Discussion on the detection method and technical application of steel corrosion in concrete

1、 Corrosion mechanism of steel bars

The corrosion of steel bars in reinforced concrete is mainly the result of electrochemical reactions. After pouring concrete, the hydration reaction of cement creates a strong alkaline environment, and steel bars undergo oxidation reaction (also known as passivation reaction) in this environment, resulting in a dense oxide layer on the outer surface of the steel bars, commonly known as passivation film. A complete passivation film can isolate the steel bars from the external environment and prevent corrosion of the steel bars.

When concrete is damaged by external forces or chemical erosion causing the passive film to partially disappear, the unprotected steel bars will gradually corrode in an environment with oxygen and water. There are mainly the following common factors:

1. Chloride

After chloride penetrates the surface of steel bars, the local acidification of C1- causes a decrease in the pH value of the steel bar surface, leading to the destruction of the passivation film.

2. Acidic environment

When concrete is in an acidic environment, ion permeation gradually changes the pH environment around the steel bars. When the pH value is below 10, the passive film is accelerated and corrosion occurs.

3. Carbonization

The carbonation reaction of concrete is the diffusion of carbon dioxide from environmental gases into the interior of concrete. When the concrete contains moisture, an acidic environment is created, which may lead to the occurrence of corrosion.

4. Stress

Generally, concrete components are used to withstand forces such as pressure, tension, and shear. When stress causes cracks on the surface of steel bars, it can lead to the destruction of the passivation film, and when there is no strong alkaline environment around the steel bars that generates a new passivation film, the steel bars lose protection and accelerate corrosion.

2、 Steel corrosion detection technology

For a long time, methods for detecting steel corrosion have been studied both domestically and internationally. Currently, there are mainly the following:

1. Electrochemical method

Electrochemical method is based on the electrochemical characteristics of steel corrosion, using certain detection methods to determine electrical parameters, summarize the corresponding laws under different corrosion conditions, and determine the degree or speed of steel corrosion. At present, such methods mainly include natural potential method, AC impedance method, linear polarization method, concrete surface resistivity method, constant electric quantity method, electrochemical noise method, etc.

1.1 Natural potential method

The natural potential method is a widely used method for detecting steel corrosion. Using reinforced concrete as an electrode, the corrosion of steel bars can be determined by measuring the equivalent potential between the steel bar electrode and the reference electrode.

The advantages of natural potential method are simple equipment and easy operation. The disadvantage is that it can only qualitatively determine the possible degree of steel corrosion and cannot quantitatively measure the proportion of steel corrosion; This method cannot be used for testing when the concrete surface is covered with insulation or cannot be wetted with water.

1.2 AC impedance method

The AC impedance method is to apply a small AC signal to a concrete component, and determine the properties of the concrete electrode system by measuring and comparing the relationship between the amplitude and phase of the input and output signals. It can not only determine various electrochemical parameters of the electrode, but also determine the control steps of electrochemical reactions. The evolution of electrode processes over time can also be studied through impedance spectroscopy. The disadvantage of this method is that the measurement time is long, the instrument equipment is relatively expensive, and low-frequency AC signals are required for low rate corrosion systems, making measurement difficult; The analysis method is complex, and the measured impedance spectrum is related to the geometric dimensions of the component, making it unsuitable for on-site testing.

1.3 Linear polarization method

The linear polarization method is based on the linear relationship between overpotential and polarization current when the overpotential is very small (<10mV). By applying a small current to the measurement area, the potential change caused by the current is measured. The ratio of the potential change to the current is called polarization resistance, which is inversely proportional to the corrosion current. Therefore, measuring the polarization resistance can obtain the polarization current, which can be converted into the loss of steel bars according to Faraday's law. This method is convenient and fast to measure, with high testing accuracy. The disadvantage is that the calculation coefficients of on-site components are not easy to accurately determine; Linear polarization measurement is based on a known measurement range. Due to the lateral diffusion of the input signal during measurement, it is difficult to accurately define the size of the measurement range; The instrument requires high precision.

1.4 Concrete surface resistivity method

When steel reinforcement rusts, electron migration occurs, resulting in differences in electron distribution on the concrete surface. The possibility of steel reinforcement rusting can be determined by measuring the electrical resistivity distribution on the concrete surface. This method is influenced by the surface moisture content and chemical composition of concrete.

2. Physical Law

It mainly reflects the corrosion status of steel bars by measuring the changes in physical properties such as resistance, electromagnetic, thermal conductivity, and sound wave propagation related to steel bar corrosion. Common methods include resistance rod method, eddy current detection method, radiation method, acoustic emission detection method, etc. The advantage of this type of method is that it is easy to operate and less affected by the environment. The disadvantage is that it is susceptible to interference from other damage factors in concrete; Moreover, it is difficult to establish a correspondence between physical measurement indicators and the amount of steel corrosion. Physical methods are currently mainly in the theoretical research stage and have not been used for on-site testing.

3、 Advantages of half cell natural potential detection method

The steel bar located in an ionic environment can be regarded as an electrode. After the corrosion reaction occurs, the potential of the steel bar electrode changes, and the magnitude of the potential directly reflects the corrosion situation of the steel bar. *A battery is composed of a cathode and an anode, and the steel electrode only has half of the characteristics of a battery, so it is called a half cell. By placing a constant potential reference electrode (copper sulfate electrode or silver chloride electrode) on the surface of concrete and forming a battery with a steel electrode, the potential distribution of the steel electrode can be obtained by measuring the relative potential difference between the steel electrode and the reference electrode. By summarizing the statistical patterns between potential distribution and steel corrosion, the degree of steel corrosion can be determined through potential measurement results.

This method is easy to operate, has fast testing speed, and is convenient for continuous measurement and long-term tracking. It is widely used in various countries and is currently the most commonly used testing method in China.

4、 Application of Half Cell Natural Potential Method

According to the electrochemical measurement method (natural potential method) in GB/T50344-2004 "Technical Standard for Testing of Building Structures", the steel corrosion detection equipment "SW-3D Steel Corrosion Detector" developed by Beijing Shengshi Weiye adopts the polarization electrode principle and measures the surface potential of concrete through a copper/copper sulfate reference electrode. The degree of steel corrosion is determined using the universal natural potential method;

This product has a graphical testing interface, which provides intuitive and convenient measurement; Automatic statistical analysis of measurement results to assist users in making judgments. The technical specifications are comparable to similar foreign instruments: measurement range ± 1000mV, resolution 1mV.

Its main performance characteristics are as follows:

1. Easy to operate, test results are displayed in numerical or graphical form;

2. The reading of the measuring point is fast and stable, and the variation of the potential reading does not exceed 2mv;

3. The degree of corrosion of steel bars is displayed in 9 levels of grayscale or color;

4. Draw a potential contour map, and the contour difference can be set (50mv~100mv);

5. Powerful professional analysis and processing software that can automatically generate initial drafts of detection reports.