Abstract: This paper briefly introduces the factors influencing the corrosion rate of metals in the pickling process and uses them as a basis for selecting the appropriate corrosion inhibitor.
Ⅰ. Preface
Pickling is widely used in different processes in various industrial fields, such as industrial equipment, pipelines, low-pressure boilers, power plant boilers, central air conditioning, and another equipment cleaning, steel rust removal, and oil and water acidification. As the pickling agent is corrosive to the metal, a pickling corrosion inhibitor should be added to reduce or eliminate the damage to the metal.
Ⅱ. the choice of pickling corrosion inhibitor
Industrial equipment and pipelines to adapt to the process and transport media often choose different materials. According to the other materials and different types and degrees of scaling, chemical pickling is used in various cleaning media. Commonly used inorganic cleaning media are hydrochloric acid, nitric acid, sulfuric acid, hydrofluoric acid. Organic cleaning media are citric acid, sulfamic acid, EDTA, hydroxyacetic acid, etc. As we all know, different acids have different characteristics, so the need for corrosion inhibitors is not the same.
The usual cleaning process is related to the cleaning medium, metal material, and corrosion inhibitor varieties and related to factors such as the amount of corrosion inhibitor, cleaning temperature, and cleaning flow rate. These factors are interrelated. Corrosion agents need to be considered comprehensively.
2.1 The influence of corrosion inhibitor dosage
Corrosion inhibitor dosage on the metal corrosion of the impact of roughly three situations: a. The corrosion rate of the metal with the amount of corrosion inhibitor.
a, the corrosion rate of the metal with the corrosion inhibitor dosage increases and decreases. Organic and inorganic corrosion inhibitors in acidic media belong to this situation. The actual use should be combined with the protection effect, taking into account the comprehensive benefits, to reasonably determine the amount of corrosion inhibitor.
b, corrosion inhibitor concentration and the relationship between the corrosion rate of the metal has a limit value. At specific attention, when the corrosion inhibition effect is the best, the concentration is too low or too high will make the corrosion inhibition efficiency reduced. Therefore, in the use of such corrosion inhibitors, we must pay attention to the corrosion inhibitor can not be excessive.
c. Insufficient amount of corrosion inhibitor will accelerate the corrosion of metals. Most oxidizing agents such as hydrogen peroxide, sodium silicate, etc., belong to such corrosion inhibitors. For this type of corrosion inhibitor add too little is dangerous. You must pay great attention.
2.2 The effect of cleaning temperature
a, the temperature increases, corrosion inhibition efficiency decreases significantly. Due to the temperature rise, corrosion inhibitor adsorption is reduced considerably, corrosion increased, most organic and inorganic corrosion inhibitors are part of this situation.
b, in a particular range, corrosion inhibitors do not change with the temperature. The corrosion inhibition efficiency does not change with the weather for some inorganic corrosion inhibitors for neutral aqueous solutions. For precipitation film-type corrosion inhibitors, generally should also be used below the boiling point of the medium to have better results.
c, with the increase in temperature, corrosion inhibition efficiency also increased. Due to the temperature rise, corrosion inhibitors can rely on chemical adsorption and metal surface bonding to generate a reaction product film. Or higher temperatures, the corrosion inhibitor is easy to form a layer of similar passivation film layer on the metal surface, thereby reducing the corrosion rate. Therefore, this type of corrosion inhibitor is the most practical value when the medium temperature is high.
2.3 The effect of media flow rate
In most cases, increasing the flow rate of the medium will lead to a reduction in corrosion inhibition efficiency. The flow rate increases, resulting in accelerated corrosion when the corrosion inhibitor becomes a corrosion activator.
2.4 The impact of cleaning materials
In a complete cleaning system, the material of pipes and equipment is sometimes not the same. For some special equipment, to achieve specific strength, toughness, plasticity, or other properties, alloys are often used. To meet the unique process requirements so that a variety of materials coexist.316L stainless steel, titanium alloy; 20G, 12CrMoV, 15CrMo, SA210C, and other materials are also present in the boiler system.
The coexistence of various materials brings difficulty to the cleaning, while the requirements of corrosion inhibitors are also higher. Sometimes, a single corrosion inhibitor does not meet the cleaning requirements, the need to add auxiliary corrosion inhibitors.
2.5 The influence of oxidizing factors
When there is a certain amount of dissolved oxygen, Cu2 +, Fe3 + in the pickling solution will significantly increase the corrosion rate.
When there is a certain amount of dissolved oxygen, Cu2+, and Fe3+ in the pickling solution, it will significantly increase the corrosion rate.; Cu2+ presence will accelerate corrosion and cause copper plating phenomenon, so in the cleaning process, must choose the appropriate auxiliary corrosion inhibitor to inhibit the adverse effects of these factors on corrosion inhibition efficiency.
Ⅲ. Summary
Corrosion inhibitor as an economical and efficient anti-corrosion method in protecting resources and reducing material loss plays an important role, especially in the chemical cleaning work. The role of corrosion inhibitors is pivotal.
Acid cleaning is a seemingly simple but complex process, and corrosion inhibitors play an important role in acid cleaning. As the corrosion rate of the metal in the acid by the cleaning medium, cleaning temperature, flow rate, and corrosion inhibitor type, and other factors, so we have to combine the previous experience, comprehensive analysis, select the most appropriate corrosion inhibitor to achieve low consumption and efficient cleaning and safety and environmental protection.
