3. Analysis and discussion

According to the test results, the mechanical properties and chemical composition of the blade meet the requirements of the standard. The metallographic structure is low carbon martensite + dispersed intermetallic strengthening phase, and the metallographic structure is normal. From the metallographic structure of the crack section, it can be seen that most of the cracks originate from the corrosion pits on the blade surface (as shown in the Fig.8,9). The main cracks originate from the inner arc and propagate to the center, while the other secondary cracks originate from the corrosion pits on the outer arc and propagate to the center. Most of the cracks in the origin region propagate along the grain. The macro morphology of blade fracture can be clearly divided into fracture source region, crack growth region and instantaneous fracture region. There are shell like patterns in the crack growth area and shear lip shape in the 45° direction in the instantaneous fracture area, which is a typical fatigue fracture morphology. According to XRF analysis, there are a lot of corrosion products, many secondary cracks, and mud like patterns in the fracture area, and there are corrosion sensitive elements such as S, Cl which indicates that the turbine blade fracture belongs to a typical corrosion fatigue fracture.
During the operation of the last stage blade, when the impeller rotates, there are two forces acting on the blade: one is the centrifugal force generated by the mass of the blade itself, the shroud and the drawing mass; the other is the steam flow force generated by the high-speed steam flow through the blade channel. Generally, the centrifugal force not only produces tensile stress but also bending stress in the moving blade because the penetration line of the centrifugal force acting point does not pass through the center of gravity of the cross section for calculation through the blade. The steam force acting on the blade is composed of two parts: one does not change with time, and the other changes with time. Among them, the component that changes with time will cause the exciting force of the blade. Centrifugal and steam forces may also cause torsional stresses in the blades. The exciting force causes forced vibration and even resonance, which makes the alternating stress of the turbine blade increase sharply [9]. Under the action of periodic alternating stress caused by the above-mentioned multiple stresses, high cycle fatigue crack on blade is created,then gradually expands, and finally breaks due to insufficient strength. The vibration intensified sharply and the impact of the broken blade accelerate the fracture of the remaining blades while the rotor dynamic balance is damaged after one blade breaks.
In addition, the content of chloride ion, calcium and silicon in the steam exceed the standard due to the unstable quality control of steam water. The saturated steam or even wet steam is finally transformed with the gradual expansion of steam whose pressure and temperature decrease. The various ions which corrode nearby parts finally gradually precipitates on the last several stages of blades while the partial pressure of various ions in the steam changes. In particular,chloride ions do great harm to stainless steel. The surface damage (corrosion pit) of the blade caused by corrosion not only destroys the hardened layer, but also forms the crack source, which provides conditions for the generation of cracks. However, the alternating stress is the stress state that the blade must bear. Under the joint action of the alternating stress and the corrosive environment, the cracks are formed and continue to expand, which eventually leads to the blade fracture.
The detailed mechanism of crack formation is analyzed as follows.
Blade cracking originates from corrosion pits on blade surface or water erosion on blade edge. As a result of water erosion, a sharp groove which changes the stress state of the blade surface is formed at the edge of the blade, and makes the crack appear and expand more easily at this position. Pitting is considered to be the precursor of corrosion fatigue damage again and again, which has been proved by many blade failure analysis [10-15]. Pitting not only reduces the actual strength of blade, but also leads to stress concentration, which can become the core of crack in the range of low cycle stress. The formation of corrosion pit begins with pitting caused by chloride ion. Chloride ion pitting can be divided into two stages: pitting nucleation stage and pitting growth stage. There are two theories of pitting nucleation, that is, the theory of passive film destruction and the theory of adsorption [16]. According to the failure theory of passivation film, it is most likely to penetrate the tiny pores in the oxide film and reach the metal surface, and interact with the metal to form a soluble compound due to the small radius and strong penetration ability of chloride ion so that the structure of the oxide film changes and the metal produces corrosion. According to the adsorption theory, the fundamental reason why chloride ion destroys the oxide film is that chloride ion has a strong ability to be adsorbed by metal, and they are preferentially adsorbed by metal, and oxygen is discharged from the metal surface. Because oxygen determines the passivation state of the metal, chloride ion and oxygen compete for the adsorption point on the metal surface, and even can replace the passivation ion in the adsorption to form chloride with the metal. The adsorption of chloride on the metal surface is not stable, forming soluble substances, which leads to the acceleration of corrosion.
Once pore erosion is formed, it develops rapidly, and there are many models of pore erosion development. It is generally recognized that acidification autocatalytic process occurs in the pores. Once the pitting is formed, the external and internal reactions of the pitting are different. The metal in the hole is in the active state (the potential is negative) and becomes the anode. The metal outside the hole is in the passive state, and the potential is positive and becomes the cathode. An activation passivation micro galvanic corrosion cell is formed inside and outside the hole.
Due to oxygen enrichment outside the pore, cathode reaction occurs:
O2+H2O+4e→4OH- (1)
Anode reaction in the hole:
M→Mn++ne (2)
Due to the increase of pH at the orifice, the secondary reaction of metal ions takes place, taking Fe2 + as an example,
Fe2++2OH-→Fe(OH)2 (3)
Fe(OH)2+2H2O+O2→4Fe(OH)3(4)
The iron hydroxide formed by the reaction is deposited on the orifice. As the corrosion proceeds, the pH outside the orifice increases gradually, and the rust layer and the scale layer together accumulate in the crater to form an occluded battery, which hinders the ion migration, finally form oxygen concentration cell. Because the metal ions in the pore are not easy to diffuse outwards, resulting in the increase of cations. In order to maintain the electrical neutrality, the Cl- ions outside the pore migrate to the pore, and the Cl- concentration inside the pore increases. Hydrolysis of metal ions in the pores results in serious acidification in the pores, which in turn causes more Cl- migration and acidification. The autocatalytic process of block cell is formed.
Mn++nH2O→M(OH)n+nH+(5)
It can be said that there is a linear relationship between the pitting potential and the concentration of the aggressive ion, as it is also reported by other authors. Eq. (6) is given based on their works and describes this linear relationship[17]:
E pit = A  − B  log Cx (6)
where A  and B  are constants, Cx  is the concentration of the aggressive ion and E pit is the pitting potential.
The increase of pitting depth mainly depends on the positive dissolution of metal in the pit. The higher the current density of anodic dissolution is, the faster the anodic dissolution speed is, the larger the corrosion pit is. In the above analysis, the blade is subjected to tension, bending, torsion, exciting force and other complex stresses. The influence of complex stress on the current density of anodic dissolution can be expressed as the sum of the change of internal energy caused by the anisotropic stress and the change of enthalpy caused by the plastic deformation. Under the action of complex load, the current density of activation dissolution of metal can be expressed as a function of complex stress state [18]
\begin{equation} i=i_{0}\exp\left(\frac{G}{\text{RT}}\right)\nonumber \\ \end{equation}
\(\ \ \ =i_{0}\exp\left[\frac{1}{\text{RT}}\left(\frac{M\left(1-2\upsilon\right)\left(\sigma_{1}+\sigma_{2}+\sigma_{3}\right)}{3E\rho}+\eta\frac{\text{Mn}}{\rho}\frac{\sigma_{1}^{n+1}+\sigma_{2}^{n+1}+\sigma_{3}^{n+1}}{P^{n}\left(1+n\right)}\right)\right]\)(7)
i is the current density of activation dissolution ,E is the modulus of elasticity,P is the pressure,σ1σ2σ3is the main stress,ν is the Poisson’s ratio,ρ is the density of metal electrode,η is the percentage of plastic strain energy density stored in metal.
From the above formula, it can be seen that the larger the complex load is, the greater the current density of anodic dissolution at the bottom of the pit is, the faster the anodic dissolution rate of the pit is, and the larger the pit is. According to fracture mechanics, the size of corrosion pit directly affects the threshold strength factor and critical cracking stress.
The relationship between crack stress field intensity factor,stress and crack half length:
\(K=Y\sigma\left(\text{πa}\right)^{2}\) (8)
Threshold nominal stress range for crack elongation[19]:
\({\sigma}_{th}=\frac{1}{\text{Y.}\sqrt{\pi}}\ \frac{{K}_{th,lc}}{\sqrt{l+l_{0}}}\)(9)
\({K}_{th,lc}\) is the threshold stress intensity factor range for long cracks, Y is a geometry factor, l is the crack length and l0 is a hypothetical “intrinsic” crack length given by:
l0= \(\frac{1}{\pi}\left(\frac{{K}_{th,lc}}{{\sigma}_{0}Y}\right)^{2}\)(10)
It was found [20,21] that using conventional solutions for the geometry factor according to the pit shape (e.g. the solution for a semi-elliptical crack provided by Newman and Raju[22,23]) leads to highly conservative results (overestimation of up to 40%). Therefore, Eqs. (11) and (12) were evaluated according to extensive fatigue results. Following equations were suggested for the threshold nominal stress range\({\sigma}_{\text{th}}\):
\({\sigma}_{\text{th}}=\frac{1}{0.42.\sqrt{\pi}}\ \frac{{K}_{th,lc}}{\sqrt{a+a_{0}}}\)(11)
\({\sigma}_{\text{th}}=\frac{1}{0.65.\sqrt{\pi}}\ \frac{{K}_{th,lc}}{\sqrt{c+c_{0}}}\)(12)
\({K}_{\text{th},\text{lc}}\): threshold stress intensity factor range for long cracks,
\({\sigma}_{\text{th}}\ \): threshold nominal stress range for crack elongation
Y: geometry factor
l: crack length
l0: hypothetical “intrinsic” crack length
a,c: a is the corrosion pit depth and c is the half corrosion pit width (a0 and c0 are calculated according to Eq. (12)).
It can be seen from the above formula that crack growth is closely related to stress, depth and width of corrosion pit.The critical cracking stress decreases when the width a and depth c increase. As the width and depth of the corrosion pit increase, the critical stress decreases and the blade cracks more easily.To sum up, on the one hand, the corrosion pit which becomes the crack source under the external conditions of complex alternating stress formed by tensile force, bending force, torsion force and exciting force forms, widens and deepens under the action of Cl -; on the other hand, the complex alternating stress directly promotes the crack growth until the fracture failure.
It has been proposed by some authors [24]that laser cladding of alloy elements which were a mixture of Cr, Ni, Co and W powder on the surface can improve the corrosion cracking resistance of blade surface. After laser alloying, the surface layer was denser and the grain refined, while the microhardness of the surface(average 610HV0.2) was about one times higher than that of the substrate material (330HV0.2). The friction coefficient of the laser-alloyed 17-4PH layer was much lower than that of the substrate.