JPH0225729A - Method of predicting corrosion of metal in contact with high-temperature water - Google Patents

Abstract

PURPOSE:To predict the corrosion rate of a metallic material by determining the influence that the oxide film consisting of an oxide crystals on the corrosion phenomenon of said material. CONSTITUTION:The nucleus formation and growth of the crystal of the oxide formed when the surface of the metallic material comes into contact with high- temp. water of various conditions (pH, temp., dissolved oxygen concn., flow rate, etc.) are first measured and quantitized. The various conditions of such high temp. are taken into the memory device section of a computer and the quantity of the oxide film and the elution quantity of the metal after the time (t) are measured by using the n-dimensional vector variable with the number of a function as the number of the next order. The elution quantity of the oxide is then calculated by using the similar function groups and the corrosion quantity after the time (t) is predicted.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to metal materials that come into contact with high-temperature water, and in particular, the present invention relates to metal materials that come into contact with high-temperature water, and in particular, such as the primary cooling system piping of an atomic couplant, where high-temperature flowing water comes into contact with steel materials, corrosion may occur. This paper relates to a method for predicting corrosion in areas where corrosion progresses.

[Conventional technology]

Conventionally, corrosion of metals in contact with high-temperature water has been discussed in Corrosion 35.5 (1979), pp. 219 to 226.
Page (Corrosion 35, 5 (1979) P
As discussed in 219-226), predictions have been made by approximating the constant on the parabolic or logarithmic side, which is obtained from the best fit of experimental values. However, with this method, if even one of the environmental and material conditions differs from the experimental conditions, a constant cannot be determined and corrosion prediction becomes impossible.

The problem with this problem is that the effect of the film formed on the metal surface due to corrosion on the corrosion rate can only be experimentally confirmed under each condition, and the effects of environmental factors and material factors on film formation cannot be quantitatively determined. This is because they were not evaluated.

The present invention makes it possible to predict the amount of corrosion of various metal materials (stainless steel, carbon steel, etc.) under various high-temperature water conditions (pH temperature, dissolved oxygen concentration, flow rate, etc.) over the long term. .

[Problem to be solved by the invention]

The above-mentioned conventional technology has the problem that the influence of the oxide crystal film layer formed on the metal surface on the corrosion rate has not been sufficiently studied, and it is not possible to quantitatively interpret the reduction in the corrosion rate. Ta.

An object of the present invention is to clarify the influence of an oxide film composed of oxide crystals on corrosion phenomena, and to provide a method for predicting the corrosion rate of metal materials.

[Means to solve the problem]

The above purpose is to nucleate oxide crystals that are formed when the surface of a metal material comes into contact with high-temperature water under various conditions.

and by quantifying its growth.

First, regarding the influence of the temperature of high-temperature water, it is known that the higher the temperature, the faster the growth rate of oxides.
Stainless steel and carbon steel show maximum corrosion rates at around 250"C and 150℃, respectively, and above that the corrosion rate decreases despite the high temperature. This is because oxides damage the metal surface to a certain extent. This is to prevent the oxidizing agent from contacting the metal surface.Therefore, if other conditions are constant, by determining the number of oxide crystals at a certain temperature, it is possible to obtain the same number of crystals at other temperatures. is formed, and its growth rate can essentially be determined as a function of temperature.

Next, regarding the dissolved oxygen concentration, it is thought that the number of oxide crystal nuclei generated is inversely affected by the temperature, and there is no relationship with the growth rate of the nuclei. If the conditions are constant, it is uniquely determined.

Thirdly, regarding pH, as seen in the pH-'M1 diagram (Pourvai X diagram), the stable state of the metal at each potential (M oxide, cation, or
Therefore, we can determine the phase of the metal at the corrosion potential in high-temperature water, and compare it with the standard (pH 7) based on the change in pH to find out whether it approaches the region where it is easy to elute. Approaching the oxide formation region, etc. can be quantitatively evaluated using the distance in the diagram.

These various high-temperature conditions are loaded into the computer storage unit, and the amount of oxide film and the amount of metal eluted after a period of time are determined using a linear vector variable in which the number of functions is the number of dimensions.

Next, the amount of oxide elution is calculated using a similar function group, and the amount of corrosion after time (1) is thereby predicted.

Finally, a function system is created for other factors (material factors, flow velocity, changes in position, etc.) and similar vector calculations are performed.

Here, in particular, if the actual measured values of the amount of corrosion are already grouped together at two or more points, the error can be reduced by comparing them with the above-mentioned calculation results.

[Effect]

Temperature,
Vector functions of dissolved oxygen concentration and pH are synthesized, and the number of oxide crystals under specific conditions and changes in particle size over time become a set of variables with a fixed range, and gradually change until the surface is covered with oxide. To increase. Here, the volume of the oxide film is determined from the particle size and number of oxide particles. Multiply this by the density to calculate the amount of oxide film. Next, the distribution of oxide crystals and 1
The proportion of pores in the oxide film (porosity) is determined from the volume, and the metal elution (diffusion transfer amount) is determined. Note that the vector function described above is used again to determine the diffusion rate in the oxide film of the oxidizing agent (Oz, 0H-) that forms the oxide crystal based on the base metal.

This will result in the vector function itself. That is, by incorporating the repetitive function into the function, the model becomes a model that more accurately reproduces reality, and errors are reduced.

〔Example〕

<Example 1> Predict the amount of corrosion when JISSTS42 carbon steel having the chemical composition (wt%) shown in Table 1 is immersed in pure water (liquid) at 288°C with adjusted dissolved oxygen concentration and corroded. In order to do this, the grain size and surface density of oxide crystals formed on the surface of carbon steel were determined according to the present invention. The results are shown in FIG. Further, FIG. 2 shows calculation results for each Do of the radius and length of the pores in the oxide film determined from these results. 1 is the oxide particle size, 2 is the number of oxides, 3 is the puncture radius, and 4 is the pore length.

Based on these results, the amount of corrosion at each Do after 1,000 hours was determined and the results are shown in FIG.

Table 1 <Example 2> The amount of corrosion of JISSTS42 carbon steel having the chemical composition (wt%) shown in Table 1 under degassing conditions and with a dissolved oxygen concentration of 1000 ppb was predicted according to the present invention, and the actual measured values were calculated. compared with. The results are shown in Table 2. It can be seen that the amount of corrosion after 1000 hours can be predicted within a range of ±20% compared to the actual measured value.

Table 2 (Example 3) The oxide film formed on the metal surface is assumed to be spherical, and its growth is determined using the following differential equation.

t t dt Equation (1) represents the outermost layer, Equations (2) and (3) represent growth toward the metal side, A1-A3 are constants, and t represents the time of contact with high-temperature water.

Next, the radius and length of the pores in the oxide film are determined from the change in particle size over time, and the amount of corrosion is determined from the diffusion constant of the oxidizing agent and the amount of movement in the pores.

<Example 4> When iron corrodes in high-temperature flowing water, the dissolved oxygen concentration on the inlet side and the dissolved oxygen on the outlet side were measured, and a linear linear coefficient between the amount of decrease in dissolved oxygen and the amount of corrosion was found. Figure 3 shows the relationship between the amount of reduction in dissolved oxygen and the amount of corrosion. From this, the following formula was obtained as a formula to display the amount of corrosion. Unit is weight (g)
It is.

Wre = 2, 33 Wo + aWFI! : Iron weight Wo : Oxygen weight Here, α is a constant greater than 0, and the value increases at high temperatures, but the value of pure iron near 100°C is as low as 0.1, and like other alloys, No need to introduce parameters.

<Example 5> In order to predict the amount of corrosion of the atomic couplant cooling system piping (carbon III), a transferred oxygen amount calculation program and a corrosion prediction display program were designed. The calculation method for the amount of oxygen is based on the solution of the diffusion equation, and the corrosion prediction display is based on the Xy
Based on the y two-dimensional drawing program.

Table 3 shows the input and output items. The acceleration factor sequence indicates the permutation of factors that make a large contribution per unit to changes in corrosion rate.

Table 3 This method is suitable for preventing pipe breakage and setting the replacement period in other plants as well.

〔Effect of the invention〕

According to the present invention, the amount of corrosion in a portion of the inner surface of a plant component that cannot be directly measured can be predicted by simple calculation.

[Brief explanation of the drawing]

FIG. 1 is a predicted diagram of the grain size and surface density of an oxide crystal according to an example of the present invention, and FIG. 2 is a predicted diagram of the dependence of the radius and length of the pores formed at that time on dissolved oxygen concentration. FIG. 3 is a diagram showing the relationship between the amount of reduction in dissolved oxygen and the amount of corrosion. 1... Particle size of oxide, 2... Number of oxides, 3... Pore radius, 4... Pore length. Furthermore, the amount of corrosion can be determined by measuring the amount of oxygen consumed before and after the measurement area and the amount of oxygen reaching the material surface. This makes it possible to predict and reduce the exposure dose rate of the atomic couplant (≧- PPb)

Claims (1)

[Claims] 1. The amount of corrosion on the water contact surface of a metal material that comes into contact with heated water exceeding 100°C is determined from the amount of oxygen that moves through the voids of the oxide film formed on the surface of the water contact surface. A method for predicting corrosion of metals in contact with high temperature water. 2. In claim 1, after the oxide crystals have grown one-dimensionally on the surface of the metal material until they are in contact with each other, oxygen moves further into the metal through the voids of the oxide. A method for predicting corrosion of metals in contact with high-temperature water, characterized by using inference that corrosion will progress. 3. In claim 2, if corrosion progresses from the oxide layer on the water contact side to the n-th layer, the ratio of the oxide crystal grain sizes is 1:2^-^1^/
^3: 3^-^1^/^3...: (n-1)^-^1^
/^3:n^-^1^/^3, and a method for predicting corrosion of metals in contact with high-temperature water, characterized by having a model that makes it possible to show a reduction in the corrosion rate due to a reduction in the movement path of oxygen. 4. A method for predicting corrosion of a metal in contact with high temperature water, as set forth in claim 1, comprising using a device that displays the amount of corrosion of the metal material. 5. In claim 4, the screen configuration of the device for displaying the amount of corrosion includes a group of environmental factors for inputting water contact temperature, flow rate, dissolved oxygen concentration, and pH, and alloy composition and water contact area for input. an input section consisting of a material factor group, and an output section consisting of a corrosion prediction diagram showing the time course of the oxide film amount and elution amount, and a material thinning prediction diagram showing the time course of the material thickness. Method for predicting corrosion of metals in contact with high temperature water. 6. Claim 4, wherein the screen configuration of the device for displaying the amount of corrosion displays changes in the shape of the oxide crystals indicating the growth of the oxide film on the surface of the metal material. A method for predicting corrosion of a metal in contact with high-temperature water, the method comprising: an output section that displays over time an image obtained by visually observing and enlarging the surface from above. 7. In claim 1, in order to detect the amount of oxygen moving to the surface of the metal material through the oxide film, a device for measuring the amount of oxygen is installed at both ends of the metal in contact with water, and a constant flow rate is provided. A method for predicting corrosion of metals in contact with high-temperature water, characterized by having a function of passing high-temperature water over the surface and measuring the amount of decrease in dissolved oxygen concentration. 8. In claim 1, applying a voltage of 100 mV or less to the surface of the metal material,
The method is characterized in that the amount of transferred oxygen is predicted by measuring the voltage change due to the generation of stray current caused by electron transfer due to the progress of the corrosion reaction, and predicting the amount of electron transfer by differentiating the absolute value of the voltage change with time. A method for predicting corrosion of metals in contact with high temperature water. 9. A method for predicting corrosion of a metal in contact with high temperature water according to claim 1, wherein the metal material is made of an alloy based on any one of Fe, Ni, and Co.