layers. Cr enriched the rust layer of both steels as it can promote the forma- tion of a protective rust layer and reduce the anodic dis- solution of the matrix alloy. Meanwhile, Cu enriched the rust layer of SDP1Cu, indicating that Cu promotes the densification and completion of the rust-layer structure. Other researchers have also found that Cu enrichment of other alloy steels can promote the densification of the rust layer, prevent oxygen diffusion to the surface of the matrix and reduce the electrical conductivity of the rust-layer surface so as to protect the material and reduce its corrosion. Figure 10 shows the variation in the surface rust- layer composition and the relative content of each phase in SDP1Cu and SDP1 steel for different salt-spray test durations. After the salt-spray test, the compositions of the rust layers on the surfaces of the two steel samples are similar. The rust layers are mainly composed of -FeOOH, -FeOOH, -FeOOH, Fe 3 O 4 and large amounts of amorphous compounds. These amorphous compounds are mainly microcrystalline oxides or hy- droxides and could not be identified by XRD. 16 -FeOOH is more continuous and compact; it is non-conductive, effectively protecting the matrix. 4 DISCUSSION The corrosion of plastic mold steel in an aqueous en- vironment is dominated by electrochemical corrosion. In etchant solutions containing dissolved oxygen, the fol- lowing anodic and cathodic reactions occur. 8,17 Anodic reactions are presented in Equations (3) to (6): Fe Fe 2+ +2e – (3) Fe 2+ + 2Cl – +4H 2 O FeCl 2 ·4H 2 O (4) FeCl 2 ·4H 2 O Fe(OH) 2 + 2Cl – +2H++2H 2 O (5) 4Fe(OH) + + 2OH – +O 2 4FeOOH +2H 2 O (6) Cathodic reactions are presented in Equations (7) and (8): O 2 +2H 2 O+2e – 4OH – (7) Fe 2+ + 8FeOOH + 2e – 3Fe 3 O 4 +4H 2 O (8) Corrosion products of low-alloy steel generally con- sist of -FeOOH, -FeOOH, -FeOOH, Fe 3 O 4 and amor- phous products. During long-term corrosion of steel, -FeOOH is first formed on the surface of the matrix. Then, it develops into amorphous FeOOH, which is eventually transformed into stable -FeOOH. -FeOOH exhibits good continuity and density, and does not con- duct electricity, effectively protecting the matrix. An ad- dition of alloying elements can enrich the rust-layer cracks and prevent the corrosive medium from contact- ing the matrix, thus reducing the corrosion rate 18 .A c - cording to a previous study, 19 an addition of Cu promotes the transformation of -FeOOH into the most stable cor- rosion phase, -FeOOH, thus reducing the electrical con- ductivity of the corroding rust layer and slowing down the electrochemical corrosion rate. The rust layer on the surface of low-alloy steel is di- vided into two sublayers: the outer rust layer, which is loose and porous exhibiting poor protection, and the in- ner rust layer, which is mainly composed of dense -FeOOH and firmly bonded to the matrix. The corro- sion products of the experimental steel without Cu are thick and loose, with many cracks, and the interface be- tween the corrosion products and the matrix is uneven with a few corrosion pits. In contrast, the corrosion prod- ucts of the experimental steel with Cu are thin and com- pact, and the interface with the matrix is smooth. From surface analyses obtained with different tech- niques, it was confirmed that a film containing elemental Cu was formed on the SDP1Cu steel surface after a cor- rosion test. Hao et al. 20 reported that elemental Cu is con- centrated on the surface of Cu-containing carbon steel immersed in a strongly acidic NaCl aqueous solution, and the results of the present study are in good agree- ment with this previous report. The mechanism for the improvement of corrosion resistance of steel due to the formation of elemental Cu on a steel surface in a strongly acidic environment was examined in detail by Hoog et al. 9 based on electrochemical measurement re- sults. A Cu-enriched layer is formed on the steel surface due to base metal dissolution and the re-deposition of Cu with lower solubility. Ishii et al. 21 suggested that Cu im- proves the corrosion resistance of steel surfaces and inhibits anodic reactions. Due to the Cu and Cr enrich- ment, the corrosion rust layer on low-alloy steel gener- ates a complex iron chromium copper oxide, refines the rust layer of particles and fills micro cracks and holes of the rust layer, thereby increasing the rust-layer density, blocking the channels of direct contact with the corrosive medium and the matrix and improving the corrosion re- sistance. Cu exists in the rust layer formed in various compound salts, being the core of FeOOH crystalliza- tion. Due to the milling and densification, the inner rust layer reduces the ion channels and anode area; it also re- duces the formation of Fe 3 O 4 , the conductivity and the corrosion rate. X. CHEN et al.: EFFECTS OF A Cu ADDITION ON THE CORROSION RESISTANCE OF LOW-ALLOY ... Materiali in tehnologije / Materials and technology 56 (2022) 4, 381–388 387 5 CONCLUSIONS The addition of Cu to low-alloy plastic mold steel significantly improves the corrosion resistance of the material. The following conclusions can be drawn from this study: 1) Cu and other elements can improve the self-corro- sion potential of SDP1Cu steel and reduce the corrosion rate and self-corrosion current density of the material. 2) During the salt-spray test, the corrosion rate of SDP1Cu steel decreased from 1.60 g/(m 2 ·h) to 0.988 g/(m 2 ·h) and that of SDP1 steel decreased from 1.579 g/(m 2 ·h) to 1.21 g/(m 2 ·h) with the increase in the salt-spray duration due to the generation of a rust layer on the surface of the material. The protective effect of the formed corrosion layer is enhanced in SDP1Cu steel. 3) An elemental Cu-enriched layer was formed on the corroded surface of Cu-containing steel. 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