Industrial Corrosion and Protection--02 Corrosion Assessment

Thread Content

This post was last edited by Haichuan Laoyu on February 15, 2026, at 15:30. Corrosion evaluation. I. Evaluation of general metal corrosion. Depending on the form of corrosion damage, there are various methods for assessing the extent of metal corrosion. For general corrosion, it is usually measured by the average corrosion rate. The corrosion rate can be expressed using the weight loss method (or weight gain method), depth method, gas volume method, resistance method, and current density method. Among them, the weight loss method is suitable for cases of general corrosion or when the surface corrosion products are easy to fall off or be removed. The weight gain method is applicable to cases of general corrosion or when the surface corrosion products are firmly attached to the specimen surface, are almost insoluble in the solution, and are not contaminated by external substances. The depth method is suitable for assessing the degree of corrosion in metals of different densities, in order to evaluate the direct impact of the corrosion depth of a material or the degree of thinning due to corrosion on the service life of the material components. The gas volume method is suitable for assessing the corrosion rate of metals under hydrogen evolution or oxygen consumption corrosion conditions; if the amount of hydrogen evolved or oxygen consumed is proportional to the amount of metal that corrodes, then the corrosion rate can be evaluated based on the amount of hydrogen evolved or oxygen consumed per unit area of the sample per unit time. The sensitivity of the gas capacity method is much higher than that of the weight method, and it enables the determination of the instantaneous corrosion rate of materials during the corrosion process. The resistance method for determining the corrosion rate of metals is based on the principle that corrosion causes a reduction in the cross-sectional area of the metal sample, thereby increasing its resistance; by measuring the change in resistance during the corrosion process, the amount of corrosion and the corrosion rate can be determined. The current density method is based on the fundamental principle that the corrosion rate of a metal is proportional to the corrosion current density; it determines the electrochemical corrosion rate of the metal by measuring the corrosion current density i. II. Evaluation of localized metal corrosion: Localized corrosion is often evaluated by examining the changes in the properties of the metal before and after corrosion. (1) Galvanic corrosion: Galvanic corrosion is related to the actual potentials of the metals in contact with each other in a solution, and it forms a macroscopic corrosion cell as a result. The driving force behind galvanic corrosion comes from the actual potential difference between two different metals in contact. Generally speaking, the greater the difference in electrode potentials between the two metals, the more severe the galvanic corrosion. Based on this, through experiments, a table that ranks various metals or alloys according to the level of their stable potentials (non-equilibrium potentials or corrosion potentials) measured under certain conditions is called the electrode series. The vertical arrangement of the metals can be used to qualitatively compare their corrosion tendencies. (2) Pitting: In this method, a specimen made of a certain material is exposed to a corrosive medium for a specified period of time, after which the number of pits formed per unit area as well as the maximum depth of these pits are measured to assess the degree of pitting. The pitting potential can also be determined by using electrochemical potentiodynamic scanning to obtain polarization curves, thereby identifying the breakdown potential and protection potential related to pitting. (3) Crevice corrosion: Samples with artificial crevices can be exposed to a corrosive medium for a specified period, after which weight loss measurement and observation of the corrosion pattern near the artificial crevices are carried out. (4) Intergranular corrosion: After exposing specimens of a certain material to a corrosive medium for a specified period, tensile, bending, and impact tests are conducted to determine changes in strength and toughness. The sensitivity to intergranular corrosion can also be measured using electrochemical methods. (5) Stress corrosion: The remaining mechanical properties can be determined by exposing a specimen under a certain stress to a corrosive medium for a specified period of time; alternatively, the lifespan of the stress-corrosion specimen until failure can be measured directly by exposing it to the medium. By measuring the stress corrosion fracture life at different stress levels, the maximum stress at which stress corrosion fracture does not occur in this corrosion system is determined, namely the critical stress for stress corrosion. (6) Corrosion fatigue: The main measurement parameter is the number of stress cycles to failure of the specimen (service life). On the oN corrosion fatigue curve, the stress amplitude corresponding to a specified corrosion fatigue life (such as N – 10 fatigue cycles) is typically taken as the maximum stress at which no corrosion fatigue fracture occurs; this is known as the corrosion fatigue critical stress, or corrosion fatigue strength. (7) Hydrogen embrittlement and hydrogen-induced cracking: Tensile and impact tests were conducted on hydrogen-charged specimens. Generally, as strength increases, toughness decreases. III. Metal Corrosion Measurement Techniques 1. Polarography. Polarography is an electrolytic analytical method carried out under special conditions; namely, electrolysis is performed in an electrolytic cell using a dropping mercury electrode, and the limiting current, which is proportional to the concentration of the ion being analyzed, is measured. 2. Ion-selective electrode analysis technology. This technique makes use of a membrane electrode that exhibits selective affinity for a specific ion (i.e., an ion-selective electrode, which is essentially an electrochemically sensitive device). The electrode potential follows the Nernst equation in relation to the concentration of the ion being measured; thus, by measuring the potential, it is possible to determine the concentration of certain ions in the solution, and from this, the corrosion rate can be calculated. 3. Atomic absorption spectroscopy method. This method utilizes the ability of the ground-state atoms of the element being tested to absorb specific radiation wavelengths; the magnitude of the absorption value is related to the concentration of those atoms, thereby enabling a qualitative and quantitative analysis of the element in question. 4. Indicator method. This method utilizes indicators composed of certain chemical reagents; the reactions between these indicators and corrosion products (metal ions, OH-, H+) result in different specific colors, which are used to identify the anodic and cathodic areas on the metal surface undergoing corrosion, as well as the local areas affected by corrosion and the type of corrosion present.