**3. Discussion**

orientation perpendicular to the erosion interface decreases sharply at first and then gradually declines to a stable level. However, the erosion-corrosion rate of the parallel sample almost linearly decreases, and it maintains the higher erosion rate after the erosion steps into the steady stage (**Figure 3a**). Compared to two erosion patterns (e.g., vertical or parallel sample), it is revealed that there exists an erosion initiation effect or incubation period of the interface ζ formation for the DS Fe-B alloy in flowing zinc (**Figure 3b**). Obviously, the adhesive film with interfacial pinning effect in the vertical sample at initial erosion-corrosion stage does not form. However, the erosion-corrosion rates decrease with the increase of erosion time, for example,

**Figure 3.** Erosion rates of DS Fe-B alloy with different Fe2B lamellar spacing in flowing zinc: (a) erosion-corrosion rate

It indicates that the interface structure may undergo continuous changes owing to the combined effect of the oriented Fe2B and epitaxial grown ζ-FeZn13 products under flow erosion condition. Clearly, epitaxial grown ζ-FeZn13 products at the interface demonstrate an accumulation/pile-up effect and synergistically generate a buffer layer with oriented Fe2B to resist the flowing zinc erosion with the prolonged erosion time. It therefore means that the erosion-corrosion interface structure is dominant by flow-accelerated diffusion of liquid zinc and ζ product accumulation at the interface during the prolonged erosion-corrosion process.

**Figure 4** shows the erosion-corrosion interface of DS Fe-B alloy in flowing zinc with different erosion times as a function of oriented Fe2B lamellar spacing. It is clear that small and large Fe2B spacing in vertical sample in flowing zinc can be damaged in the form of both Fe2B dissolution and numerous spallation at the front of the erosion-corrosion interface for 5 h (**Figure 4a**). Evidently, only suitable lamellar size of oriented Fe2B can resist the erosion corrosion (e.g., λFe2B = 3.67 μm) at the initial erosion stage. However, after erosion for 30 h at a steady erosion stage (**Figure 4b**), there exists an adhesive film with the interfacial pinning effect in DS Fe-B alloy with Fe2B lamellar spacing λFe2B = 3.67 μm. An adhesive product film comprising the oriented Fe2B and epitaxial grown columnar ζ-FeZn13 builds up at the erosion-corrosion interface as a buffer layer to resist flowing liquid zinc damage. Smaller size

**2.3. Interface morphological evolution during flowing zinc erosion corrosion**

erosion from 5 to10 h (**Figure 3b**).

126 Cavitation - Selected Issues

vs. erosion time; (b) erosion-corrosion rate vs. Fe2B spacing.

#### **3.1. EBSD analysis on microstructure and erosion-corrosion interface**

**Figure 5** shows the EBSD analysis of DS Fe-B alloy before and after erosion in flowing zinc. Clearly, the DS Fe-B alloy is mainly composed of α-Fe and Fe2B to form dual-phase textured microstructure (**Figure 5a**). The {002} poles of oriented Fe2B grains are located at Y-axis in forms of lightest spot area, while {100}, {110}, and {111} poles of Fe2B grains distribute within quadrants, which indicates that Fe2B [002] orientation dominates its preferred growth direction, as recorded as the strong peak (002) plane in XRD of the transverse section (**Figure 2c**). Furthermore, the {110} poles of α-Fe grains display concentrated spot area in the vertical axis, which may reveal that α-Fe grains generate an orientation growth in the [110] direction.

**Figure 5c** and **d** show the interfacial orientation map and grain boundary distribution collected by EBSD in a well-distributed directional area of Fe-B alloy in flowing zinc. The

than 200. In the present rotating disk, the bulk flowing pattern should be regarded as turbulent flow owing to the estimated Re value (here Re = 8066). Therefore, there should be a violent and sustaining fluid force of flowing zinc, which can strongly impinge on the erosion interface to break the Fe2B skeleton (**Figure 4**). The damaged erosion interface indicates that hydraulic effects from fluid scouring force and momentum transfer may strongly destroy the surface film and sweep ζ products, thus accelerating the further localized corrosion [15, 41–44].

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**Figure 6** shows the occurrence of cracks along the α-Fe/Fe2B boundary and transgranular cracking of Fe2B. Obviously, no corrosion products generate along the α-Fe/Fe2B phase boundary, and only initiation and propagation of cracking occurs ahead of the erosioncorrosion interface in some weak sites [25–28, 42–46]. The separation of α-Fe/Fe2B phase boundary indicates that high zinc potential and penetration under capillary action can induce the weakening of phase boundary [13, 21, 25–28, 40]. Actually, the high zinc potential at the interface may induce the reduction of the cohesion strength along the weak site of the phase boundary [21, 40]. Essentially, a highly concentrated stress zone ahead of the erosioncorrosion interface can produce because of the growth of products. Thus, the combined effect from scouring force of flowing zinc and high zinc potential can stimulate this stressed zone to initiate cracks without products at the interface owing to the chemical and mechanical effect

From **Figure 6b**, it reveals that the Fe2B (i.e., nonwetting with liquid zinc) is directly prone to cracking through transgranular pattern with a main crack plus some network of microcracks to release the high zinc potential energy. Obviously, the location of cracking initiation occurs in the reduced bonding site at the front of the erosion interface, and there is almost no products existing on the cracking interface (**Figure 6b**). Therefore, once the cracks take place under the hydrodynamic effect and high potential of flowing liquid zinc, the strong capillarity and

**Figure 6.** Cracking along α-Fe/Fe2B boundary and transgranular cracking of Fe2B in flowing zinc: (a) phase boundary

**3.2. Erosion-corrosion synergistic effects on interfacial morphologies**

(**Figure 6a**) [15, 21, 25–29, 40, 42].

cracking; (b) transgranular cracking of Fe2B.

**Figure 5.** EBSD analysis of DS Fe-B alloy before and after erosion: (a) EBSD image before erosion; (b) pole figures of Fe2B and α-Fe; (c) erosion interfacial Euler angle orientation; (d) grain boundary distribution (black lines indicates misorientations larger than or equal to 10°, green lines indicate grain boundaries with smaller misorientations less than 10°, and red lines indicate phase boundaries).

different colored areas of α-Fe and Fe2B indicate that Fe2B can strongly resist the flowing zinc erosion (**Figure 5c**). However, α-Fe grains in [110] orientation are maintained adjacent to the Fe2B [21], which probably reveals it possesses better corrosion resistance to flowing zinc. Moreover, no selective and preferential erosion-corrosion path occurs on α-Fe/Fe2B phase boundary, and α-Fe still displays uniform dissolution in flowing zinc (**Figure 5d**). That means the phase boundary in DS Fe-B alloy does not demonstrate obvious corrosion sensitivity in liquid zinc. Obviously, the erosion corrosion of DS Fe-B alloy in flowing zinc largely relies on the adhesive interface structure controlled by Fe2B lamellar spacing and dense pile-up effect of epitaxial grown ζ-FeZn13 at the interface.

Nevertheless, such an interface structure can be forcefully affected by the flow pattern, which is based on the dimensionless parameters, for example, Reynolds number (Re), Sherwood number (Sh), and Schmidt number (Sc) [13, 15]. According to the Refs. [6, 15, 41–43], the fluid flow under rotating condition should be turbulent flow if the Reynolds number Re is more than 200. In the present rotating disk, the bulk flowing pattern should be regarded as turbulent flow owing to the estimated Re value (here Re = 8066). Therefore, there should be a violent and sustaining fluid force of flowing zinc, which can strongly impinge on the erosion interface to break the Fe2B skeleton (**Figure 4**). The damaged erosion interface indicates that hydraulic effects from fluid scouring force and momentum transfer may strongly destroy the surface film and sweep ζ products, thus accelerating the further localized corrosion [15, 41–44].

#### **3.2. Erosion-corrosion synergistic effects on interfacial morphologies**

**Figure 6** shows the occurrence of cracks along the α-Fe/Fe2B boundary and transgranular cracking of Fe2B. Obviously, no corrosion products generate along the α-Fe/Fe2B phase boundary, and only initiation and propagation of cracking occurs ahead of the erosioncorrosion interface in some weak sites [25–28, 42–46]. The separation of α-Fe/Fe2B phase boundary indicates that high zinc potential and penetration under capillary action can induce the weakening of phase boundary [13, 21, 25–28, 40]. Actually, the high zinc potential at the interface may induce the reduction of the cohesion strength along the weak site of the phase boundary [21, 40]. Essentially, a highly concentrated stress zone ahead of the erosioncorrosion interface can produce because of the growth of products. Thus, the combined effect from scouring force of flowing zinc and high zinc potential can stimulate this stressed zone to initiate cracks without products at the interface owing to the chemical and mechanical effect (**Figure 6a**) [15, 21, 25–29, 40, 42].

From **Figure 6b**, it reveals that the Fe2B (i.e., nonwetting with liquid zinc) is directly prone to cracking through transgranular pattern with a main crack plus some network of microcracks to release the high zinc potential energy. Obviously, the location of cracking initiation occurs in the reduced bonding site at the front of the erosion interface, and there is almost no products existing on the cracking interface (**Figure 6b**). Therefore, once the cracks take place under the hydrodynamic effect and high potential of flowing liquid zinc, the strong capillarity and

different colored areas of α-Fe and Fe2B indicate that Fe2B can strongly resist the flowing zinc erosion (**Figure 5c**). However, α-Fe grains in [110] orientation are maintained adjacent to the Fe2B [21], which probably reveals it possesses better corrosion resistance to flowing zinc. Moreover, no selective and preferential erosion-corrosion path occurs on α-Fe/Fe2B phase boundary, and α-Fe still displays uniform dissolution in flowing zinc (**Figure 5d**). That means the phase boundary in DS Fe-B alloy does not demonstrate obvious corrosion sensitivity in liquid zinc. Obviously, the erosion corrosion of DS Fe-B alloy in flowing zinc largely relies on the adhesive interface structure controlled by Fe2B lamellar spacing and dense pile-up effect

**Figure 5.** EBSD analysis of DS Fe-B alloy before and after erosion: (a) EBSD image before erosion; (b) pole figures of Fe2B and α-Fe; (c) erosion interfacial Euler angle orientation; (d) grain boundary distribution (black lines indicates misorientations larger than or equal to 10°, green lines indicate grain boundaries with smaller misorientations less than

Nevertheless, such an interface structure can be forcefully affected by the flow pattern, which is based on the dimensionless parameters, for example, Reynolds number (Re), Sherwood number (Sh), and Schmidt number (Sc) [13, 15]. According to the Refs. [6, 15, 41–43], the fluid flow under rotating condition should be turbulent flow if the Reynolds number Re is more

of epitaxial grown ζ-FeZn13 at the interface.

10°, and red lines indicate phase boundaries).

128 Cavitation - Selected Issues

**Figure 6.** Cracking along α-Fe/Fe2B boundary and transgranular cracking of Fe2B in flowing zinc: (a) phase boundary cracking; (b) transgranular cracking of Fe2B.

penetration of liquid zinc along these defects as attacking channel occur, thus leading to large cracking spreading and spalling of Fe2B as well as film breakdown of layers [14–17, 41–46].

grains and penetrates into the inner grains far away from the erosion-corrosion interface. Surrounding these penetration areas, a small amount of corrosion product generates, and more cracking initiation and propagation occur at the edge of the corrosion areas (**Figure 7d**). It is revealed that the cracking produces at the phase boundary without any products, which further indicates that a film stress and decohesion at phase boundary occur since the segregation energy under high zinc potential [45, 46]. In addition, numerous plastic slip bands and spalling of Fe2B happen on the surface of the products (ζ-FeZn13), and larger cracks appear ahead of the Fe2B rupture fronts, which is attributed to the stress concentration in the flowing

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**Figure 8** shows the flowing zinc scouring effect on the damage of the erosion-corrosion layers. It is clear that the flowing zinc can scour and sweep the interface adhesive film, peeling off the interface (**Figure 8a**). The overall fracture and rupture of eroded Fe2B skeleton indicate that the flow force of liquid zinc can pull apart and drag films from the interface. Besides, typical erosion-corrosion pits and plastic slips as well as plenty of cracking spalling Fe2B around the erosion interface can coexist. That is to say, an interface adhesive film withstands severe liquid zinc erosion corrosion, which also aggravates a phase transition layer of Fe2B skeleton (**Figure 8a**) [21, 40]. Obviously, three zones, that is, I-zone with uncorroded Fe2B, II-zone with the corroded Fe2B (namely, transition phase in dark gray color in **Figure 8a** and **b**), and IIIzone with columnar ζ-FeZn13 products [40], are included. Obviously, a strong combined effect between erosion and corrosion by flowing zinc concurrently happens to damage the interface film. The repaired film behind the spalling blocks (**Figure 8b**) implies that a synergistic role of flow and corrosion can accelerate the zinc diffusion reaction and product accumulation to rebuild the broken films. Actually, strong local turbulent flow can assist the occurrence of corrosion, thus leading to the flow-induced localized corrosion (FILC) [38, 39, 41–46]. From **Figure 8c**, it is clear that numerous teardrop- and horseshoe-shaped continuous erosion pits (i.e., micromechanical pits by flow) intensively appear on the surface of ζ products. Obviously, each erosion pit has a sharp inverted triangle at the bottom of the sunken pits, and they can arrange linearly and regularly along the plastic slip line. Besides, some big pits emerge accompanying with some slips in it from the combination of small pits. The aggregation cracking

**Figure 8.** Effects of flowing zinc scouring on the damage of erosion layers: (a) and (b) rupture and phase transition of

Fe2B; (c) pits and cracking along the slip as well as the aggregation of small erosion pits.

circumstance owing to the shear force of local liquid zinc turbulence [41–44].

**3.3. Flow-induced corrosion cracking and pits in erosion-corrosion layers**

**Figure 7** shows the changes of the interface and layers of DS Fe-B alloy under the flowing zinc erosion corrosion. It is clear that the plenty of columnar Fe2B produces cracking and fracture at the erosion interface, which results from the local stress concentrations produced by high zinc potential, fluid force effect, and a small quantity of growth stress of products. The localized corrosion accelerated by the flowing erosion and corrosion cracking should be responsible for the overall damage of the interfacial films [43–46]. The slip bands of products and cracks of Fe2B in the layers reveal the powerful combined effects of chemical and mechanical damage on the films (**Figure 7a**). In nature, local corrosion and flow regime may fully induce and stimulate to emit the dislocation motion and crack initiation in such viscous flowing media [26–29, 45, 46]. **Figure 7b** and **c** show the corrosion of matrix at the front of the erosion-corrosion interface. Evidently, some spalling debris of matrix separated from the substrate can scatter among the corrosion products (**Figure 7b**). Meanwhile, the interfacial front of the erosion reveals a thin loose and porous structure of ferritic layer (**Figure 7c**), which can collapse into small broken pieces and debris before its corrosion owing to the flow effect. Essentially, a deformed sublayer with higher stress concentration at the erosion-corrosion interface may generate under local turbulent flow, which makes the sublayer of ferrite matrix porous to facilitate the further corrosion [41, 42]. Besides, the flowing role can enhance the penetration of liquid zinc into the substrate. **Figure 7d** shows a classic penetration of liquid zinc through the grain boundary. It reveals that the flowing zinc runs through several

**Figure 7.** Interface changes of DS Fe-B alloy (λFe2B = 3.67 μm): (a) microcracks; (b) and (c) matrix zone with porous structures; and (d) penetration, surrounding cracking, and slip bands of ζ products.

grains and penetrates into the inner grains far away from the erosion-corrosion interface. Surrounding these penetration areas, a small amount of corrosion product generates, and more cracking initiation and propagation occur at the edge of the corrosion areas (**Figure 7d**). It is revealed that the cracking produces at the phase boundary without any products, which further indicates that a film stress and decohesion at phase boundary occur since the segregation energy under high zinc potential [45, 46]. In addition, numerous plastic slip bands and spalling of Fe2B happen on the surface of the products (ζ-FeZn13), and larger cracks appear ahead of the Fe2B rupture fronts, which is attributed to the stress concentration in the flowing circumstance owing to the shear force of local liquid zinc turbulence [41–44].

#### **3.3. Flow-induced corrosion cracking and pits in erosion-corrosion layers**

penetration of liquid zinc along these defects as attacking channel occur, thus leading to large cracking spreading and spalling of Fe2B as well as film breakdown of layers [14–17, 41–46].

130 Cavitation - Selected Issues

**Figure 7** shows the changes of the interface and layers of DS Fe-B alloy under the flowing zinc erosion corrosion. It is clear that the plenty of columnar Fe2B produces cracking and fracture at the erosion interface, which results from the local stress concentrations produced by high zinc potential, fluid force effect, and a small quantity of growth stress of products. The localized corrosion accelerated by the flowing erosion and corrosion cracking should be responsible for the overall damage of the interfacial films [43–46]. The slip bands of products and cracks of Fe2B in the layers reveal the powerful combined effects of chemical and mechanical damage on the films (**Figure 7a**). In nature, local corrosion and flow regime may fully induce and stimulate to emit the dislocation motion and crack initiation in such viscous flowing media [26–29, 45, 46]. **Figure 7b** and **c** show the corrosion of matrix at the front of the erosion-corrosion interface. Evidently, some spalling debris of matrix separated from the substrate can scatter among the corrosion products (**Figure 7b**). Meanwhile, the interfacial front of the erosion reveals a thin loose and porous structure of ferritic layer (**Figure 7c**), which can collapse into small broken pieces and debris before its corrosion owing to the flow effect. Essentially, a deformed sublayer with higher stress concentration at the erosion-corrosion interface may generate under local turbulent flow, which makes the sublayer of ferrite matrix porous to facilitate the further corrosion [41, 42]. Besides, the flowing role can enhance the penetration of liquid zinc into the substrate. **Figure 7d** shows a classic penetration of liquid zinc through the grain boundary. It reveals that the flowing zinc runs through several

**Figure 7.** Interface changes of DS Fe-B alloy (λFe2B = 3.67 μm): (a) microcracks; (b) and (c) matrix zone with porous

structures; and (d) penetration, surrounding cracking, and slip bands of ζ products.

**Figure 8** shows the flowing zinc scouring effect on the damage of the erosion-corrosion layers. It is clear that the flowing zinc can scour and sweep the interface adhesive film, peeling off the interface (**Figure 8a**). The overall fracture and rupture of eroded Fe2B skeleton indicate that the flow force of liquid zinc can pull apart and drag films from the interface. Besides, typical erosion-corrosion pits and plastic slips as well as plenty of cracking spalling Fe2B around the erosion interface can coexist. That is to say, an interface adhesive film withstands severe liquid zinc erosion corrosion, which also aggravates a phase transition layer of Fe2B skeleton (**Figure 8a**) [21, 40]. Obviously, three zones, that is, I-zone with uncorroded Fe2B, II-zone with the corroded Fe2B (namely, transition phase in dark gray color in **Figure 8a** and **b**), and IIIzone with columnar ζ-FeZn13 products [40], are included. Obviously, a strong combined effect between erosion and corrosion by flowing zinc concurrently happens to damage the interface film. The repaired film behind the spalling blocks (**Figure 8b**) implies that a synergistic role of flow and corrosion can accelerate the zinc diffusion reaction and product accumulation to rebuild the broken films. Actually, strong local turbulent flow can assist the occurrence of corrosion, thus leading to the flow-induced localized corrosion (FILC) [38, 39, 41–46]. From **Figure 8c**, it is clear that numerous teardrop- and horseshoe-shaped continuous erosion pits (i.e., micromechanical pits by flow) intensively appear on the surface of ζ products. Obviously, each erosion pit has a sharp inverted triangle at the bottom of the sunken pits, and they can arrange linearly and regularly along the plastic slip line. Besides, some big pits emerge accompanying with some slips in it from the combination of small pits. The aggregation cracking

**Figure 8.** Effects of flowing zinc scouring on the damage of erosion layers: (a) and (b) rupture and phase transition of Fe2B; (c) pits and cracking along the slip as well as the aggregation of small erosion pits.

of small erosion pits along slip indicates that large stress concentration occurs along the slip direction [28, 45]. Meanwhile, a small tiny crevice surrounded by some cracks generates at the bottom of the larger erosion pits, which actually reveals that each pit may incubate and initiate microcracks at the bottom of the erosion pits related to the flow-induced stress cracking along slip bands. Evidently, the erosion damage is much larger than that of corrosion, and flow-assisted localized corrosion can enhance. Besides, the dislocation motion may emit to induce some deformations, and cracks initiate in the layers, which is responsible for the stress concentration based on the flow force and film stress [45, 46]. Therefore, a fierce synergistic effect of erosion and corrosion in flowing zinc occurs at the erosion interface.

turbulent intensity, but also relies on thickness of boride (i.e., lamellar spacing) to sustain flow scouring. Therefore, the local turbulence and induced pitting corrosion can strongly determine the damage of the interface films and mass transfer owing to the size effect of Fe2B

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**Figure 10** shows the EBSD erosion-corrosion morphology and the corresponding electron blackscattered pattern (EBSP) in interface layers. It is clear that the boride lateral displays an obvious cavernous sculpture or fish-scale shaped craters, which reveals that the erosion corrosion of Fe2B occurs gradually (**Figure 10a**). At the Fe2B/FeZn13 interface, numerous erosion pits and some slip steps as well as cracks ahead of the slips indicate that the laterals of Fe2B (e.g., (110) prism) are prone to more eroded than that of the basal plane (e.g., (001) plane), which may be related to

Theoretically, the breakdown and subsequent repair of the protective films depend on the localized turbulence at the erosion-corrosion interface. That means flow-induced localized corrosion (FILC) may emerge in turbulent zone during the erosion process [43–46]. Essentially, there exist velocity and concentration boundary layers determined by the rate-controlled step

**Figure 11** depicts a schematic representation of interface damage and flow-accelerated corrosion (FAC) of the DS alloy in flowing zinc with the increase of fluid velocity or shear intensity. It is revealed that when an adhesive interfacial pinning film (i.e., oriented Fe2B plus epitaxial grown ζ-FeZn13) exists at the erosion-corrosion interface, a tiny fluctuation of fluid velocity may result in the occurrence of microturbulence among the orientation grains or gaps of product films. Once the liquid zinc velocity reaches the threshold level, the interface film will destroy, and then the thinning and breakaway of the films may occur [6, 29]. Actually, local microturbulence is likely to generate in the form of fluid eddies or reversed flow owing to the presence of the obstacles. That means a steady erosion-corrosion stage will be suppressed (i.e., stage-I in **Figure 11**). In this situation, the film will be constantly thin and attenuate fast, thus leading to the increase of the erosion-corrosion rate. Besides, the penetration of

**Figure 10.** EBSD erosion-corrosion morphology and electron black-scattered pattern (EBSP) of Fe2B in erosion layers: (a)

the zinc-atom penetration potential and corroded anisotropy of Fe2B (**Figure 10b**) [40].

**3.5. Erosion-corrosion mechanism determined by interface structure and flow**

during the erosion-corrosion process [41, 42, 47–49].

morphology at the interface; (b) EBSP of Fe2B.

lamellae.

#### **3.4. Effect of Fe<sup>2</sup> B lamellar spacing on the interface structure in flowing zinc**

**Figure 9** shows the bending deformation and corrosion pitting (i.e., chemical pits from pitting corrosion) of Fe2B in DS Fe-B alloy with different lamellar size in flowing zinc. From **Figure 9a**, it reveals the micromechanical effect on the bending deformation and fracture of smaller borides at the front of the erosion interface. Clearly, a lot of bending deformation of borides occurs at the Fe2B/ζ erosion interface, and little corrosion products generate among the oriented Fe2B (e.g., λFe2B = 1.87 μm). Instead, numerous small gaps among Fe2B facilitate the occurrence of the liquid zinc flowing scouring effect in columnar grains. The cracking and bending deformation at the erosion interface along the flow direction can fully indicate that attacking shear force strongly impinges on the interface to destroy the corrosion-resistant phase and film microstructure (**Figure 9a**). Actually, the large turbulent flow (e.g., occurrence of fluid whirlpool when encountering obstacles) can induce the cracking initiation of Fe2B and result in its rupture. However, no obvious deformation of borides with moderate lamellar spacing (i.e., λFe2B = 3.67μm) takes place, and only numerous pitting corrosions (e.g., smallpox petechial or freckles of pitting corrosion) triggers on the Fe2B surface (**Figure 9b**). It also infers that large absorption under high potential leads to decohesion firstly, and then chemical reaction occurs between free atoms or ions to produce Fe2B pitting corrosion stimulated by flow (e.g., free iron and boron atoms releasing from Fe2B lattice to complete Fe2B/ FeB phase transition through atomic configuration) [14, 40]. The visible cracking and pitting of borides at the erosion interface indicate that the erosion damage not only depends on local

**Figure 9.** Bending deformation and pitting corrosion of Fe2B in DS Fe-B alloy with different lamellar sizes in flowing zinc: (a) deformation of Fe2B at erosion interface front (λFe2B = 1.87 μm); (b) pitting corrosion of Fe2B (λFe2B = 3.67 μm); and (c) cracking and fracture of Fe2B in layers (λFe2B = 5.94 μm).

turbulent intensity, but also relies on thickness of boride (i.e., lamellar spacing) to sustain flow scouring. Therefore, the local turbulence and induced pitting corrosion can strongly determine the damage of the interface films and mass transfer owing to the size effect of Fe2B lamellae.

**Figure 10** shows the EBSD erosion-corrosion morphology and the corresponding electron blackscattered pattern (EBSP) in interface layers. It is clear that the boride lateral displays an obvious cavernous sculpture or fish-scale shaped craters, which reveals that the erosion corrosion of Fe2B occurs gradually (**Figure 10a**). At the Fe2B/FeZn13 interface, numerous erosion pits and some slip steps as well as cracks ahead of the slips indicate that the laterals of Fe2B (e.g., (110) prism) are prone to more eroded than that of the basal plane (e.g., (001) plane), which may be related to the zinc-atom penetration potential and corroded anisotropy of Fe2B (**Figure 10b**) [40].

#### **3.5. Erosion-corrosion mechanism determined by interface structure and flow**

Theoretically, the breakdown and subsequent repair of the protective films depend on the localized turbulence at the erosion-corrosion interface. That means flow-induced localized corrosion (FILC) may emerge in turbulent zone during the erosion process [43–46]. Essentially, there exist velocity and concentration boundary layers determined by the rate-controlled step during the erosion-corrosion process [41, 42, 47–49].

**Figure 11** depicts a schematic representation of interface damage and flow-accelerated corrosion (FAC) of the DS alloy in flowing zinc with the increase of fluid velocity or shear intensity. It is revealed that when an adhesive interfacial pinning film (i.e., oriented Fe2B plus epitaxial grown ζ-FeZn13) exists at the erosion-corrosion interface, a tiny fluctuation of fluid velocity may result in the occurrence of microturbulence among the orientation grains or gaps of product films. Once the liquid zinc velocity reaches the threshold level, the interface film will destroy, and then the thinning and breakaway of the films may occur [6, 29]. Actually, local microturbulence is likely to generate in the form of fluid eddies or reversed flow owing to the presence of the obstacles. That means a steady erosion-corrosion stage will be suppressed (i.e., stage-I in **Figure 11**). In this situation, the film will be constantly thin and attenuate fast, thus leading to the increase of the erosion-corrosion rate. Besides, the penetration of

**Figure 10.** EBSD erosion-corrosion morphology and electron black-scattered pattern (EBSP) of Fe2B in erosion layers: (a) morphology at the interface; (b) EBSP of Fe2B.

**Figure 9.** Bending deformation and pitting corrosion of Fe2B in DS Fe-B alloy with different lamellar sizes in flowing zinc: (a) deformation of Fe2B at erosion interface front (λFe2B = 1.87 μm); (b) pitting corrosion of Fe2B (λFe2B = 3.67 μm);

of small erosion pits along slip indicates that large stress concentration occurs along the slip direction [28, 45]. Meanwhile, a small tiny crevice surrounded by some cracks generates at the bottom of the larger erosion pits, which actually reveals that each pit may incubate and initiate microcracks at the bottom of the erosion pits related to the flow-induced stress cracking along slip bands. Evidently, the erosion damage is much larger than that of corrosion, and flow-assisted localized corrosion can enhance. Besides, the dislocation motion may emit to induce some deformations, and cracks initiate in the layers, which is responsible for the stress concentration based on the flow force and film stress [45, 46]. Therefore, a fierce synergistic

**B lamellar spacing on the interface structure in flowing zinc**

**Figure 9** shows the bending deformation and corrosion pitting (i.e., chemical pits from pitting corrosion) of Fe2B in DS Fe-B alloy with different lamellar size in flowing zinc. From **Figure 9a**, it reveals the micromechanical effect on the bending deformation and fracture of smaller borides at the front of the erosion interface. Clearly, a lot of bending deformation of borides occurs at the Fe2B/ζ erosion interface, and little corrosion products generate among the oriented Fe2B (e.g., λFe2B = 1.87 μm). Instead, numerous small gaps among Fe2B facilitate the occurrence of the liquid zinc flowing scouring effect in columnar grains. The cracking and bending deformation at the erosion interface along the flow direction can fully indicate that attacking shear force strongly impinges on the interface to destroy the corrosion-resistant phase and film microstructure (**Figure 9a**). Actually, the large turbulent flow (e.g., occurrence of fluid whirlpool when encountering obstacles) can induce the cracking initiation of Fe2B and result in its rupture. However, no obvious deformation of borides with moderate lamellar spacing (i.e., λFe2B = 3.67μm) takes place, and only numerous pitting corrosions (e.g., smallpox petechial or freckles of pitting corrosion) triggers on the Fe2B surface (**Figure 9b**). It also infers that large absorption under high potential leads to decohesion firstly, and then chemical reaction occurs between free atoms or ions to produce Fe2B pitting corrosion stimulated by flow (e.g., free iron and boron atoms releasing from Fe2B lattice to complete Fe2B/ FeB phase transition through atomic configuration) [14, 40]. The visible cracking and pitting of borides at the erosion interface indicate that the erosion damage not only depends on local

effect of erosion and corrosion in flowing zinc occurs at the erosion interface.

**3.4. Effect of Fe<sup>2</sup>**

132 Cavitation - Selected Issues

and (c) cracking and fracture of Fe2B in layers (λFe2B = 5.94 μm).

**3.** Local turbulence of liquid zinc can cause the formation of slip bands and erosion-corrosion pits on the surface of ζ-FeZn13, subsequently leading to the aggregation and crack initia-

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**4.** The erosion-corrosion mechanism dominates the combined effects of breakdown of films, the rupture of Fe2B, and flow-accelerated corrosion, which depends on the Fe2B lamellar

The authors thank the financial support for this work from the Natural Science Foundation of China (No. 51771143, 51301128 & 51475005), and also appreciate the Open Fund of National Joint Engineering Research Center for abrasion control and molding of metal materials (Grant

The entirety of this text with the 'conflict of interest' declaration is indicated.

, Hanguang Fu2

and Engineering, Xi'an Jiaotong University, Xi'an, Shaanxi Province, P.R. China

Science and Engineering, Beijing University of Technology, Beijing, P.R. China

Materials, Henan University of Science and Technology, Luoyang, P.R. China

[1] Fernandes PJL, Jones DRH. International Materials Reviews. 1997;**42**:251-261 [2] Ina K, Koizumi H. Materials Science and Engineering A. 2004;**387-389**:390-394 [3] Zhang J, Hosemann P, Maloy S. Journal of Nuclear Materials. 2010;**404**:82-96

[4] Suzuki T, Ohno K, Masuda S, Nakanishi Y, Matsui Y. Journal of Nuclear Materials.

\*Address all correspondence to: sqma@mail.xjtu.edu.cn and shengqiang012@163.com

1 State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science

2 Research Institute of Advanced Materials Processing Technology, School of Materials

3 National Joint Engineering Research Center for Abrasion Control and Molding of Metal

and Shizhong Wei<sup>3</sup>

tion of erosion pits along slips.

No. HKDNM201801) for this work.

\*, Jiandong Xing1

**Acknowledgements**

**Conflict of interest**

**Author details**

Shengqiang Ma1

**References**

1987;**148**:230-234

spacing controlled interface structures and morphology.

**Figure 11.** Schematic erosion-corrosion mechanism and interface damage under flow-accelerated corrosion (FAC) of the DS alloy in flowing zinc.

liquid zinc in boundaries and stressed zones ahead of the products may strongly induce interatomic decohesion and segregation [14, 26–28]. The flow-induced localized corrosion on the bare and uncovered matrix as well as some microcracks generating at weak cohesion can burst out, which in turn roughens the interface (e.g., the stage-II in **Figure 11**) [6, 15, 29, 41–44]. Meanwhile, the corrosion products are swept away endlessly by strong flowing zinc in order to reject their deposition and accumulation at the interface. Therefore, a strong synergistic effect of microturbulence and FILC can generate, which depends on the interface film structure and morphology. Accordingly, the present work reveals the importance of interface morphology and effect of Fe2B size on interfacial film damage.

### **4. Conclusions**

This work reveals the interface structure and film damage of DS Fe-B alloy with various Fe2B lamellar spacing in flowing zinc as well as the relationship between the interfacial morphology and local flow. The main conclusions are as follows:

