-
-
HaiGang Yunyi
-
-
Tianjin Haigang Steel Coil
Service first, quality assured. More dedicated and focused on crafting even better boards.
05
2020
-
03
Do you know how galvanized strip steel is de-rusted?
Acid pickling for rust removal is a critically important step in the hot-dip galvanizing process. Whether or not a high-quality surface finish can be achieved on the workpiece prior to plating—and how to improve quality and reduce consumption during the hot-dip galvanizing process—both hinge on the pivotal role played by this acid pickling stage! At the same time, the environmental pollution and ecological pressures generated by this process have become major headaches and sources of concern for our hot-dip galvanizing enterprises! In recent years, China's galvanizing industry has been growing rapidly, and along with this growth, issues such as environmental pollution, wastewater treatment, and emissions have become increasingly prominent.
Since the vast majority of domestic enterprises use hydrochloric acid pickling, our discussion will take hydrochloric acid pickling as an example.
Generally speaking, iron surfaces are covered with rust and oxide scale. The main components of rust are ferrous hydroxide and ferric hydroxide, while the primary constituents of the oxide scale are magnetite (Fe3O4) and hematite (Fe2O3). Both iron oxides and rust readily react with hydrochloric acid and dissolve; the reaction equations are as follows:
FeO + 2HCl → FeCl₂ + H₂O Reaction Equation (1)
Fe(OH)₂ + 2HCl → FeCl₂ + H₂O (Equation 2)
2Fe(OH)3 + 6HCl → 2FeCl3 + 6H2O Reaction (3)
The acid pickling process for oxide scale, however, is somewhat more complicated. This is because ferric tetroxide and ferrous trioxide are relatively insoluble in hydrochloric acid solutions. However, when these oxides coexist with iron, they form a corrosion cell, with iron serving as the anode. The iron in contact with the oxide scale dissolves first, simultaneously generating hydrogen gas and promoting the detachment of the oxide scale from the surface of the steel substrate. This process is accompanied by a mechanical stripping effect that occurs as hydrogen gas is produced. The reaction equation is as follows:
2Fe + 6HCl → 2FeCl₃ + 3H₂↑ Reaction (4)
At this point, the hydrogen that is released reduces ferric tetroxide and ferric oxide to ferrous oxide. The reaction equation is as follows:
Fe3O4 + H2 → 3FeO + H2O Reaction Equation (5)
Fe2O3 + H2 → 2FeO + H2O, Reaction Equation (6)
Then, the reaction described in Equation (1) is repeated and dissolved in the acidic solution.
In the reactions described above, reaction (4) continues even after the oxide scale has been removed. Excessive consumption in reaction (4) can lead to over-corrosion of the metal substrate, a phenomenon commonly referred to as "overpickling" or "overacid washing." Moreover, excessive acid washing results in the accumulation of large amounts of iron ions, causing the pickling solution to lose its effectiveness prematurely. The reaction process also generates significant quantities of hydrogen gas; hydrogen atoms readily diffuse into the metal interior, inducing hydrogen embrittlement and thereby reducing the metal's toughness, ductility, and plasticity, and adversely affecting its mechanical properties. Additionally, when hydrogen molecules carry acid droplets out of the acid bath, they release substantial amounts of acid mist, contaminating the workshop environment.
Regarding the overreaction in controlling Reaction (4), the traditional approach has been to add corrosion inhibitors. However, simply adding corrosion inhibitors can affect the pickling rate. From the perspective of pickling quality, we aim for high corrosion-inhibition efficiency—meaning that after the workpiece has had its oxide scale removed, the weight loss should be minimized as much as possible. Yet, higher corrosion-inhibition efficiency often leads to a reduction in the pickling rate and production efficiency; in severe cases, it can even result in the workpiece becoming impossible to clean properly, leading to a phenomenon known as "underpickling."
The purpose of pickling to remove rust is to eliminate rust and oxide scale without corroding the metal substrate itself. Excessive acid can cause the metal surface to become rough, with a dark black or dark gray appearance. When viewed under magnification, the surface after pickling exhibits a jagged texture and uneven, pitted features, significantly increasing the surface area. This leads to a greater deposition of zinc during hot-dip galvanizing and also adversely affects the quality of the galvanized surface.
In the aforementioned basic pickling reaction equations, Reaction (4) represents an important step. Our key objective is to precisely control this step. To achieve this, we have employed specialized stripping agents and penetrants that enhance and accelerate the detachment of oxide scale from the steel substrate, thereby resolving the challenge of maintaining a high pickling rate while ensuring high corrosion inhibition efficiency. Under these conditions, we can appropriately increase the dosage of corrosion inhibitors and use special adsorbents to form an adsorptive layer on the surface of the steel substrate—much like applying a thin film onto the metal surface after the oxide scale has been removed. This adsorptive layer effectively prevents further reaction between the metal substrate and the acid solution, completely eliminating the problem of "over-pickling" in the hydrochloric acid pickling process. As a result, the dissolution and corrosion of iron are fundamentally inhibited and reduced. Consequently, the amount of iron ions generated in the acid solution is significantly decreased, leading to lower acid consumption due to fewer reactions. At the same time, we have also introduced an appropriate amount of complexing agents that can form complexes with iron ions, thereby minimizing the iron ion concentration in the pickling solution to the greatest extent possible.
Cold-rolled strip steel, pickling to remove rust, galvanized strip steel
Previous page
We primarily produce various products including galvanized strip steel, pickled strip steel, and cold-rolled strip steel. These products are widely used in industries such as construction, mechanical manufacturing, automotive, and household appliances.
Contact usContact Information
Company address:
Shuangtang High-end Hardware Products Industrial Park, Jinghai District, Tianjin City
Contact phone number:
Corporate email:
tjhaigang@tjhaigang.com
Quick Navigation
Page Copyright: All rights reserved by Tianjin Haigang Steel Coil Co., Ltd.