廠房通風管道腐蝕原因分析！

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采用鍍鋅碳鋼作為廠房通風風管材料時，局部區(qū)域會發(fā)生優(yōu)先腐蝕，從而導致風管失效，根據(jù)現(xiàn)場情況調查并進行分析，研究表明局部區(qū)域氣壓變化導致空氣中水分在局部區(qū)域凝結，空氣中氯離子在凝結水中聚集，導致局部腐蝕速度遠大于一般大氣腐蝕速度?？梢酝ㄟ^在局部有凝結水區(qū)域采用耐蝕性更好的材料代替鍍鋅碳鋼以延長風管整體壽命。

When galvanized carbon steel is used as ventilation duct material in factory building, preferential corrosion will occur in local area, which leads to failure of ventilation duct. According to field investigation and analysis, the study shows that the change of pressure in local area leads to the condensation of water in local area, and chloride ions in air accumulate in condensate water, leading to local corrosion. The corrosion rate is much larger than the general atmospheric corrosion rate. The overall life of the duct can be prolonged by replacing galvanized carbon steel with better corrosion resistance materials in locally condensed water areas.

核電站通風系統(tǒng)主要功能是為各廠房通風散熱，由風機房內的風機及分布在廠房內的通風管道組成，國內某核電站通風管道采用鍍鋅鋼板，隨著服役年限延長部分區(qū)域出現(xiàn)腐蝕情況，且由于腐蝕及設計不足多次出現(xiàn)脫落問題，該系統(tǒng)雖與核無關，但腐蝕脫落會帶來工業(yè)問題，因此有必要對風管腐蝕特點、原因進行分析并給出解決方案，同時匯總結構設計不足問題及應對措施，可為現(xiàn)役電站糾正維修及新建電站通風系統(tǒng)設計提供指引。

The main function of the ventilation system of nuclear power plant is to ventilate and heat dissipation for each workshop. It is composed of the fan in the fan room and the ventilation ducts distributed in the workshop. Galvanized steel sheet is used in the ventilation ducts of a nuclear power plant in China. Corrosion occurs in some areas along with the extension of service life, and the problems of shedding occur many times due to corrosion and insufficient design. Although the system has nothing to do with nuclear safety, corrosion shedding will bring about industrial safety problems. Therefore, it is necessary to analyze the characteristics and causes of duct corrosion and give solutions. At the same time, the problems of structural design and countermeasures are summarized, which can provide guidance for corrective maintenance of existing power stations and ventilation system design of new power stations.

1 腐蝕特征與原因分析

1 corrosion characteristics and cause analysis

1.1 通風管道中風管腐蝕特征調查

Investigation on corrosion characteristics of 1.1 ventilation pipes

從電站維修管理系統(tǒng)中查閱歷史風管腐蝕相關事件，對腐蝕部位類型進行統(tǒng)計，主要包括六類：風門前后(相連風管、鉚釘、風門)、風管轉向位置(彎頭、導風裝置)、風機出口風管、風門本體及內部組件、過濾器前后，各腐蝕類型在調查的兩座核電站主要分布情況如圖1所示，該結果與現(xiàn)場調查結果一致，其中風機出口、風門及前后風管、風管改變方向(靠近風機出口)位置是較主要的腐蝕失效部位。

From the maintenance and management system of power plant, the corrosion related events of air duct are consulted, and the types of corrosion sites are statistically analyzed, which mainly include six types: the front and rear of air duct (connected air duct, rivet, air door), the turning position of air duct (elbow, air guide device), the outlet air duct of fan, the main body of air valve and its internal components, and the front and rear of filter, and each type of corrosion. As shown in Fig. 1, the results are consistent with the field investigation. The location of fan outlet, air valve, front and rear air duct and air duct changing direction (near the fan outlet) is the main corrosion failure site.

1.2 腐蝕原因分析

1.2 corrosion cause analysis

鋅在室外海洋大氣環(huán)境中均勻腐蝕速率約為0.5～8μm/年，與碳鋼材料相比，在大多數(shù)環(huán)境中鍍鋅層腐蝕速度平均比碳鋼低22倍，且均大于10倍，因此選擇鍍鋅層作為風管材料防腐層的設計是合理的。然而現(xiàn)場的鍍鋅層卻在局部位置快速腐蝕，導致風管腐蝕不可用，結合現(xiàn)場風管腐蝕調查結果，風機出口、風門及前后風管、風管改變方向(靠近風機出口)位置是腐蝕敏感部位，這些位置的共同點主要表現(xiàn)在該區(qū)域氣體壓力發(fā)生明顯變化，風機出口由于風機提供動能導致風壓升高，風管轉向位置、風門等有阻擋作用位置，風在風門表面、風管轉向位置由于阻擋作用風速降低，根據(jù)流體能量守恒，流體動能降低時其靜壓能升高，即此位置均存在風壓力升高，導致空氣中的水蒸汽分壓升高，當水蒸汽分壓超過形成凝結水，而空氣中氯離子溶解于水中形成侵蝕性環(huán)境。鋅在含有氯離子的水溶液介質中典型腐蝕速度為20～70μm/年，遠遠高于大氣環(huán)境中十倍以上，而風管鍍鋅層厚度僅20μm，將在短時間內消耗，無法起到保護作用。

The uniform corrosion rate of zinc in outdoor marine atmospheric environment is about 0.5-8 micron/year. Compared with carbon steel, the corrosion rate of galvanized layer in most environments is 22 times lower than that of carbon steel and more than 10 times. Therefore, it is reasonable to choose galvanized layer as the anticorrosion layer of air duct material. However, the galvanized layer on the spot corrodes rapidly in local places, which results in the corrosion of air ducts not available. Combining with the results of Corrosion Investigation of air ducts on the spot, the position of fan outlet, air valve, front and rear air duct and air duct changing direction (near the fan outlet) is the sensitive part of corrosion, and the common point of these positions is mainly manifested in the occurrence of gas pressure in this area. Obviously, the wind pressure increases at the outlet of the fan due to the kinetic energy provided by the fan, and the wind pipe turning position and the wind gate have blocking effect. The wind speed decreases at the surface of the wind valve and the turning position of the wind pipe due to the blocking effect. According to the conservation of fluid energy, the static pressure energy increases when the kinetic energy decreases, that is, there is wind pressure at this position. When the partial pressure of water vapor exceeds that of condensed water, chloride ions in the air dissolve in the water to form an erosive environment. The typical corrosion rate of zinc in aqueous solution containing chloride ion is 20-70 micron/year, which is much higher than that in atmospheric environment by 10 times. However, the thickness of galvanizing layer on air duct is only 20 micron, which will be consumed in a short time and can not play a protective role.

1.3 腐蝕模式

1.3 corrosion mode

通風管道中風管中輸送的介質是來自于室外的高濕度、高鹽份的空氣，同時局部存在上述描述的凝結水環(huán)境，風管材料在這些環(huán)境中主要表現(xiàn)為陰極吸氧去極化控制的電化學腐蝕，腐蝕形式上有均勻腐蝕、電偶腐蝕、縫隙腐蝕。

The medium in the air duct is from the outdoor air with high humidity and high salinity, and the condensate environment described above exists locally. The air duct materials in these environments are mainly electrochemical corrosion controlled by cathodic oxygen absorption and depolarization. The corrosion forms are uniform corrosion, galvanic corrosion and crevice corrosion.

鍍鋅層在大氣環(huán)境和凝結水中表現(xiàn)為均勻腐蝕，但在有凝結水環(huán)境腐蝕速度顯著增大，因此現(xiàn)場易產(chǎn)生凝結水的位置鍍鋅層很快消耗完，碳鋼開始腐蝕，如圖2所示。當鍍鋅層消耗出現(xiàn)局部基體鐵裸露時，碳鋼與鍍鋅層出現(xiàn)電偶腐蝕。在風管材料連接部位，采用搭接方式錨固，從而有縫隙存在，當處于干燥環(huán)境時，空氣中氯離子及濕氣并不容易進入縫隙，不會發(fā)生嚴重腐蝕，但當存在凝結水時，含氯離子的凝結水進入縫隙，促進了縫隙腐蝕發(fā)生，其腐蝕速度遠大于均勻腐蝕，

The galvanized layer exhibits uniform corrosion in atmospheric environment and condensate water, but the corrosion rate increases significantly in condensate water environment. Therefore, the galvanized layer is quickly depleted and the carbon steel begins to corrode, as shown in Figure 2. The galvanic corrosion of carbon steel and galvanized steel occurs when the galvanized layer is consumed to expose the partial matrix iron. When the air is in a dry environment, the chloride ion and humidity in the air are not easy to enter the cracks, and serious corrosion will not occur. But when there is condensate water, the condensate water containing chloride ion enters the cracks, which promotes the occurrence of crack corrosion and its corrosion rate. The degree is far greater than that of uniform corrosion.

風管材料之間通過鉚接方式連接，所采用的鉚釘材質為工業(yè)純鋁，當鍍鋅層在風管制作過程中破損或因腐蝕消耗完時，鉚釘會與鐵基體發(fā)生電偶腐蝕，如圖4所示，且在有凝結水環(huán)境時電偶腐蝕效應會加劇，使得鉚釘被迅速消耗，從而導致連接強度不足，連接構件脫落，如風門脫落，風管連接部位脫落等現(xiàn)象發(fā)生，是腐蝕掉落的主要原因之一。

The rivets are made of industrial pure aluminum. When the galvanized layer is damaged or depleted due to corrosion, the rivets will galvanically corrode with the iron substrate, as shown in Fig. 4, and the galvanic corrosion effect will be intensified in condensate water environment, making the rivets rapidly depleted. As a result, the connection strength is insufficient, the connection components fall off, such as the wind door fall off, the wind pipe connection parts fall off and so on, which is one of the main reasons for corrosion fall.