Effect of Surface Oxide Scale on Corrosion Resistance of Class A Ship Plate Steel

Using the neutral salt spray corrosion test, the corrosion resistance of the corroded steel with and without scale on the surface was tested in NaCl medium. The corrosion morphology, element distribution and phase structure of corrosion products were analyzed by field emission scanning electron microscopy (FESEM) , energy dispersive spectroscopy (EDS) and X -ray diffractometer (XRD) . The research shows that the corrosion resistance of the sample without scale on the surface is obviously better than that of the sample with scale, which is mainly because the existence of scale leads to the phenomenon of large cathode and small anode in the corrosion process of the sample, which accelerates the corrosion process. In addition, the surface of the sample without scale mainly suffered from comprehensive and uniform corrosion, while the surface of the sample with scale had general corrosion and local severe corrosion.
Key words: salt spray corrosion; ship plate steel; scale; corrosion resistance
In the process of shipping and storage, ship plate steel is in contact with seawater or exposed to the atmosphere, and corrosion defects will occur on the surface, which reduces the quality of the product. In this paper, salt spray corrosion experiments are used to simulate the corrosive environment of Class A ship plate steel during sea transportation and storage, and the defects such as pits and pitting on its surface are analyzed and studied.
The experimental material is A -class ship plate (AH32, thickness 14 mm), and its chemical composition is: cut (C) = 0.15% , s(Si) = 0.23% , w(Mn) = 0.67 %, w(S) = 0.013 %, w(P) = 0.016%, b(A1) = 0.041 % o
The appearance of the defective plate after sea transportation and the sample after the salt spray corrosion test. There are many pitting spots on the surface of the defective ship plate steel after sea transportation; after the salt spray corrosion test, there are also many pitting spots on the surface of the sample, and a small amount of oxide scale remains.
According to the national standard GB/T 10125-1997 ( artificial atmosphere corrosion test - salt spray test), the salt spray corrosion is similar to the operating environment of the defective plate after sea transportation, and can effectively simulate the corrosion of ship plate steel sea transportation and storage environment. The specific implementation scheme is as follows: the pH value of the experimental solution is between 6-5-7.2 , and the temperature in the salt spray chamber of the experiment is 35+2 ° C. The size of the corrosion test sample is 100 mm * 50 mm * 5 mm, and the sample is in the experimental After being degreased with acetone, rinsed with distilled water, and dehydrated with alcohol, put them in a desiccator for use. The samples are divided into two groups, that is, samples with oxide skin on the surface and samples without oxide skin, and each group has 3 samples , weigh the mass of each group of samples before corrosion with an electronic balance, accurate to 0.1 mg . After the experiment, use a cleaning solution (500 mL hydrochloric acid + 500 mL deionized water + 20 g hexamethylene tetra Amine) to soak the sample to remove corrosion products, then wash the sample with water at room temperature, then with acetone, dry and weigh.

(XRD), using a Cu target, for phase composition analysis of corrosion products and oxide scales. Cut the sample after removal of corrosion products into small pieces with a size of 15 mm× 15 mm , clean the surface with alcohol, and use a JSM-7001F field emission scanning electron microscope (FESEM) with energy spectrum ( EDS ) after drying to observe the sample . The surface morphology of the sample was analyzed and its composition was analyzed.
The peel is mainly composed of Fe oxides. Through the XRD and EDS analysis of the oxide scale, it is shown that the oxide scale on the surface of the ship plate steel in this study is mainly composed of FesO . and FezOs phase composition. According to the literature [2,3] , the composition of the scale on the steel surface at high temperature is very complex, mainly Fe oxides, which are FeO, Fe 3 O 4 and FezOs phases from the inside to the outside. Obviously, the phase composition of the scale on the steel plate surface in this study is not consistent with that reported in the literature.

Analysis of surface oxide surface composition of class A ship plate ship plate steel

In order to study the corrosion performance of ship plate steel with different surface quality, two types of ship plate with different surface quality were prepared in the experiment, and the oxide scale in the local area of the surface of the sample with original oxide scale appeared to fall off; the oxide scale on the surface was due to The oxide scale on the surface of the steel plate is not removed during the rolling process. Scale-free samples are machined from samples with scale on the surface.

The surface macroscopic appearance of two different surface quality types of A -grade steel plates, and the XRD analysis results of the surface oxide phase composition of ship plate steel . It can be seen that the oxide scale on the steel plate surface is mainly composed of FeQ* and FeQs phases. Since the experimental material is a steel matrix, the diffraction peak of the Fe phase in the XRD diffraction pattern is very strong.

Appearance and line-scan analysis of oxide scale on the cross-section surface of ship plate steel. The thickness of the oxide skin is about 6 ~ 8 "m, and there is a more obvious interface between the oxide skin and the matrix, indicating that the combination between them is not very tight. Line scanning analysis shows that,
The surface has the morphology of the scale sample after corrosion, and the surface is distributed with relatively uniform corrosion pits, but there are relatively serious corrosion areas locally, and some obvious pitting pits appear in these areas; in addition, the test Some scale remains on the surface. The above shows that the sample has the characteristics of overall uniform corrosion and local severe corrosion.

Surface appearance of ship plate steel after salt spray corrosion test The surface appearance of the corroded skin sample shows that there are no obvious pitting pits on the surface of the sample, and its surface presents the characteristics of comprehensive and uniform corrosion.
The samples with scale on the surface corroded more severely than the samples without scale. This indicates that the samples without scale on the surface have better corrosion resistance.

Analysis and Discussion on Surface Oxidation of Class A Ship Plate Steel :
At present, studies have shown that the corrosion products of Class A ship plate steel mainly include a-FeO(OH) , 〃 - FeO(OH), J-FeO(OH ) and Fe 3 O 4 wait. In the initial stage of salt spray corrosion, under the action of CL , the anode reaction in the active zone occurs: Fe + 2CH-* Fe 2+ +2Cl-+2e-, Fe 2 + combines with OPT to form Fe(OH) 2 ; in Q, H2 . With the participation of Fe(OH) 2 , rust is formed; when the corrosion rate is fast, it is easy to cause anoxic, and a reaction occurs: Fe?+f FeO — Fe 3 O 4 , that is, FesO is formed .
At the same time, Fe exists in the form of [ Fe(OH ) 2 ]+ , and can be rapidly oxidized to 7-FeO ( OH ) when the pH value is neutral . decrease, Fe" can be adsorbed on the surface of r-FeO(OH) to promote its dissolution and gradually transform into a-FeO(OH) and Fe 3 O 4 , the reaction formula is: /-FeO(OH) «-FeO(OH)+ Fe 3 O 4 ; gradually acidic medium is conducive to the dissolution transformation of FeO (OH ) , that is, p-FeO(OH) -* a-FeO (OH)+Fe 3 0 4o
However, continuous spraying is adopted in this study, and the change of pH value on the sample surface is not particularly large. Therefore, the amount of fine a-FeO(OH) and FeQ< formed by the slow conversion of y-FeO(OH) is very small. less; since CL can be dissolved into the lattice of 0-FeO (OH) , and the crystal structure of /3-FeO (OH) is unstable, it is easy to transform into «-FeO (OH) and FeQj . Therefore, the surface of the steel plate in this study The vast majority of a-FeO(OH) and FesO in the corrosion products . It is transformed from 0-FeO(OH) . The structure of a-FeO(OH) and FeQ< formed by A-FeO(OH) is loose due to the loss of soluble CL , and it is easy to accelerate corrosion 8~ Gong. Therefore, the corrosion products of a-FeO(OH) in this study have a strong The matrix cannot provide effective protection. Since the surface corrosion products of the non-scale samples in this study contain less «-FeO(OH) phase, the corrosion resistance of the non-scale samples is better than that of the samples with scale.
In addition, during the salt spray corrosion test, the rust layer of the steel plate is mixed with a large amount of foreign ions such as CL , which destroys the uniformity of the rust layer and causes the structure of the rust layer to be incompact. The voids in the chemical scale and the pressure release in the scale also cause discontinuity in the scale. Therefore, the intrusion of C1 - ions can form pitting nuclei in the defective parts of the oxide scale, induce pitting corrosion, and form a large cathode-small anode galvanic couple electrode. In addition, the potential of the oxide scale is much more positive than that of the steel matrix. Obvious local corrosion holes can be formed inside. Therefore, the existence of scale changes the corrosion form of the steel surface, which is more harmful and leads to the generation of ship plate steel surface defects.
To sum up, the state of oxide scale on the steel plate surface has a direct impact on the occurrence of surface defects in the corrosive environment during subsequent shipping and storage. Therefore, the main measures to reduce its surface oxide scale are as follows:

1. Optimize the descaling effect, use a suitable high-pressure water descaling device, and optimize the device system parameters;
2. Optimize the heating system to reduce the formation of scale;
3. During sea transportation, the steel plate should be sealed as much as possible to avoid contact with seawater;
4. Before stacking the steel plates, the water on the surface should be cleaned, and the steel plates should be stored away from the open air to prevent them from getting wet in the rain.

Conclusions on the influence of scale on the surface of Class A ship plates :

1. The surface of the sample without scale mainly suffered from general and uniform corrosion, while the surface of the sample with scale had general corrosion and local severe pitting corrosion.
2. During the sea transportation and storage of ship plate steel, the surface corrosion defects are mainly caused by the existence of scale on the surface of the steel plate. The anode phenomenon accelerates the corrosion process.
3. To reduce the surface defects of ship plate steel is mainly to remove the scale on the surface of the steel plate and prevent the scale from being pressed in.

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