| Taiyuan,as one of the energy and heavy industry bases in China,the problem of water resources shortage has severely restricted its rapid economic development,the use of reclaimed water is particularly important.The safe and stable delivery of reclaimed water to water users will involves the laying of supporting pipeline networks,it takes a long time and high cost to build the new reclaimed water pipeline network,and the utilization of reclaimed water will be restricted in a short time.The heat-supply period of the heat-supply pipeline network in Taiyuan is about 5 months per year,and the rest of the time is idle,so the mature heat-supply pipeline network can be used to transmit and distribute reclaimed water in idle period,but it is necessary to ensure that there is no harmful effect on the heat-supply pipeline network in the process of transmission and distribution.Most of the existing heat-supply pipeline network in Taiyuan is made of carbon steel and stainless steel,carbon steel is easily corroded after long-term transmission and distribution of high-temperature heat-supply medium,but stainless steel,as a pipe with stronger corrosion resistance,is more suitable for the transmission and distribution of reclaimed water.Although the transmission and distribution of reclaimed water by stainless steel heat-suppy pipeline network is theoretically feasible,but it is urgent to study the corrosion influence of the reclaimed water quality on the stainless steel pipelines in the transportation process.In this paper,five water quality factors(chloride ion,sulfate,alkalinity,hardness and p H)affecting corrosion in reclaimed water were selected to investigate the effects of simulated solutions with different concentrations of water quality factors and reclaimed water on the corrosion resistance of 304 and 316 L stainless steel materials by using electrochemical method and gravimetric method,and the corrosion influence law and mechanism of action were also explored through analysis and characterization methods such as SEM,the results and conclusions obtained are as follows:1.Through the electrochemical method,the results show that:(1)In the chloride ion concentration range of 25-400 mg/L,chloride ion obviously promotes the corrosion of two kinds of stainless steel,if the concentration is controlled from 200 mg/L to less than 25mg/L,the corrosion effect will be greatly weakened.(2)In the presence of chloride ions,both sulfate ions and bicarbonate ions will have a certain impact on the corrosion of chloride ions,sulfate ion has a critical concentration value of 150 mg/L for the corrosion effect of two kinds of stainless steels.Below this value,it has a promoting effect,above this value,it has an inhibitory effect.However,the effect of bicarbonate ion on corrosion is related to the material,when the concentration is lower than 150 mg/L,it has an inhibitory effect on 304 stainless steel,otherwise it has an accelerating effect,while it has always been an accelerating effect on316L;(3)In the presence of chloride ions,calcium ions alone has almost no effect on the corrosion of chloride ions,but calcium carbonate sediment will be formed when it coexists with the same concentration of bicarbonate ions,which will affect the corrosion,and its impact also has a critical concentration value,this value for 304 stainless steel is 150 mg/L,and for 316 L is 200 mg/L,lower than the critical concentration value is inhibiting function,and higher than that is promoting function.(4)The corrosion resistance of two kinds of stainless steels in the weakly acidic simulated solution containing only chloride ions is obviously worse than that in the neutral and weakly alkaline environment,and the weakly alkaline environment has a significant inhibitory effect on the corrosion of 316 L stainless steel.(5)The corrosiveness of reclaimed water is not as strong as that of simulated solution.The above results appear because the high concentration of chloride ion accumulated on a certain weak point of the passive film of stainless steel.After the chloride ion invaded,a series of electrochemical reactions were triggered,leading to the spread of local corrosion,thereby forming pitting corrosion pits.The bumps formed around the pits were the deposition of reactants.But if the concentration of chloride ion was controlled within a certain range,the chloride ion in the water could not be fully aggregated at a certain point,so as to achieve the effect of corrosion inhibition.Other water quality factors affect the corrosion process of chloride ions through competitive inhibition or reaction with metal ions to generate corresponding corrosion products,thereby playing a certain role in the corrosion of stainless steel.2.Through the gravimetric method,the results show that:(1)The corrosion rate of two kinds of stainless steels in simulated water is basically stable at a low level in each period of the first 40 days,but has an upward trend,and there is a significant increase of corrosion rate in the period of 50 days,but a certain degree of reduction of 60 days,this shows that the chloride ion corrosion process has a certain regularity with time.(2)The influence of different reclaimed water quality factors on corrosion is almost the same as that under the electrochemical method,but there are also differences within a certain error range.For example,150 mg/L of bicarbonate ion exhibits an inhibitory effect on the corrosion of 304 under the gravimetric method,while it exhibits an accelerating effect under the electrochemical method;with the increase of the calcium carbonate sediment amount,the corrosion rate of two stainless steels both decreases under the gravimetric method,but the corrosion effect of the two kinds of stainless steels is first inhibited and then promoted under the electrochemical method,the appearance of this difference in conclusion is inseparable from the experimental error of the weight method itself and the setting of experimental conditions.(3)The corrosion rate of the two kinds of stainless steels in the dynamic immersion experiment in the reclaimed water was obviously greater than that in the static immersion experiment in the simulated solution,this is because the scouring effect of the reclaimed water in the dynamic experiment cleans up the corrosion products generated on the surface of the stainless steel in time,making it easier for the corrosive ions to reach the surface of the stainless steel.3.By comparing the corrosion behavior of 304 and 316 L stainless steels under the same experimental conditions,the results show that:(1)In most cases of electrochemical method,316 L stainless steel has a more positive self-corrosion potential than 304 stainless steel,with lower corrosion current density and greater charge transfer resistance.(2)In most cases of gravimetric method,the corrosion rate of 316 L stainless steel in the same time period is lower than that of 304.(3)From the macro and micro angles,the pitting pit formed on the surface of 316 L stainless steel is significantly smaller.The above comparisons all prove that316 L stainless steel has stronger corrosion resistance and is more suitable than 304 stainless steel as a heat-supply pipeline to transport and distribute reclaimed water. |
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