Point corrosion testing, point corrosion testing, pipe point corrosion detection, small hole corrosion detection, metal point corrosion detection
Tianjin Rong Da Testing Co., Ltd. contact: Wang Jingfu
Cell phone: 13102238283 QQ:3144667033
Telephone: 022-23892103
Pitting corrosion is that most of the surface of the metal material is not corroded or slightly corroded. The average point of corrosion is less than 1mm, and the depth is less than that of the aperture. Stainless steel prone to corrosion in the environment containing Cl.
The metal material in some environments, after a certain period of time, most of the surface corrosion or corrosion is very slight, but in the small area of the surface, appear pitting or pitting, and with the passage of time, pits continued to develop in depth, to form a small hole shaped corrosion pit. This phenomenon is called point corrosion, also known as pitting corrosion, pitting corrosion, Kong Shi.
The structure of the large cathode small anode is formed on the geometry of the pitting corrosion, which leads to a rather large anodic dissolution rate, which can lead to the failure of the corrosion and perforation. In addition, pitting can aggravate other types of local corrosion, such as intergranular corrosion, stress corrosion cracking, corrosion fatigue and so on.
Important characteristics of point corrosion
Pitting occurs mostly on the metal material of the surface generated oxide film or passive film, or on the metal of the cathode coating.
Pitting corrosion often occurs in the presence of specific ions in the presence of an oxidizing agent and a reactive anion in the presence of a passive solution. A necessary condition for the occurrence of pitting.
Pitting corrosion is a kind of local corrosion which is concealed and destructive.
Pitting corrosion occurs above the critical potential.
Factors affecting pitting corrosion
Environmental factors:
1 halogen ions and other anions: in chloride, iron, nickel, aluminum, titanium, zirconium and their alloys are likely to produce pitting corrosion. Zinc, copper and titanium in solution containing chloride ions, can also suffer from passive damage.
A lot of non corrosive anions containing oxygen, such as NO3-, CrO42-, SO42-, OH-, CO32-, etc., can be added to the solution containing Cl-, which can play the role of corrosion inhibitor.
The thiocyanate, perchlorate, hypochlorite, can promote pitting.
Cation and gaseous substances in 2 solution: in the corrosive medium, metal cations and corrosive halide anions coexist, the oxidation of metal ions, such as Fe3+, Cu2+ and Hg2+ to promote the role of pitting.
PH value of 3 solution:
The pH value was lower than 9 ~ 10, the price of two metals, such as iron, nickel, cadmium, zinc and cobalt, the pitting potential of pH and almost independent, higher than the pH value, the pitting potential is changed, due to the passivation effect induced by OH- ion.
The conditions of pitting corrosion and pitting potential are not affected by the pH value of the solution, such as aluminum, which is caused by the buffer action of the various steps of aluminum ion hydrolysis.
4 ambient temperature: the pitting potential of iron and its alloys usually decreases with the increase of temperature.
5 medium velocity: the flow of the solution has a certain beneficial effect on the inhibition of pitting corrosion.
Material factor
1, the nature of metal - pure metal resistance to pitting corrosion resistance: 25 C NaOH 0.1M in the pitting potential size: A