PICKLING AND OVER-PICKLING MECHANISMS OF LOW CARBON STEEL.

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Hot strip mill (HSM) scales

At the exit of the finishing mill, steel sheets are recovered with scale. They are coiled immediately and stocked. The coils of the hot strip mill are commonly called black coils due to the dark colour of scale. Scale composition depends on coiling temperature, cooling rate and position on the strip.

Low carbon steel (LCS) scales

Scales of black coils of LCS are mainly composed of a wüstite layer covered by magnetite and hematite superficial layers.
Depending on the cooling rate, wüstite is transformed partially or completely into magnetite and iron eutectoid (see also I.4.2.2 section and Figure I.9). Water quenching and high temperature coiling favor wüstite conservation. Air cooling and low temperature coiling enhances its transformation. Scale is generally thin (of 5-20 μm in thickness), spread and brittle due to the mechanical pressure in the hot strip mill. Wüstite is plastic at high temperatures; it is rolled softly in the hot strip mill conditions and at lower temperatures it is brittle which facilitates de-scaling. Magnetite is less plastic than wüstite at high temperatures and less brittle at room temperatures, its cohesion and adherence are higher. Hematite is brittle and extremely hard. It is undesirable during rolling, cooling and also at room temperature [55-62].

Silicon alloyed steel (SiAS) scales

Black coils of silicon alloyed steels coming from the hot strip mill have scales with fayalite grains and sometimes with fayalite infiltrations in steel at scale/metal interface. The outer scale is composed of thin and dense wüstite similar to that of low carbon steels. This is due to mechanical pressure of rollers. As fayalite is resistant to pickling, its infiltrations stay on pickled sheets surface and lead to later defects in cold rolling [56, 63].

LCS model scales

Model scales of low carbon steel grades obtained in laboratory furnaces at high temperature, humid oxidizing atmosphere and relatively short oxidation times (below 15 min) reproduce well the chemical composition of hot rolled scales. They are composed mainly of iron oxides; the effect of traces of alloying elements is neglected. Three oxides are present as continuous parallel sub-layers classified by increasing oxidation degrees on steel: wüstite FeO, magnetite Fe3O4 and hematite Fe2O3. They are formed by iron ions vacancies diffusion and contact with oxygen. According to [23], the mean thickness ratio of hematite, magnetite and wüstite thicknesses in the total scale is 1:4:95 respectively (Figure I. 8).

Influence of pickling parameters on pickling time

The objective of pickling is to dissolve the entire scale layer in a short time to increase productivity and at the same time, to protect steel from mass loss and high surface roughness due to an excessive over-pickling. Actually, optimal pickling time is defined when all scale is removed with pickling and over-pickling. Pickling time depends on the metal and scale characteristics and the pickling conditions:
– Scale thickness: it increases with the rolling and coiling temperature and is different at the centre and tail of the sheet due to slower cooling. The required pickling time depends on the scale thickness and chemistry [77-79].
– Scale composition: for low carbon steel scales, hematite and magnetite dissolution is very slow compared to that of wüstite. For example, in a 88 g.L-1 HCl solution at 80°C, the dissolution rates of Fe3O4 and FeO are 0.1 g.m-2.min-1 and 1.6-2.0 g.m-2.min-1 respectively. FeO decomposition into Fe3O4 and Fe can accelerate its dissolution up to a factor 10 [79-81].
– Scale morphology: scale cracking and elongation of 1 to 2 % accelerates pickling with a factor 2 to 3 [82].
– Bath temperature: pickling efficiency increases with bath temperature. Pickling is usually performed at 70-90°C [79, 83].
– Acidic solution: scale is dissolved quickly in highly concentrated and hot HCl or H2SO4 acid solutions. For given concentration and temperature, HCl is more efficient [75, 79, 84].
– Iron cations concentration (Fe2+ and Fe3+): in an HCl medium, the unfavorable effect of Fe2+ does not appear below 80 g.L-1, and remains in every case more moderate than in H2SO4 medium. The ferric ions Fe3+ are known to decrease the pickling time and facilitate the attack reaction of the base metal. But their concentration must be well controlled to avoid overpickling, mainly for grades which can be pickled rapidly. Generally, the salt concentrations are 50-100 g.L-1 for FeCl2 and 3-5 g.L-1 for FeCl3 at the end of pickling [74, 76, 79].
– Alloying elements concentration: PO4 3-, Mn2+, Cr3+ coming from steel dissolution increase slightly pickling time.

Pickling mechanism of low carbon steels

Scale of low carbon steels is composed of iron oxides. These oxides form parallel layers in the following order from the interface with steel: wüstite, magnetite and hematite. Scale of black coils is a mixture of wüstite, magnetite and iron coming from the wüstite transformation (see also §I.4).

Pickling steps of LCS

The evolution of scale morphology and electrochemical state during pickling and over-pickling were observed by coupling an open circuit potential OCP monitoring with a series of cross-section micrographs of scale at significant OCP values [88]. Figure I. 14 shows the evolution of the open circuit potential and scale morphology of a continuous and thick model scale of low carbon steel, dissolved in H2SO4 acid. The pickling and over-pickling steps are well separated with an open circuit potential drop once the acid reaches the steel/scale interface [88-90].

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Table of contents :

I. LITERATURE REVIEW
I. 2. Oxidation and pickling in steelmaking process
I. 2. 1. Overview of steelmaking process
I. 2. 2. The hot strip mill
I. 2. 3. The pickling baths
I. 3. Oxidation of steel
I. 3. 1. Steel substrate properties
I. 3. 1. 1. Low carbon steel
I. 3. 1. 2. Silicon alloyed steel
I. 3. 2. Oxidation conditions
I. 3. 2. 1. Oxidation in the Hot Strip Mill
I. 3. 2. 2. Oxidation in a pilot furnace
I. 3. 3. Oxidation of low carbon steel
I. 3. 3. 1. Oxidation kinetics
I. 3. 3. 2. LCS oxidation mechanism
I. 3. 4. Oxidation of silicon alloyed steels
I. 3. 4. 1. Influence of silicon on the kinetics of scale growth
I. 3. 4. 2. Influence of silicon on oxidation mechanism
I. 4. Scale and metal oxides properties
I. 4. 1. Hot strip mill (HSM) scales
I. 4. 1. 1. Low carbon steel (LCS) scales
I. 4. 1. 2. Silicon alloyed steel (SiAS) scales
I. 4. 2. Model scales
I. 4. 2. 1. LCS model scales
I. 4. 2. 2. Iron oxides properties
I. 4. 2. 3. SiAS model scales
I. 5. Pickling and over-pickling
I. 5. 1. Pickling of hot mild steels
I. 5. 1. 1. Pickling conditions
I. 5. 1. 2. Influence of pickling parameters on pickling time
I. 5. 2. Pickling mechanism of low carbon steels
I. 5. 2. 1. Pickling steps of LCS
I. 5. 2. 2. Pickling reactions of LCS
I. 5. 2. 3. Iron oxides dissolution
I. 5. 3. Effect of Silicon on pickling mechanism
I. 5. Conclusions
II. 1. Elaboration of steel grades samples
II. 1. 1 Steel chemical composition
II. 2. 2. Steel grains size
II. 2. Oxidation of steels
II. 2. 1. Oxidation experiments
II. 2. 2. Oxidation methods
II. 3. Scale Characterization after oxidation
II. 3. 1. Optical observations
II. 3. 2. Spectrometric methods
II. 4. Scale dissolution characterization
II. 4. 1. Electrochemical measurements
II. 4. 1. 1. Electrochemical set-up
II. 4. 1. 2. Electrochemical methods
II. 4. 2. ICP-AES setup and method
II. 5. Scale characterization after pickling and over-pickling
III. OXIDATION AND SCALE CHARACTERIZATION OF SILICON ALLOYED STEELS
III. 1. Introduction
III. 2. Steel substrate properties
III. 3. Oxidation mechanisms
III. 3. 1. Low carbon steel oxidation
III. 3. 1. 1. Oxidation kinetics
III. 3. 1. 2. Oxidation mechanism
III. 3. 2. Effect of Silicon content on oxidation
III. 3. 2. 1. Oxidation kinetics
III. 3. 2. 2. Oxidation mechanism
III. 4. Scale characterization
III. 4. 1. Low carbon steel scale properties
III. 4. 2. Silicon steel scales properties
III. 5. Conclusions
IV. PICKLING AND OVER-PICKLING MECHANISMS OF LOW CARBON STEEL.
IV. 1. Introduction
IV. 2. Model scale characteristics before pickling
IV. 3. Electrochemical dissolution of scale
IV. 3. 1. Corrosion potential Ecorr
IV. 3. 2. Corrosion current Icorr
IV. 3. 2. 1. Tafel curves
IV. 3. 2. 2. Corrosion current evolution during scale dissolution
IV. 3. 3. Electrochemical Impedance Spectroscopy EIS
IV. 3. 3. 1. EIS diagrams
IV. 3. 3. 2. Evolution of charge transfer resistance R
IV. 3. 3. 3. Evolution of CPE parameters
IV. 3. 3. 4. Evolution of effective capacitance
IV. 4. Total dissolution of scale
IV. 5. Pickling and over-pickling mechanisms
IV. 5. 1. Pickling thermodynamics
IV. 5. 2. Pickling steps and reactions
IV. 5. 3. Over-pickling reactions
IV. 6. Influence of some parameters on picking and O-P mechanism of LCS scales .
IV. 6. 1. Scale composition and morphology
IV. 6. 1. 1. Influence of hematite on pickling
IV. 6. 1. 2. Pickling of an industrial scale
IV. 6. 2. Influence of acid concentration
IV. 6. 3. Influence of pickling bath temperature
IV. 6. 4. Influence of a cathodic applied potential
IV. 7. Steel surface after pickling and over-pickling
IV. 7. 1.Steel surface after insufficient pickling
IV. 7. 2 Steel surface after long over-pickling:
IV. 8. Conclusions
V. PICKLING AND OVER-PICKLING MECHANISMS OF HIGH SILICON ALLOYED STEEL GRADES
V. 1. Introduction
V. 2. Model scale characteristics before pickling
V. 3. Electrochemistry of scale dissolution
V. 3. 1. Corrosion potential Ecorr
V. 3. 2. Corrosion current density Icorr
V. 3. 2. 1. Current-potential curves
V. 3. 2. 1. Evolution of the corrosion current density
V. 4. Total dissolution of scale
V. 4. 1. Total dissolution rate (TDR)
V. 4. 2. Electrochemical contribution in scale dissolution
V. 5. Pickling and over-pickling (O-P) mechanism
V. 5. 1. Pickling steps and reactions
V. 5. 2. Over-pickling steps and reactions
V. 6. Influence of some parameters on picking and O-P mechanism
V. 6. 1. Scale morphology and composition
V. 6. 1. 1. Influence of fayalite morphology
V. 6. 1. 2. Behaviour of industrial scale
V. 6. 2. Influence of pickling bath temperature
V. 6. 3. Influence of acid concentration
V. 7. Steel surface after pickling and over-pickling
V. 7. 1. Steel surface after insufficient pickling
V. 7. 2. Steel surface after over-pickling
V. 8. Conclusions
GENERAL CONCLUSION
ANNEXES
REFERENCES 

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