In this study, Cu-bearing SS coatings with varying Cu contents were fabricated using pre-alloyed powders and LDED technology, and their antifouling performance was evaluated using a comprehensive dynamic antifouling evaluation system. Based on this, the microstructure and distribution of Cu in the coatings were analyzed, and the causes of cracking under different Cu content conditions were investigated. Subsequently, the antifouling performance of the coatings was comprehensively evaluated using macro- and micro-characterization techniques, and changes in the structural and adhesive properties of biofilms at the early stage. Additionally, the release kinetics of Cu ions were analyzed using electrochemical techniques. Combined with the expression of reactive oxygen species (ROS) during the release process, the antifouling mechanism of the Cu-bearing SS coatings was elucidated. This study provides new insights and solutions for fabrication and performance evaluation of metal-based antifouling coatings, and addresses the issues of weak corrosion resistance and unstable Cu ion release observed in previous work.
The longitudinal sectional microstructures of the coatings are shown in Fig. 1. As shown in Fig. 1a, the 304 L SS coating exhibited a uniform and dense microstructure with a thickness of 304.4 ± 15.3 μm and no visible defects, and formed a strong metallurgical bond with the substrate. With the addition of Cu, the coating thickness increased to 385.4 ± 2.1 μm for 304L-5Cu SS and 470.8 ± 2.3 μm for 304L-10Cu SS, which resulted from a further reduction in melting point and improved wettability due to the higher Cu content. The microstructure exhibited a small number of pores within the coating. However, excessive Cu (20 wt%) increased thermal conductivity and laser reflectivity, leading to a reduced thickness (~220.7 ± 6.8 μm), accompanied by through-thickness cracks. During the LDED process, variations in melting point, thermal expansion coefficient, and lubricity of the powder material can result in the formation of defects such as cracks and pores in the coating. Moreover, by comparing the composition difference between the powders and the coatings (Table S1 and Table 1), it could be found that the Cu content in the 304L-20Cu SS coating was significantly reduced, and the gasification of Cu led to a serious heat loss. Several through-thickness cracks were visible along the grain boundaries, compromising the structural integrity of this coating.
As shown in Fig. 1e, Cu was distributed uniformly in the 304L-5Cu SS coating. As shown in Fig. 1f, the uniformity of Cu distribution slightly decreased in the 304L-10Cu SS coating. It tended to accumulate around the periphery of the columnar crystals. As Cu content increased further, the distribution of Cu in the 304L-20Cu SS coating became highly uneven, with distinct regions of Cu aggregation (Fig. 1g). These Cu enrichment areas were associated with the observed through-thickness cracks, indicating that uneven Cu distribution resulted in significant local stress differences. The accumulation of Cu exacerbated these stresses, leading to the crack initiation and propagation. The increased oxygen content at the crack locations indicated that the cracks formed during the coating preparation process, as these cracks facilitated the oxidation reactions. Due to the severe compromise of the structural integrity of 304L-20Cu SS coating, further studies on this coating stopped.
The antibacterial performance of the coatings was evaluated by measuring the growth of P. aeruginosa colonies on the surface of each sample after an incubation period of 24 h. Figure S1 illustrates the colony growth on the surfaces of 304 L SS, 304L-5Cu SS, and 304L-10Cu SS coatings. It was evident that the number of bacterial colonies on the Cu-bearing SS coatings was significantly reduced compared to the 304L SS coating. The 304L-5Cu SS coating achieved an antibacterial rate of 99.1%, while the 304L-10Cu SS coating exhibited a higher antibacterial rate of 99.9%. The addition of Cu into the coatings greatly enhanced their antibacterial properties. The results indicated that the antibacterial performance of the coatings was improved with the increase of Cu content, which was a critical factor in inhibiting early biofilm formation on the coating surface.
Figure 2a shows the macroscopic morphologies of biofouling attachment on various coating surfaces. The 304 L SS coating exhibited significant biofouling attachment throughout the experiment, and its surface was heavily covered with brownish-yellow fouling after 1 day of co-cultivation in the dynamic antifouling evaluation system (DAES). As the co-cultivation time was prolonged to 4 and 7 days, the fouling transitioned from microalgae to filamentous algae, accompanied by a substantial increase in attachment percentage. On the 7th day, the 304 L SS coating displayed the heaviest fouling coverage among all samples. In contrast, the 304L-5Cu SS coating showed a significant improvement in biofouling resistance. After 4 days of co-cultivation, its surface did not exhibit any brownish-yellow fouling, and the filamentous algae fouling was also absent. However, some corrosion occurred on the 304L-10Cu SS coating, leading to visible color contrasts on the macroscopic surface.
As shown in Fig. 2b, c, microfouling attachment increased over time for all samples. Consistent with the macroscopic observations, the 304 L SS coating exhibited the highest biofouling attachment percentage at each time point, which rose sharply from the 4th to the 7th day, more than doubling to 14.56%, indicating that once a critical threshold was reached, the fouling process is significantly accelerated. In contrast, the 304L-5Cu SS and 304L-10Cu SS coatings showed much lower adhesion, with biofouling coverages of 6.81% and 2.46% on the 7th day, respectively, highlighting their superior antifouling capability. Notably, the 304L-10Cu SS coating maintained the lowest adhesion throughout the test, suggesting that the higher Cu content not only suppressed initial organism attachment but also damaged the few cells that adhered, thereby slowing further colonization. The 304L-5Cu SS coating showed a similar but less pronounced effect, with more biofouling than that on the 304L-10Cu SS coating, but still less than that on the 304L SS coating. These observations suggest that Cu-bearing SS coatings, particularly the 304L-10Cu SS coating, not only resisted the initial adhesion of fouling organisms but also delayed the formation of stable biofouling layers.
Figure 2e presents field emission scanning electron microscope (FE-SEM) images detailing the progression of biofouling attachment on the coatings over 7 days. The 304L SS coating consistently showed the densest biofouling layer, dominated by large filamentous algae by the 7th day, confirming its weak antifouling resistance at both macro and micro scales. Throughout the 7-day period, the 304L-10Cu SS coating maintained minimal biofouling, with extensive fragmentation observed on both the 4th day and 7th day, suggesting that its higher Cu content suppressed initial adhesion and damaged the limited number of attached organisms, thereby slowing further colonization. The 304L-5Cu SS coating displayed a similar but weaker effect, with more attachment than the 304L-10Cu SS coating but less than the 304L SS coating, accompanied by noticeable algal disruption.
Figure 2d further illustrates the relative inhibition abilities of the 304L-5Cu SS and 304L-10Cu SS coatings over time. Both coatings exhibited strong initial inhibition on day 1, with 304L-10Cu SS coatings showing a superior effect. As immersion progressed, the inhibition rate of 304L-5Cu SS coatings declined markedly, dropping to 58.3% by the 7th day, indicating a gradual loss of effectiveness as biofilm formation advanced. In contrast, 304L-10Cu SS coatings maintained a consistently high inhibition rate throughout the 7-day period, remaining at 82.1% on the 7th day with only a slight decrease from its initial value. These results suggest that while 304L-5Cu SS coatings primarily delayed early-stage attachment, 304L-10Cu SS coatings provided more sustained resistance, effectively suppressing biofouling even during the biofilm development stage.
To investigate how Cu-bearing SS coatings influence the early formation stage of biofilms, the surfaces of samples after 6 h and 24 h of co-cultivation were stained to visualize protein and polysaccharide adsorption (Fig. 3a, c, d). Quantitative analysis revealed that at both time points, the 304 L SS coating exhibited the highest levels of attachment. The percentages of polysaccharides increased from 1.84% at 6 h to 2.36% at 24 h, and those of proteins increased from 0.74% at 6 h to 1.21% at 24 h. The 304L-5Cu SS coating showed markedly lower values (0.51%/0.14% at 6 h; 0.98%/0.31% at 24 h), while the 304L-10Cu SS coating maintained the lowest adsorption (0.26%/0.18% at 6 h; 0.32%/0.21% at 24 h). Notably, the early-stage adhesion of polysaccharides was significantly higher than that of proteins for all samples, suggesting that polysaccharides play a crucial role in the initial formation of the conditioning film. The 304L-10Cu SS coating consistently showed the most significant inhibition effect, ensuring better performance in the subsequent stages of biofouling inhibition.
Furthermore, the colonization degree of the biofilm, composed of polysaccharides and proteins, can provide a strong foundation for subsequent marine biofouling formation. To assess biofilm adhesion, AFM height images and force-distance curves (Fig. 3b, e, f, g) were analyzed. The surface profiles revealed increasing topographical complexity and biofilm accumulation with prolonged immersion, indicating progressive biomass deposition on the coating surfaces. Compared with the 304L-5Cu SS and 304L-10Cu SS coatings, the biofilm formed on the 304 L SS coating appeared more continuous and exhibited greater accumulation. This observation aligned with previous results, confirming the stronger tendency of the 304 L SS coating toward the early formation stage of biofilm, which facilitated subsequent organism attachment. Statistical analysis based on force-distance measurements (Fig. S2) demonstrated significant differences in adhesion strength. After 6 h of co-cultivation, the adhesion force of the biofilm on the 304 L SS coating reached 21.4 nN, which was 4.3 times and 5.8 times that of the biofilms on the 304L-5Cu SS and 304L-10Cu SS coatings, respectively. As the cultivation time was extended to 24 h, the adhesion force of the biofilm on the surface of the 304 L SS coating increased to 36.6 nN, still significantly higher than that of the biofilms on the Cu-bearing SS coatings. These findings indicated that the addition of Cu can reduce the adhesion ability of biofilms on the surface. With the increase of Cu content, the antifouling potential of the coatings was enhanced.
After 42 days of immersion in synthetic seawater (Fig. 4a), the self-polishing copolymer (SPC) coating exhibited visible discoloration and peeling, whereas both Cu-bearing SS coatings remained intact without notable corrosion or morphological differences. As shown in Fig. 4b and Table S2, all coatings displayed relatively high initial Cu ion release rates, with the SPC coating peaking at approximately 12 μg (cm²·d), which was markedly higher than 6.10 μg (cm²·d) and 8.43 μg (cm²·d) for the 304L-5Cu SS and 304L-10Cu SS coatings at the 4th day of immersion, respectively. As the immersion time increased, the release rates of all coatings decreased, and the SPC coating consistently released more Cu ions with fluctuating levels. Specifically, the SPC coating maintained a release rate approximately 8 to 10 times higher than those of the 304L-5Cu SS and 304L-10Cu SS coatings, the latter two stabilized at less than 1 μg (cm²·d) after the initial peak. The cumulative release data (Fig. 4c) further confirmed this trend: by day 42, the SPC coating reached 330.88 μg cm², compared with 37.11 μg cm² for 304L-5Cu SS coating and 39.74 μg cm² for 304L-10Cu SS coating. These results demonstrate that the SPC coating continuously released Cu ions at a much higher rate, which may enhance short-term antifouling efficacy but risks premature depletion of active agents and greater environmental impact, while the 304L-5Cu SS and 304L-10Cu SS coatings showed a more controlled and gradual release, which was favorable for long-term antifouling performance.
To evaluate the relationship between corrosion behavior and Cu ion release, electrochemical tests were conducted in synthetic seawater. Figure 4d and Table S3 show the corrosion current density (i) of Cu-bearing SS coatings in synthetic seawater obtained via linear polarization resistance (LPR) test, which directly reflects the changes of corrosion rate. The results showed that the current density of both Cu-bearing SS coatings exhibited a rapid increase followed by a sustained decrease, reaching a peak on the 4th day of immersion and stabilizing after 14 days. This trend was consistent with the changes in the release rate of Cu ions. Figure 4e presents the charge transfer resistance (R) fitted by equivalent circuit modeling of the electrochemical impedance spectroscopy (EIS) data (Fig. S3, Table S4). The results showed that the R reached a minimum value on the 4th day of immersion and then gradually increased, consistent with the role of R in regulating the corrosion current density of the coating. Furthermore, these findings were confirmed by potentiodynamic polarization curves (Fig. 4f). Tafel fitting revealed that the E value of the 304L-10Cu SS coating shifted positively, indicating a reduced tendency for the coating to corrode. Additionally, as shown in Fig. 4g and Table S5, compared to the 304L-5Cu SS coating, the 304L-10Cu SS coating exhibited higher anodic and cathodic polarization slopes, which is consistent with the LPR test results. Moreover, an increased cathodic polarization slope indicated that the coating also limited the reduction process of oxygen in the solution, thereby providing a protective effect on the coating. Furthermore, the current density of the 304L-10Cu SS coating in the passive region was significantly lower than that of the 304L-5Cu SS coating, indicating a higher corrosion resistance of the passive film. Therefore, although the 304L-10Cu SS coating had a higher Cu content, its lower corrosion rate resulted in a slightly higher cumulative Cu ion release rate compared to the 304L-5Cu SS coating, without a significant difference between them. Additionally, as the potential was increased, the 304L-5Cu SS coating demonstrated a superior pitting corrosion resistance. This indicated that although increasing Cu content can enhance the protective ability of the passive film, according to the Pourbaix diagram of Cu-HO and Cr-HO, CuO preferentially dissolves into Cu(OH) at higher potentials, leading to the loss of protective components in the passive film. Consequently, the 304L-10Cu SS coating exhibited higher pitting sensitivity.
Figure 4h shows the ROS distribution on 304 L SS coating and Cu-bearing SS coatings after 7 days of immersion in the DAES. The results indicated that Cu-bearing SS coatings exhibited stronger fluorescence signals, which increased with Cu content, suggesting that these coatings can induce high ROS expression in algal biofilms. Figure 4i presents the quantified results of ROS production. Compared to the positive control, a weaker ROS fluorescence signal was detected on the surface of the 304 L SS coating. However, on the surface of Cu-bearing SS coating, ROS expression was obviously strong, and the fluorescence signal ratio increased with the immersion time in the DAES. Specifically, after 4 days of immersion, the fluorescence signal intensities of the 304L-5Cu SS and 304L-10Cu SS coatings increased by 2.23-fold and 2.77-fold, respectively, consistent with the significant increase in the release rate of Cu ions. Furthermore, the 304L-10Cu SS coating showed the most significant fluorescence signal ratio, indicating that the intensity of ROS fluorescence signals in the biofilm increased with the increase of the cumulative release of Cu ions.
The formation of marine biofouling involves sequential stages, including the development of a conditioning film, colonization by microbial biofilms, establishment of microfouling communities, and subsequent macrofouler attachment. The condition film, composed of adsorbed organic matter and minerals, is difficult to prevent and serves as a substrate for microbial adhesion. Bacteria and microalgae then colonize this layer, forming an irreversible biofilm through extracellular polymeric substances (EPS) secretion, which provides a favorable environment for further fouling. The Cu-bearing SS coatings markedly inhibited bacterial activity within 24 h (Fig. S1), leading to disruption of bacterial biofilm formation and elimination of the substrate required for algal colonization. Moreover, microalgae can also adhere directly to the conditioning film by secreting EPS; however, analyses of biofilms formed in the DAES demonstrated that Cu-bearing coatings significantly reduced algal protein and polysaccharide secretion, thereby decreasing EPS coverage and hindering stable colonization, with the inhibitory effect strengthening as Cu content increased. AFM measurements (Fig. 3) further revealed that the viscoelastic properties of biofilms on Cu-bearing surfaces corresponded to delayed development stages, confirming that Cu addition interfered with EPS secretion and structural integrity, preventing the formation of a mature matrix. Further observations of the algal morphology on the coating surfaces after 7 days of co-cultivation (Fig. 5) focused on the same algal species co-existing on different coating surfaces, including centric diatoms, pennate diatoms, ceratium furca, and pinnularia. The algal cells on the 304 L SS coating surface maintained their complete and full morphology. In contrast, the algal cells on the Cu-bearing SS coating surface exhibited clear signs of damage, which intensified with increasing the Cu content, progressing from membrane lysis to complete fragmentation. This phenomenon explains the significant reduction in biofouling observed in the macroscopic examinations (Fig. 2a).
The cumulative release of Cu ions is positively correlated with the inhibition rate of algal coverage area (Fig. 2b and e). The release of Cu ions is a key factor leading to the damage of algal cells. On one hand, Cu ions can interfere with photosynthesis and disrupt the energy metabolism of algae, significantly reducing cellular activity. Additionally, Cu ions directly affect the functionality of photosystem II (PSII) in marine biofouling algae. Specifically, Cu ions disrupt the MnCaO cluster, a critical component of the oxygen-evolving complex (OEC) in PSII. This cluster is essential for water splitting and oxygen evolution during photosynthesis, and its destabilization by Cu ions leads to impaired electron transfer and reduced photosynthetic efficiency. The interaction can be represented by the following reaction (Formula 1):
On the other hand, the oxidation of Cu ions, which results in the loss of electrons, can react with oxygen in the environment to form superoxide anions (Formula 2):
These superoxide anions can further react with protons during algal respiration to generate hydrogen peroxide (Formula 3):
While hydrogen peroxide is a relatively stable ROS and does not directly damage cells, the Cu ions released from the coating can reduce the hydrogen peroxide to generate highly reactive hydroxyl radicals via the Fenton reaction (Formula 4 and 5):
Hydroxyl radicals, with their high oxidative capability, can cause extensive damage to the lipid membrane of algal cells, proteins, and DNA, ultimately leading to the lysis and death of cells. The continuous formation and accumulation of ROS create an adverse microenvironment on the coating surface, inhibiting the stable adhesion and growth of algae (Fig. 4h, i). This explains why, despite showing a lower Cu ions release rate compared to SPC coatings, the Cu-bearing SS coating still demonstrated a significant antifouling performance.
The dynamic release process of Cu ions from Cu-bearing SS coatings is influenced by the evolution of their uniform corrosion resistance in the service environment. Initially, the 304L-10Cu SS coating, which has a higher Cu content, exhibits lower i (Fig. 4d). According to the standard electrode potentials of Cr, Fe, and Cu in the formation of oxides, the Cu in the Cu-bearing SS coating typically undergoes significant oxidation reactions after Cr and Fe have formed a passive film, generating CuO, which coexists with Fe oxides in the outer layer of the passive film, providing protection to the substrate. Subsequently, the oxides of Cr, Fe, and Cu in the passive film undergo ion exchange reactions in aqueous solutions (Formula 6, 7 and 8), the nomenclature is shown in the supplementary material Note 1:
Compared to the highly protective Cr₂O₃, the CuO in the passive film has a stronger reaction driving force and can react more rapidly in solution. Consequently, the 304L-10Cu SS coating showed a significant decrease of R values on the 4th and 14th days of immersion. The dissolution of the passive film is accompanied by a repair process, where Cr, Fe, and Cu ions formed during dissolution react with chloride and hydroxide ions in the solution to re-form the metal oxides in the passive film. Therefore, the addition of Cu caused the CuO layer on the surface to react more easily, leading to fluctuations in corrosion resistance. However, compared to the 304L-5Cu SS coating, the 304L-10Cu SS coating exhibited higher R value, making it more difficult for charge transfer during corrosion reactions and thus slowed down the corrosion rate, as consistent with the polarization curve results. Figure 6 schematically illustrates the biofouling process on conventional SS coatings and the antifouling mechanism of Cu-bearing SS coatings. As a result, Cu-bearing SS coatings offer significant advantages by extending the service life, reducing the maintenance costs, and minimizing the environmental impact through optimized Cu content and self-repair mechanisms.