Research On The Mechanism Of Corrosion And Corrosion Inhibition Of Galvanized Steel Pipes In Water Supply System

Galvanized steel pipe is the most commonly used water supply pipe material in China.However,long-term use of galvanized steel pipes can lead to pipe corrosion,which can result in water leakage,bursts,increased water transportation energy consumption,and compromised drinking water quality.Therefore,ensuring the safety of drinking water has become an issue that cannot be ignored.However,research on the corrosion of galvanized steel pipes for water supply and the mitigation or inhibition of corrosion has not been perfected.This study will investigate the corrosion of galvanized steel pipes under the influence of different p H values,chloride ion concentrations and water flow,as well as the corrosion inhibition of galvanized steel pipes under the influence of different sodium silicate corrosion inhibitor concentrations;the process of corrosion and corrosion inhibition of galvanized steel pipes in water supply is further verified by simulating actual pipes,which provides a theoretical basis for the corrosion prevention of galvanized steel pipes in China’s water supply system.The main research results are as follows:（1）Through analysis of the released metal quantities from the samples,it was observed that the zinc release exceeded the limit set by the"Hygienic Standard for Drinking Water"（GB5749-2022）of 1000μg/L,under three different corrosion influencing factors,namely p H value,sodium chloride concentration,and changes in water flow conditions.Simultaneously,as the zinc release increased,the release of iron ions was inhibited.In all experimental groups,the release of iron remained below the standard limit requirement of 300μg/L.The minimum zinc ion release was observed at p H 7.5（3055.40μg/L）;however,due to the absence of corrosion inhibitors,the zinc release in the p H7.5 experimental group still exceeded the national standard limit by a factor of3.06.After a 14-day static test,the change in corrosion current density was 3.99μA/cm²,and the absolute value of the corrosion potential increased by 0.030 V.These values represented the smallest changes among the experimental groups,suggesting that the lower zinc ion release could be attributed to the lower corrosion current density and corrosion potential changes.In the chloride ion concentration influence experiment,the corrosion degree of galvanized steel pipe samples reached its maximum at 150 mg/L Cl^-in the experimental group,resulting in a zinc release of 7908.70μg/L.The corrosion current density increased by 10.85μA/cm²,and the absolute value of the corrosion potential rose by 0.047 V.However,due to the enrichment of chloride ions,the zinc ion release showed a certain degree of decrease（4903.67μg/L）when the chloride ion concentration in the experimental group increased to 250 mg/L.In the water flow condition experiment,the zinc ion release in the intermittent flow experimental group（1698.70μg/L）was significantly lower than that in the static experimental group（3055.40μg/L）and the continuous flow experimental group（2520μg/L）,which might be related to the stability of corrosion scale formed during the corrosion process of galvanized steel pipe samples.Characterization through electron microscopy revealed that a denser corrosion scale layer,mainly composed of spherical zinc oxide（Zn O）,was formed after the completion of the intermittent flow experiment group.（2）The inhibition effects on the release of zinc and iron varied with different concentrations of sodium silicate.When the initial concentration of sodium silicate was 50 mg/L（as Si O₂）,the best inhibitory effect on zinc was observed on the fifth day of the experiment（56.12μg/L）.Compared with the blank group without the addition of sodium silicate（2302.33μg/L）,the release of zinc was reduced by 97.56%,preventing the concentration of zinc ions in the water from exceeding the national standard limit.The best inhibition effect on the release of iron was observed on the second day of the experiment at a sodium silicate concentration of 150 mg/L（0.06μg/L）.Compared with the blank group without the addition of corrosion inhibitor（34.90μg/L）,the release of iron was reduced by 99.83%.In all other experimental groups,the concentration of zinc and iron released from the galvanized steel pipes did not exceed the national standard limit.Based on overall consideration,the sodium silicate concentration of 50 mg/L was found to be the optimal corrosion inhibition group.When the concentration of sodium silicate was 50 mg/L,the corrosion current density I_corrof the metal samples on the 14th day（1.03μA/cm²）was compared with that of the blank group（4.44μA/cm²）,and the corrosion inhibition rate reached 76.76%.Furthermore,by calculating the corrosion potential differenceΔE_corr（greater than 0.085 V）,it can be inferred that sodium silicate is an anodic corrosion inhibitor.In this environment,the charge transfer resistance R_ct value（54.06KΩ·cm²）was higher than that of other experimental groups,and the double-layer capacitance C_dl value was the smallest(0.52μF·cm^-2).Further verification through scanning electron microscopy and other morphological analyses revealed the formation of a uniformly textured protective film on the surface of the metal samples in an environment with a sodium silicate concentration of 50mg/L.This film effectively prevents other corrosive media from contacting the metal surface,with zinc silicate（Zn₂Si O₄）being the primary component.（3）Two different water circulation systems were set up,one with added corrosion inhibitors and one without,to simulate the corrosion and corrosion inhibition processes that occur in actual galvanized steel pipes.During the pretreatment stage,the contact area between the galvanized steel pipe and the electrolyte was much larger than that in the beaker experiment,resulting in an average zinc release of 6 mg/L,which exceeded the national standard by six times.After the pretreatment stage,corrosion inhibitors were added to the group with added corrosion inhibitors.When the concentration of sodium silicate in the water solution was 50 mg/L,the concentration of zinc ions released（0.60mg/L）was found to be 92.51%lower than the zinc content in the device without added corrosion inhibitors（8.01 mg/L）.However,when the concentration of sodium silicate increased to 100 mg/L（2.03 mg/L）and 200 mg/L（7.57 mg/L）,the concentration of zinc ions in the water began to gradually increase.Combined with water quality analysis,it was found that this was due to the hydrolysis of silicates exacerbating the conductivity of the water.After adding200 mg/L of sodium silicate,the conductivity increased from 731.30μS·cm^-1 to985.30μS·cm^-1.Surface morphology analysis of the pipeline revealed that a protective film was formed on the surface of the galvanized steel pipe after the addition of corrosion inhibitors,and its main chemical component was zinc silicate（Zn₂Si O₄）.The inner wall of the galvanized steel pipe without added corrosion inhibitors,under long-term corrosion,mainly consisted of iron oxide（Fe₂O₃）and some zinc oxide（Zn O）.