# Effect of Laser Surface Treatment on the Corrosion Resistance of Zircaloy-4 at High Temperature
## 1 Introduction  
After the Fukushima nuclear power plant accidents in 2011, a number of studies on accident-tolerant fuels (ATFs) have been conducted to cope with loss-of-coolant accidents (LOCAs) [1; 2; 3]. Up to now, coating technology is mainly used to prepared coated zirconium alloy cladding. Plasma electrolytic oxidation, magnetron sputtering vacuum arc plasma deposition, arc ion plating, cold spraying, a special physical vapor deposition system and laser beam scanning are widely utilized deposition technologies. Nitride coatings, MAX phase coatings, carbide coatings, pure metal coatings and high-entropy alloy coatings are the main coatings [4; 5; 6; 7; 8; 9; 10]. To the best of our knowledge, the metallic Cr coating is considered to be the best choice for pressurized water reactor (PWR) due to their optimal corrosion resistance in high-temperature steam [4; 11; 12; 13; 14; 15; 16]. However, many kinds of defects are formed during the corrosion of Cr-coated zirconium alloys in steam at high temperature. For example, many pores are formed at the interface of Cr\({}_{2}\)O\({}_{3}\) and Cr due to the Kirkendall effect [11; 16; 11; 17; 18; 19; 20]. Small cavities are also present inside the Zr substrate and the ZrCr\({}_{2}\) layer adjacent to the ZrCr\({}_{2}\)/Zr interface due to the Kirkendall effect [16; 19; 21]. Bubbles are formed in Zr substrate near the Cr-Zr layer. And nanocavities appear into the Cr\({}_{2}\)O\({}_{3}\) grains [19; 20; 22]. Moreover, Cr diffuses into the Zr matrix to form Zr(Cr) solid solution and brittle ZrCr\({}_{2}\) precipitation [7; 23; 24]; ZrCr\({}_{2}\) is about five times harder than the Zr matrix, and the modulus of elasticity of ZrCr\({}_{2}\) is 1.5 times lower that of the Zr matrix [25; 26]. During the corrosion, ZrCr\({}_{2}\) tends to cause crack formation; then, these cracks extend to the coating, and the matrix causes the coatings to lose their protectiveness [27; 28]. Consequently, the corrosion rate is enhanced. Basedon the discussion presented above, although a metallic Cr coating is a promising option for practical application, it also has many deficiencies. We can sum up two key points that cause the coatings lose their effectiveness. One is that Cr is a heterogeneous element to Zr. Another is the interface that forms between the coating and the Zr matrix. So, we must think about that whether it is possible to achieve the effect of improving the high-temperature corrosion resistance of zirconium alloy cladding without coatings by only treating the surfaces of zirconium alloy cladding, so that neither heterogeneous elements can be introduced nor obvious interfaces can be present, thus avoiding the above-mentioned problems of Cr coatings during actual high-temperature corrosion, which is the main starting point of this work. Some works on the effect of laser surface treatment (LST), as one type of surface treatment technology, on the corrosion resistance of zirconium alloys have been reported. [PERSON] et al. [29] found that LST can significantly increase the nodular corrosion resistance of the Zircaloy-4 plate specimens. [PERSON] et al. [30] firstly prepared an oxide film on the surface of a Zr-1 Nb alloy via the micro-arc oxidation (MAO) method because the oxide film layer formed by MAO was loose and the surface was not flat. The subsequent treatment of the material surface using s laser was expected to improve the surface state of zirconium alloy tubes after MAO. The results showed that the specimens treated with a laser of 500 mJ and a scanning speed of 0.13 mm/s achieved the most obvious improvement in corrosion resistance of the specimens corroded in LiOH aqueous solution. It was concluded that this was mainly related to the fact that LST could reduce the surface roughness and increase the density of the oxide film and promoted the transformation of the monoclinic zirconia (m-ZrO\({}_{2}\)) phase to the tetragonal zirconia (t-ZrO\({}_{2}\)) phase. [PERSON] et al.