The causes of the corrosion problem of the 150 t continuous casting mold system in a steel plant were analyzed and improvement measures were proposed. By adding sodium hydroxide, pre-film treatment, adjusting the content of corrosion inhibitors, installing analytical deaerators, corrosion monitoring equipment and stainless steel filters, the water quality of the crystallizer was improved and the stable operation of the continuous casting crystallizer system was ensured.
Keywords: crystallizer; water treatment; iron oxide; pre-film; deaerator
The crystallizer is called the heart of the continuous casting machine and plays a vital role in continuous casting production. The cooling water of the continuous casting crystallizer is the blood of the crystallizer, and the water quality is soft water. Its function is to accelerate the solidification of the liquid steel in the crystallizer into a safe shell of a certain thickness and shape. Once the stability of the cooling water quality cannot be guaranteed, the crystallizer will suffer from scaling, corrosion, clogging, etc. during production. The continuous casting production process will also cause slab cracks, frequent bonding, and serious and vicious steel leakage accidents.
From the end of 2013 to June 2014, in a 150 t continuous casting mold system of a steel plant, due to corrosion of the crystallizer itself and its water tank, rust flakes fell off, causing blockage of the crystallizer flow channel, affecting production for 74 hours and affecting product quality by 760 t.
In order to reduce the corrosion of the 150 t continuous casting crystallizer system, reduce the blockage rate of the flow channel, stabilize production, and improve the output and quality, the causes of corrosion of the crystallizer system were investigated and analyzed. Starting from the system water quality and reducing the dissolved oxygen content of the system, a method to avoid corrosion of the crystallizer system was found.
The overheating problem of the mold copper plate has always plagued the 150 t continuous casting system. The process flow is shown in Figure 1. Overheating of the copper plate will cause rapid aging of the crystallizer, and the copper plate needs to be replaced after 200 furnaces (about 1 time/week). From the end of 2013 to June 2014, the copper plates of two crystallizers were overheated and damaged due to blockage of the cooling water channels of the crystallizer system and scaling of the crystallizers. In order to find the cause and propose a solution, we sampled and analyzed the blockages in the cooling water channels of the crystallizer system and the crystallizer scaling deposits, and investigated the main components of the corrosion products.
Fig．1 Process flow of 150t continuous casting mold system
Analysis of clogged particles in the cooling water channel of the crystallizer system
Energy spectrum analysis (Figure 3) was performed on the clogging particles (Figure 2) taken out from the blocked cooling water channel. It can be seen that the clog particles are mainly composed of iron and oxygen, that is, the main component is rust. Rust is caused by peeling and corrosion of the inner wall of the crystallizer water tank. Exfoliation corrosion is the corrosion of metal starting along planes parallel to the surface, typically at grain boundaries, where the corrosion products force the metal to separate from the bulk, resulting in a layered appearance. The crystallizer water tank is exposed to air and moisture for a long time during the shutdown period. If corrosion protection and deoxidation treatment are not carried out, spalling corrosion will occur.
Fig．2 Blocked cooling channels and blockage particles
Fig．3 Energy spectrum composition analysis of blockage particles
Analysis of crystallizer scaling deposits
The scale deposit (Fig. 4b) was removed from the scale crystallizer (Fig. 4a) and analyzed and found to be composed of 27.7% organic matter and 72.3% mineral matter. The fouling sample was analyzed using inductively coupled plasma optical emission spectrometry (ICP), and the results are shown in Table 1. It can be determined from Table 1 that the main mineral components are: ZnO, Ca(OH) 2 and Fe2O3. Analyze the origin of the main mineral components:
(1) Zinc is an amphoteric metal and can be corroded under acidic or alkaline conditions. Once zinc is dissolved in water, a pH value >9 will cause zinc hydroxide to be unable to dissolve in water, thus forming a precipitate. Therefore, if the crystallizer is made of a metal alloy containing zinc, zinc may precipitate to form hydroxide when dissolved in water.
(2) The above situation does not occur with copper because its oxidation potential is lower than zinc, which is why zinc is used as a consumable electrode.
(3) Ca(OH) 2 most likely comes from hydration.
(4) Fe2O3 is most likely to come from iron oxide, that is, the physical deoxidation treatment efficiency is insufficient.
Fig．4 Mold and its deposits
Tab．1 Composition analysis of deposits mg / L
By analyzing the blockage particles in the cooling water channels of the crystallizer system and the scale deposits in the crystallizer, it can be determined that the corrosion component of the continuous casting crystallizer system is mainly iron oxide. Corrosion will accumulate inside the water distribution holes of the grooves of the copper plate, thereby reducing the cooling water flow and heat exchange efficiency, causing the copper plate to overheat, and eventually the copper plate needs to be replaced.
Reduce the Rezner index of water bodies
The Rezner index of water bodies can be used to judge the stability of water bodies. If the Rezner Index of a water body is >7, the water is corrosive. The peeling rust scale analysis and monitoring results show that the cooling water is extremely corrosive: when the temperature is 25°C, the Rezner Index is 9.8; when the temperature is 50°C, the Rezner Index is 8.9. By adding sodium hydroxide, the pH value of the water body is maintained between 9.5 and 10.2, and the Rezner index of the water body in the crystallizer system is reduced, thereby reducing the risk of corrosion in the loop. Sodium hydroxide is added through a manually adjusted pump. The operator checks the pH value of the water every 1 hour and adjusts the stroke of the dosing pump to control the pH value. To achieve the best results, the loop should be kept completely sealed to prevent frequent pH adjustments while reducing water and chemical consumption. In order to maintain the pH value of different water bodies between 9.5 and 10.2, the dosage of sodium hydroxide is shown in Table 2.
Tab．2 Sodium hydroxide dosage
|Original pH range of water||Dosage of sodium hydroxide|
|8.5～9.0||Continuous addition of NaOH: 5 kg / day|
|9.0～9.5||Continuous addition of NaOH: 4 kg / day|
|9.5～9.7||Continuous addition of NaOH: 3 kg/day|
|9.7～9.9||Continuous addition of NaOH: 2 kg/day|
|9.9～10.0||Continuous addition of NaOH: 1 kg/day|
|10.0～10.5||Continuous addition of NaOH: 0.5 kg / day|
|10.5 or above||Stop dosing until pH = 9.8 and then continue continuous dosing: 2 kg/day|
Protect the crystallizer water tank
Because the water tank did not undergo daily corrosion protection and deoxidation treatment, and was exposed to air and humidity for a long time during the shutdown period, spalling corrosion occurred. Pre-film treatment is an effective method to protect water tanks from corrosion. The most effective pre-filming method for the crystallizer water tank is to first sandblast the water tank and then passivate it. Since it is difficult to enter the water tank manually, the implementation of this solution is difficult. Therefore, the protective measures of pickling first and then passivation are taken.
Prepare 1m3 of citric acid solution with a concentration of about 10%, and circulate it from its storage tank into the water tank at a flow rate of 5 to 10 m3/h for 6 hours, and the return liquid flows into the storage tank. Empty the water tank and rinse with water until the pH value of the water coming out of the tank is equal to the pH value of the incoming water. Flushing adopts countercurrent flushing method.
Prepare 1 m3 of a solution containing 1 kg of potassium pyrophosphate and 100 g of sodium methylbenzotriazole (TTA) in a cleaning tank (a sodium hydroxide solution containing 50% TTA can also be used). Circulate the passivation solution from its storage tank into the water tank at a flow rate of 5 to 10 m3/h for 8 hours. The return liquid flows into the storage tank, empty the water tank and rinse with water.
Adjust the dosage of chemicals
The main components of the corrosion inhibitor currently in use are based on molybdate, copper corrosion inhibitor MBT·Na and amine, and the dosage concentration is 100 to 150 ppm. Compared with the same industry, the dosage is lower. Because the molybdate content required for this type of closed-loop cooling water system is usually at least 230 ppm based on MoO4-.
In order to increase corrosion inhibitor efficiency, molybdate requires an oxygen content in the water of at least 7 ppm as O2-. If there is insufficient O2- in the water, NO2- needs to be added to provide oxygen. The corrosion inhibitor formula is as follows:
NO2－ = 500～1 000 ppm
MoO4－ = 30～100 ppm
TTA (Other formulations) = 5～10 ppm
In order to increase the efficiency of this treatment solution, the following conditions must be met: Calcium hardness <10 ppm.
Reduce oxygen content in water
In the crystallizer feed water treatment process, oxygen removal is a very critical link. Oxygen is the main corrosive substance in the water supply system and crystallizer water tank, and the oxygen in the water supply should be removed quickly. Otherwise, it will corrode the components of the crystallizer water supply system, and the corrosion will be deposited or attached to the wall of the crystallizer water tank and the water supply hole, blocking the crystallizer flow channel and affecting product quality. In order to remove oxygen from the water, the 150 t continuous casting crystallizer water system is equipped with an analytical deaerator, which uses physical methods to reduce the oxygen content in the water from 12 mg/L to 6 mg/L.
Install corrosion monitoring equipment
Since there was no equipment to monitor corrosion on site, early warning of corrosion could not be given, which was one of the reasons for this production accident. Considering that it is difficult to enter the inside of the water tank manually, in order to perform visual control inside the water tank, it is necessary to use an endoscopic camera to evaluate the corrosion degree of the inner wall surface during maintenance. Implement this measure as part of prevention and routine maintenance.
Install stainless steel filter
In order to prevent the copper plate groove water supply hole from being blocked, a stainless steel filter is installed inside the water tank, as shown in Figure 5.
Fig．5 Diagram of water block filter screen
By analyzing the blockage particles in the cooling water channels of the crystallizer system and the scale deposits in the crystallizer, it can be determined that the corrosion component of the continuous casting crystallizer system is mainly iron oxide. In order to avoid corrosion of the crystallizer system, it is proposed to start with adding sodium hydroxide, pre-film treatment, adjusting the content of corrosion inhibitors, installing analytical deaerators, installing corrosion monitoring equipment and stainless steel filters. Improve the water quality of the 150 t crystallizer system. Since the implementation of the improvement measures, no accidents have occurred due to the corrosion of the crystallizer system or the peeling off of corroded rust flakes that blocked the flow path of the crystallizer and affected production, thus ensuring the stable operation of the continuous casting mold system.