The cause of acid corrosion damage of stainless steel electric heating tube

- Apr 23, 2020-

The surface temperature of the outer wall of the electric heating tube in the acid solution is greater than 8O ℃, and the temperature of the acid solution is 4O ~ 5O ℃. The reason for the failure of Ming electric heating tube is to analyze the cause of acid corrosion damage.

2 Inspection content and results
Macro inspection
Take one piece of unused and put into use electric heating tube (part). After observation, the unused electric heating tube is intact and no defects are found; the used corrosion is serious, and the electric
There is a clear difference in the appearance of the heat pipe, but the outer pipe wall is corroded. The part immersed in the liquid surface starts from the liquid surface. disappear completely. At the same time, it was also found that the outer surface of the outer tube wall located under the liquid surface is attached with a copper shell of a certain thickness. No copper shell was seen on the upper part of the liquid surface. Most internal and external surfaces of copper shells are covered with black rusty corrosion products and green patina.

Metallographic examination
Observe the polished longitudinal sample (the axial direction of the tube wall), its inclusions are mainly Ti (N, c), and the number is large, distributed and distributed in a chain shape, and on the polished lateral sample ( Cross section of the outer tube wall) The inner and outer surfaces of the tube wall are subject to different degrees of corrosion. The low magnification observation shows that the uneven thickness is a smooth arc-shaped pit. See Figure 3. The magnification observation at the thinnest wall thickness at this part shows that there are discontinuous small arcs at the edge of the smooth arc-shaped pit at a low magnification Further observation of the pit, most of the copper shell formed on the outer surface of the pipe wall is detached from the pipe wall, but to varying degrees, some become complete copper trousers completely detached from the outer pipe wall, and some have a certain gap. On the base of the outer surface of the wall.

A comprehensive observation of the polished test surface of the entire cross section did not reveal any cracks and other defects extending from the outside to the inside. The part that is not immersed in the acid solution has a few shallow pits but no copper shell
The surface of the sample after abrasion and polishing with aqua regia has a single austenite structure with twins in individual grains. The grain boundaries of austenite are intact and no intergranular corrosion is found.

3 Result analysis and conclusion

The metallographic structure is a single austenite, without any stress corrosion and intergranular corrosion characteristics, it is a normal structure state. The characteristics of the round blunt pits of different depths show that the corrosion of the pipe wall is first from the outer surface, that is, the surface in contact with the acid solution is first corroded. After partial penetration, the inner and outer surfaces of the pipe wall are simultaneously corroded. The corrosion process conforms to the following rules;
① Due to the presence of Ti-containing material points (and a large number) in the matrix, which is a single-phase austenite structure, it is different from the electrode potential of the matrix. In the sulfuric acid aqueous solution (electrolyte) constitutes a microbattery, electrochemical corrosion occurs, causing The substrate with lower electrode potential loses electrons and dissolves continuously, forming etch pits.
②Because the components containing copper or pure copper components are in a certain temperature (usually greater than 80 "C) during acid leaching and rust removal, and the components are turned over during the pickling process, the Cu and the acid H SO chemically reacts to produce a certain amount of CuSO .; and the copper oxides unavoidably present on the surface of Cu-containing members or Cu parts can react with H SO even at room temperature to form CuSO. These CuSO are left in the acid According to the law of chemical reaction, Fe and cr Ni in the matrix of the pipe wall are more active than Cu, so it reacts with CuSO. The reaction reduces Cu and adsorbs on the surface of the pipe wall to form a copper shell with a certain thickness (the thickness of which varies with the actual acid solution). Existing CuSO. Depends on how much.) This constitutes the diode of Cu and Fe (the main element in 1cr18Ni9Ti), that is, the diode composed of the cu shell and the substrate separated from the substrate. Similar to the wire forming a loop, forming a macro "battery", the same principle as in ①, the same electrochemical corrosion, to further accelerate the dissolution of the matrix, and this reaction even under the conditions of no production (ie, the electric heating tube has been soaked in acid (In liquid) can also In view of this, there are two reasons for the serious corrosion of the electric heating tube; one is that the Ti material point present in the substrate and the substrate have different electrode potentials, and the formation of micro-batteries causes the substrate to continuously dissolve to form erosion pits and continue to expand ; The second is due to the formation of a copper shell
Forming a "battery" with the substrate accelerates the dissolution of the substrate. Compared with the two cases, the latter has a more prominent corrosion effect, but both belong to electrochemical corrosion, which causes the electric heating tube to be immersed in an acid solution for a long time and is seriously damaged by acid corrosion.