Shreir's Corrosion vol III

813 Pages·2007·29.87 MB·English

by  Editor-in-Chief: Tony J.A. Richardson

3.01 Corrosion of Carbon and Low Alloy Steels
S. B. Lyon
Corrosion and Protection Centre, School of Materials, University of Manchester, Oxford Road, Manchester M13 9PL, UK
ß 2010 Elsevier B.V. All rights reserved.
3.01.1 Introduction 1695
3.01.1.1 Historical Perspective 1695
3.01.1.2 Iron–carbon Alloys 1695
3.01.1.2.1 Phase diagram 1695
3.01.1.2.2 Equilibrium microstructures 1697
3.01.1.2.3 Nonequilibrium microstructures 1697
3.01.1.3 Mechanical and Physical Properties 1699
3.01.1.4 Processing 1699
3.01.1.4.1 Heat treatment 1699
3.01.1.4.2 Mechanical deformation 1700
3.01.1.4.3 Metallurgical influences on corrosion 1700
3.01.2 Electrochemistry 1702
3.01.2.1 Thermodynamics 1702
3.01.2.2 Anodic Dissolution 1704
3.01.2.2.1 Oxygen-free (deaerated) conditions 1704
3.01.2.2.2 Oxygen containing (aerated) conditions 1704
3.01.2.2.3 Anion adsorption effects on the mechanism of dissolution 1704
3.01.2.3 Passivity 1705
3.01.2.3.1 Passive oxide films 1705
3.01.2.3.2 Nonoxide passive films 1706
3.01.2.4 Cathodic Reactions 1707
3.01.2.4.1 Hydrogen evolution reaction 1707
3.01.2.4.2 Oxygen reduction reaction 1707
3.01.2.5 Corrosion in Aqueous Environments 1708
3.01.2.5.1 Anode and cathode separation 1708
3.01.2.5.2 Mass transport 1708
3.01.2.5.3 Effect of flow rate on corrosion 1708
3.01.3 Corrosion Processes 1709
3.01.3.1 Corrosion Products 1709
3.01.3.2 Aqueous Corrosion 1710
3.01.3.2.1 General corrosion 1710
3.01.3.2.2 Concentration cell corrosion: Differential aeration 1710
3.01.3.2.3 Pitting and crevice corrosion 1711
3.01.3.2.4 Galvanic corrosion 1711
3.01.3.2.5 Flow-assisted corrosion (FAC) 1712
3.01.3.2.6 Erosion–corrosion 1712
3.01.3.3 Environmentally Assisted Cracking 1712
3.01.3.3.1 Environments 1712
3.01.3.3.2 Hydrogen embrittlement 1713
3.01.3.4 Microbiologically Influenced Corrosion 1713
3.01.3.5 Aqueous Corrosion Protection 1713
3.01.3.6 High Temperature Oxidation 1713
3.01.4 Atmospheric Corrosion 1714
3.01.4.1 Environmental Influences 1714
3.01.4.1.1 Humidity 1714
3.01.4.1.2 Air-borne pollutants 1715
1693
1694 Ferrous Metals and Alloys
3.01.4.1.3 Particulates 1717
3.01.4.2 Mechanism of Atmospheric Corrosion of Iron 1718
3.01.4.2.1 Acid regeneration cycle 1718
3.01.4.2.2 The electrochemical mechanism 1719
3.01.4.2.3 The wet–dry cycle 1719
3.01.4.3 Corrosion Product Composition 1719
3.01.4.4 Atmospheric Corrosion Kinetics 1720
3.01.4.4.1 Climatic variation 1720
3.01.4.4.2 Conditions of exposure 1721
3.01.4.4.3 Damage functions 1722
3.01.4.5 Weathering Steel 1723
3.01.4.5.1 Alloying effects 1723
3.01.4.5.2 Wetting and drying 1723
3.01.4.5.3 Applications 1724
3.01.4.5.4 Next generation weathering steels 1725
3.01.4.6 Classification of Atmospheres 1725
3.01.5 Corrosion in Water 1726
3.01.5.1 Water Composition 1726
3.01.5.1.1 Dissolved gases 1726
3.01.5.1.2 Dissolved solids 1727
3.01.5.1.3 Microbial effects 1728
3.01.5.2 Deposits and Scales 1728
3.01.5.2.1 Fouling of surfaces 1728
3.01.5.2.2 Under-deposit corrosion 1728
3.01.5.3 Natural Waters 1728
3.01.5.3.1 Corrosion rates 1728
3.01.5.3.2 Piped fresh water systems 1729
3.01.5.3.3 Structural steel in waters 1729
3.01.5.3.4 Variation of corrosion with height 1730
3.01.5.4 Process Waters 1730
3.01.5.4.1 Heating and cooling systems 1730
3.01.5.4.2 Boiler waters 1731
3.01.6 Underground Corrosion 1731
3.01.6.1 Controlling Factors 1731
3.01.6.2 Corrosion of Buried Steel 1732
3.01.6.2.1 Piling 1732
3.01.6.2.2 Pipelines 1733
3.01.6.2.3 Long-term burial 1733
References 1733
Lepidocrocite Metastable from of hydrated iron
Glossary
oxide, FeOOH and commonly found during
Akaganeite Hydrated iron oxide, b-FeO(OH,Cl),
atmospheric corrosion of iron-based alloys.
that is stable in the presence of
chloride ions and thus generally forms
in seawater.
Goethite Stable form of hydrated iron
oxide, FeOOH and thus commonly found Abbreviations
in nature. ALWC Accelerated low water corrosion
LAMM phase The structure of the passive film BISRA British Iron and Steel Research Association
on iron. BS EN British Standard European Norm
Corrosion of Carbon and Low Alloy Steels 1695
content were made by cementation type processes,
FAC Flow-assisted corrosion
effectively by successively placing the semifinished
ISO International Standards Organisation
object in hot charcoal or air.
MIC Microbiologically assisted corrosion
This method of iron production remained, essen-
NACE National Association of Corrosion Engineers
tially unchanged in Europe, for 1500 years.However, in
NBS National Bureau of Standards
China development of iron smelting techniques that
RH Relative humidity

were able to reach temperatures of 1150 C and,
SIMS Secondary ion mass spectrometry
consequently, were able to melt cast iron (when com-
bined with 4% carbon) was achieved in 500 BC.
Methods for reducing the carbon content of such cast
Symbols irons were necessary in order to achieve a malleable
ads Adsorbed material, and this was achieved by heating the molten
C Concentration of species material in air with stirring. During this process iron
F The Faraday or Faraday’s constant oxide, formed by oxidation of the molten metal, was
ilim Diffusion limited current density stirred into the melt and reacted with dissolved carbon
k Mass transfer coefficient producing carbon monoxide, thus lowering the overall
n Number of electrons transferred in an carbon content. In Europe, the development of water
electrochemical reaction power was applied to the bloomery forging process in
t Time order to increase production of steels from 1000 AD
a Ferrite onwards. However, cast irons were not generally pro-
g Austenite duced as knowledge of how to reliably reduce their
v Angular velocity carbon content was not introduced until the Middle
Ages (i.e., from 1100 to 1300 AD onwards) where a
process similar to the Chinese one was used in so-called
‘puddling’ furnaces. Later developments included the
3.01.1 Introduction manufacture of limited quantities of high quality steels
via crucible and similar methods.
3.01.1.1 Historical Perspective
Large scale cast iron manufacture in blast furnaces
Prior to the sustained and deliberate production of developed only after the switch from wood charcoal
iron, there is some evidence that ferrous materials (a limited resource) to coke derived from coal in the
(i.e., iron–nickel) derived from meteors were used late seventeenth and early eighteenth centuries,
intermittently in antiquity although they must have while mass production of steel had to wait until
been relatively rare. The development of iron produc- Bessemer’s invention of the converter in 1855,
tion dates back more than 3000 years (1500–1200 BC) which utilized a hot air draught from below to
when ferrous ores began to be smelted in the ancient remove carbon by reaction with oxygen. Until these
Near East civilizations (i.e., Iran, India, Mesopotamia, developments, steel was an expensive commodity
and Anatolia), which apparently coincided with a used only for niche applications where its combina-
shortage of tin for the production of bronze. In tion of properties was essential. The widespread pro-
Europe, iron began to be produced somewhat later, duction of steel lowered its cost such that it could be
1
in the period from the eight to the sixth century BC. used for an increasing number of applications, and
A feature of early iron production was the rela- eventually mild steel completely replaced wrought
tively limited temperature that the furnaces of the iron. Advances in the production of steel to further
time could achieve. In practice, this was not necessar- lower costs have continued as have alloy develop-
ily a disadvantage as the process involved the use of ments to further expand the use of ferrous materials.
wood charcoal to reduce iron ore in the solid state Nowadays, steel is a ubiquitous and essential compo-
leaving a porous mass of relatively pure solid iron (of nent of modern life.
variable composition) mixed with the ore residues
(slag) resulting in a ‘bloom.’ Subsequently, the skill
3.01.1.2 Iron–carbon Alloys
of the smith was required to repeatedly forge the hot
bloom in order to remove the majority of the slag 3.01.1.2.1 Phase diagram
inclusions, resulting in a product known as ‘wrought’ Carbon is generally present in steel at room temper-
(i.e., forged) iron. Subsequent adjustments in carbon ature as iron carbide (Fe3C or cementite). This phase
1696 Ferrous Metals and Alloys
is strictly metastable to decomposition to graphite although some specialized alloys may have composi-
and iron, however, the reaction is very sluggish at tions that lie outside these values. Steel also contains
lower carbon contents although graphite evidently elements such as silicon, phosphorus, and sulfur that
forms preferentially in, for example, grey cast irons. arise inevitably from the steel-making process and
The iron–carbon phase diagram (drawnwith cement- which may affect properties detrimentally unless lim-
ite as the stable phase) is reproduced in Figure 1. ited or controlled. For example, sulfur forms a low
The room temperature allotrope of unalloyed iron melting point eutectic with iron, and hence, limits the
is known as ferrite (a-iron) and has a body-centered ability of the steel to be processed at higher tempera-

cubic structure; above 910 C, this transforms to g-iron tures. Thus, plain carbon steels traditionally contain
or austenite (face-centered cubic) that, in turn, trans- sufficient added manganese (15–20 times that of sulfur)
forms to d-iron (also body-centered cubic) above to ‘mop-up’ the sulfur via the formation of MnS pre-

1394 C prior to melting at 1538 C. Alloying with cipitates. However, increased amounts of manganese
carbon lowers the melting point, eventually to the are also beneficial in, for example, solid solution hard-

Fe–C eutectic temperature of 1140 C forming effec- ening of ferrite, and improving the ductility and tough-
tively cast iron. Note that the solubility of carbon in ness of the alloy.

ferrite is extremely low (around 0.03% at 723 C and ‘Plain carbon steel’ may be defined as an alloy of
<0.01% at room temperature). iron with carbon where the total quantity of alloying
For practical purposes, iron may be defined as a elements is less than 2% by mass with compositional
material that contains carbon only up to its solubility limits of 0.6% for copper, 1.65% for manganese, 0.04%
limit in ferrite (i.e., <0.03% C by mass), while steel for phosphorus, 0.6% for silicon, and 0.05% for sulfur
contains carbon within its solubility limits in austen- and where no other elements are deliberately added in
ite (i.e., from 0.03% to 2.05% C by mass). In prac- order to provide a specific property or attribute. This
tice, most steels contain typically from 0.05% to somewhat convoluted definition is necessary to exclude
1.0% of carbon, with the majority of alloys lying at some low-alloyed steels (e.g., with small amounts of
the lower end of this scale (i.e., 0.05–0.5% carbon), chromium, cobalt, niobium, molybdenum, nickel,
δ+ liquid
1600
δ Liquid
1400
δ + γ γ
+liquid
1200
Fe C + liquid
3
Austenite
1000 γ
γ + Fe
3C Fe3C
α + γ +ledeburite +ledeburite
Cementite
800
Fe C
3
Ferrite α
600
400 Fe3C Fe C
3
+ledeburite
+ledeburite
+perlite
200
0
2 4 6
Perlite Ledeburite
(eutectoid) (eutectic)
Percent carbon (by mass)
Figure 1 Iron–carbon phase diagram (note ‘perlite’ is an alternative spelling of ‘pearlite’). Reproduced here under the
Gnu Free Documentation License from its original source at www.wikipedia.org.
Temperature (⬚C)
α + perlite
Corrosion of Carbon and Low Alloy Steels 1697
titanium, vanadium, etc.) that otherwise might be At carbon content below the eutectoid composition
classed as ‘plain carbon.’ In contrast, ‘low alloy steels’ (hypoeutectoid <0.8% C), ferrite will form first,
contain deliberate additions of alloying elements up while at higher carbon content cementite will form
to 10% by weight so as to develop enhanced mechan- first (hypereutectoid >0.8% C); both phases nucleat-
ical properties. Finally, ‘high alloy steels’ contain ing preferentially at the austenite grain boundaries.
more than 10% by weight of alloying additions and Pearlite (or perlite) is not a phase itself but it is rather
include materials such as stainless, tool, and maraging a two-phase mixture of ferrite (88%) and cementite
steels. Alloying additions may also be classed with (12%) that forms in alternating laths (strips); it
respect to their effects on the stability of the ferrite is so-called because of its characteristic pearl-like
and austenite phase regions. Thus, carbon, nitrogen, appearance. Figure 2 shows representative steel
manganese, nickel, and cobalt all tend to expand the microstructures of varying compositions. The indi-
austenite phase region (i.e., are austenite stabilizers), vidual laths of ferrite and cementite are often not
while silicon, chromium, molybdenum, niobium, easily resolved in commercial alloys using optical
vanadium are ferrite stabilizers. microscopy, however, they are visible in the higher
Carbon steels typically comprise more than 85% carbon content material, Figure 2(c).
of steels produced and shipped worldwide and are, Since the transformation is diffusion controlled,
therefore, by far the most frequently used iron– the spacing between the ferrite and cementite laths in
carbon alloy. It is usual to categorize steels by their pearlite varies as a function of cooling rate with slow
carbon content, but the specific boundaries are not (i.e., furnace) cooling giving the widest spacing and
well-defined. Generally, low-carbon steel (‘dead mild’ faster cooling giving closer spacing. Ferrite itself has a
steel) contains up to 0.15% carbon and 0.3–0.6% rather low yield stress, so the overall strength of the
manganese by mass. It has relatively low strength steel is dependent on the nature and spacing of sec-
but high formability, and is used typically in sheet ond phase particles, including the individual pearlite
and strip products. Mild steel contains from 0.15% to colonies as well as the pearlite lamellae and any other
0.3% carbon and is used in flat rolled products where phase that happens to be present.
higher strengths are required. For structural steel-
work, plates and rolled sections, forgings and stamp- 3.01.1.2.3 Nonequilibrium microstructures
ings of the manganese content can be increased to If steel is cooled faster than the rate at which carbon can
1.5% to improve toughness. Medium-carbon steels be rejected by diffusion from the austenite lattice, the
with 0.3–0.6% carbon and 0.60–1.65% manganese consequent formation of equilibrium iron carbide is
allow the use of quenched and tempered heat treat- partially or wholly suppressed. Under these circum-
ments with applications in axles, gears, forgings, rails, stances, the austenite cannot retain the excess of carbon
etc. Finally high-carbon steels containing 0.6–1.0% within its structure due to its thermodynamic instabil-
carbon and 0.3–0.6% manganese are used for high ity and must transform via an alternative mechanism.
strength applications such as springs andwires. Mate- At sufficiently low temperatures where essentially no
rials with carbon content greater than 1% are typi- significant diffusion of carbon can occur, the thermo-
cally insufficiently tough to be used for structural dynamic driving force is able to overcome the lattice
purposes, but find application where high hardness strains inherent in a diffusionless (shear) transforma-
and abrasion resistance is required, for example, as tion andmartensite, which is a distorted body-centered
machine tools, saw blades, etc. tetragonal structure, will form directly. At intermediate
temperatureswhere limiteddiffusion of carbon can still
3.01.1.2.2 Equilibrium microstructures occur, the bainite structure forms by transformation of
The iron–carbon phase diagram can be seen to be austenite to carbon-supersaturated ferrite with the
dominated by the pearlite eutectoid reaction (impor- subsequent diffusion of carbon and the precipitation
tant for steel) and the ledeburite eutectic reaction of carbides either in untransformed austenite (upper
(important for cast iron, and not considered further bainite) or within the ferrite (lower bainite). The
here). The pearlite reaction comprises the diffusion- detailed mechanisms of these transformations and
controlled decomposition of austenite to ferrite and their microstructures are complex and beyond the
iron carbide at the eutectoid composition (0.8% scope of this work, however, the concept is important

C by mass) and temperature (723 C): in understanding the properties of steel and particu-
larly how they may be altered beneficially by heat
g-Fe ! a-Fe þ Fe3C treatment.
1698 Ferrous Metals and Alloys
The advantage in rapid cooling (or quenching) colonies) and present in thin strips that are more
of steel is that carbon is then held uniformly in the likely to act as crack initiators. Figure 3(a) shows a
martensite phase in supersaturated solid solution. quenched martensitic structure, while Figure 3(b)
Martensite itself is very brittle and hard and, con- shows the same material but after aging (tempering)
sequently, has limited uses. However, when mar- at an elevated temperature in order to precipitate
tensite is reheated sufficiently, the retained carbon the carbide particles. The even distribution of
is able to diffuse and precipitate as fine carbides carbides is evident and compared with a pearlitic
that are relatively evenly distributed in the material. microstructure of similar carbon content, results
In contrast, in pearlitic steel, the strengthening in greatly increased fracture toughness at similar
phase is both unevenly distributed (i.e., in pearlite yield stress.
200 μm 200 μm
(a) (b)
200 μm
(c)
Figure 2 Pearlitic microstructures in steel (air cooled) (a) Hypoeutectoid (0.2% C; ferrite, light, with pearlite colonies, dark,
elsewhere in the structure). (b) Eutectoid composition (0.8% C; fully pearlitic). (c) Hypereutectoid (1.3% C; cementite has
nucleated on former austenite grain boundaries with pearlite elsewhere in the structure). Reproduced by kind permission of
Cochrane, R. F. University of Leeds and the DoITPoMS Micrograph Library (www.doitpoms.ac.uk).
100 μm 100 μm
(a) (b)
Figure 3 Annealed, compared with quenched and tempered, steel microstructures. (a) 0.31% C annealed showing pearlite
colonies of ferrite and cementite between grains of ferrite. (b) as (a) but quenched to form martensite then tempered to
precipitate a fine carbide distribution of cementite. Reproduced by kind permission of Cochrane, R. F. University of Leeds and
the DoITPoMS Micrograph Library (www.doitpoms.ac.uk).
Corrosion of Carbon and Low Alloy Steels 1699
3.01.1.3 Mechanical and Physical It is beyond the scope of this chapter to discuss the
Properties detailed effect of microstructure, composition, etc. on
the overall mechanical properties of steels and,
Alloying greatly decreases the thermal and electrical
hence, interested readers are directed to Llewellyn
conductivities of pure iron but has little effect on other 3 4
et al. and Bhadeshia et al. for further information.
physical properties such as the elastic modulus. Regard-
ing mechanical properties, pure iron (ferrite) is soft and
malleable but work-hardens rapidly, Table 1. Ferrite
3.01.1.4 Processing
can be solid–solution strengthened by either interstitial
(e.g., C, N, and P) or substitutional (e.g., Si and Mn) 3.01.1.4.1 Heat treatment
alloying additions. Silicon and manganese, which are The main purpose of heat treatment is to optimize
always present in iron at levels of 0.3–0.5%, provide the mechanical properties of a particular steel grade.
some solid–solution strengthening of the ferrite; phos- This typically involves a single or a series of heating
phorus gives much stronger solid-solution strengthen- and cooling operations designed to produce an opti-
ing but is not commonly added deliberately as it mum microstructure for the particular end use.
can greatly reduce toughness. Carbon and nitrogen These processes can be divided conveniently into:
have the greatest potential effect but have very low softening (or annealing), normalizing, hardening,
solubilities in ferrite. and tempering treatments.
As noted above, carbon-containing alloys (i.e., General process annealing is carried out on cold-
steels) are mainly strengthened by the formation of worked materials in order to relieve internal stresses
second phase carbide precipitates. In plain carbon and/or to soften them prior to further cold work. Full
steels, these comprise iron carbides that may form annealing is carried out by heating the steel into the
as pearlite colonies or, after quenching and temper- austenite phase region (if a hypoeutectoid steel), or
ing, as a fine carbide distribution in the microstruc- just above the eutectoid temperature (if hypereutec-
ture. In low alloy steels, the addition of elements such toid) followed by slow (e.g., furnace) cooling that
as molybdenum, titanium, vanadium, chromium, nio- results in a relatively coarse lamellar pearlite. Nor-
bium, and nickel either promote the formation of malizing involves the same heat treatment, however,
alloy carbides or control the formation of martensite the cooling is more rapid and carried out in air, which
and/or the favorable precipitation of iron carbides. results in a decrease in the size of microstructural
Like other body-centered cubic metals, steels are features (grain size and pearlite lamellae spacing) and
subject to a ductile-to-brittle transition and this may consequent increased final hardness.
occur close to ambient temperatures depending upon Hardening of hypoeutectoid steels involves heating
the type of steel, its alloying contents (including into the austenite phase region followed by rapid cool-
carbon, manganese, etc.), and how it has been pro- ing (or quenching). As the cooling rate is increased, the
cessed. Clearly, it is usually advisable for the ductile- formation of pearlite occurs at lower temperatures
to-brittle transition temperature to fall well below resulting in an increasingly finer lamellar structure,
operating temperatures in order to ensure adequate until at a critical cooling rate that depends on the alloy
fracture toughness during service. Key factors that content of the steel, martensite is formed directly.
influence the transition temperature include micro- Tempering of hardened steel is achieved by reheating
structure, carbide distribution, internal stress, and the to various temperatures below the austenite boundary
composition of the ferrite phase. with the intention to relieve internal stresses
Table 1 Generic properties for annealed ferrous alloys
Property Iron (>99.9% Fe) Carbon steel (0.15%C) Stainless steel (18%Cr, 10%Ni)
3
Density (Mgm ) 7.86 7.86 8.00
Elastic modulus (GPa) 200 200 195
1 1
Thermal conductivity (Wm K ) 76.2 20–65 16.2
6 1 1
Electrical conductivity (10 O m ) 11.2 6.23 1.45
Ultimate tensile strength (MPa) >200 385 565
Proof Stress at 0.2% strain (MPa) 70 285 210
Elongation (%) >40 35 55
Source: Data taken from Smithells Metals Handbook.
1700 Ferrous Metals and Alloys
induced by quenching and to permit the diffusion of For example, pits up to 1.25mm deep were found on
carbon retained in the martensite matrix in order to as-rolled steel specimens after 6months immersion in
6
precipitate a relatively even distribution of carbides. sea-water at Gosport. It follows that for most practi-

Tempering at 100–200 C is sufficient to relieve cal purposes where steel is exposed without a protec-
quenching stresses only. However, at temperatures tive coating, or indeed to achieve effective coatings

between 200 C and 450 C the martensite will adhesion to the substrate, it is essential to remove all
decompose into ferrite by precipitation of fine par- millscale either before putting components into ser-
ticles of carbide throughout the structure decreasing vice or prior to application of a protective coating.
yield strength but increasing toughness. At higher

temperatures still (i.e., 450–650 C) fewer but larger 3.01.1.4.2 Mechanical deformation
carbide particles are produced further increasing The vast majority of steel products are produced by
the toughness and reducing the strength. Micro- mechanical deformation either while ‘hot’ (i.e., above
structures formed in this way are known as tem- the recrystallization temperature of the alloy) or
pered martensites and vary in microstructure from ‘cold’ (i.e., below the recrystallization temperature);
relatively large ferrite grains containing second in the latter case, if continued processing is required,
phase carbides to small, fine-grained structures sim- periodic annealing is necessary in order to remove the
ilar to bainite. Generically, these steels are known as effects of work-hardening. Such processes include:
quenched and tempered. rolling (plate, strip, and bar products, etc.), forging,
The details for steel heat treatments are complex stamping, wire drawing, etc. Both hot and cold defor-
and those given above merely summarize the main mation will produce a varying degree of banding and
elements; further details can be found in Steel Heat texture in the resultant microstructure, which may
5
Treatment Handbook. In some cases, heat treatment result in properties that vary according to the defor-
alone cannot provide the desired structure, and mation direction, Figure 4. Nonmetallic second-phase
some form of thermo–mechanical treatment is neces- inclusions that originally derive typically from slag
sary. For example, some low alloy and microalloyed materials incorporated during the steel-making pro-
steels (high-strength low-alloy steels) develop excep- cess will tend to form stringers in the metal during
tional combinations of strength, toughness, and low rolling operations. These can form planes of weak-
ductile-to-brittle transition temperature by virtue of a ness in the steel, although modern clean steel making
controlled process combining a gradually decreasing technology has greatly reduced the volume fraction
temperature with simultaneous rolling of the steel. and distribution of such unwanted second phases.
After processing (rolling, forging, forming, etc.) at
elevated temperatures, a layer of oxide, called mill- 3.01.1.4.3 Metallurgical influences on
scale, inevitably would have formed on the metal corrosion
surface. The structure of millscale consists of three Generally, the process of manufacture has no appre-
superimposed layers of iron oxides in progressively ciable effect on the corrosion characteristics of car-
higher states of oxidation from the metal side out- bon steel. Slight variations in composition that
wards: ferrous oxide (FeO) on the inside, magnetite inevitably occur from batch to batch in steels of the
(Fe3O4) in the middle, and ferric oxide (Fe2O3) on the same quality have little effect with the exception of a
outside. The relative portions of the three oxides vary limited number of elements in a small (but impor-
with the processing temperatures. A typical millscale tant) number of applications. For example, the addi-
on 9.5mm mild steel plate would be 50 mm thick, tion of 0.2% of copper results in a two- to threefold
and contain 70% FeO, 20% Fe3O4, and 10% Fe2O3. reduction in the atmospheric corrosion rate com-
7,8
If millscale was perfectly adherent, continuous, and pared with a copper-free steel. Variation in other
impermeable, it would form a good protective coat- alloying additions in carbon steel affects the corro-
ing, but in practice millscale is liable to crack and sion rate to a marginal degree, the tendency being for
flake off exposing the underlying metal. During the rate to decrease with increasing content of car-
atmospheric exposure, the presence of millscale on bon, manganese, and silicon. Thus, steel containing
the steel may reduce the corrosion rate over compar- 0.2% of silicon rusts in air 10% slower than an
atively short periods, but over longer periods, the rate otherwise similar steel containing 0.02% of silicon.
tends to rise as the oxide flakes off the surface. In Otherwise, all ordinary ferrous structural materials,
water, severe pitting of the steel may occur if large that is, carbon and low-alloy steels, corrode at virtu-
amounts of millscale are present on the surface. ally the same rate when immersed in natural waters.
Corrosion of Carbon and Low Alloy Steels 1701
As shown in the historic data of Table 2, the process The main elements that alter the rate of corrosion
of manufacture and the composition of mild steel do of low alloy steels when immersed in natural waters
9
not affect its corrosion rate appreciably. are aluminum, copper, chromium, molybdenum, and
In carbon steels, the effect of microstructural nickel, but other additions, for example, manganese,
anisotropy caused by processing is also generally silicon, phosphorus, and sulfur, may have minor roles.
not significant. Thus, in seawater immersion tests, The action of some alloying elements can be benefi-
carried out to determine the effects of rolling direc- cial, neutral, or detrimental, depending upon whether
tion and tensile stress on the corrosion of a steel localized or uniform corrosion is being considered
10
containing 0.14% C, 0.47% Mn and 0.04% Si, and whether the steel is fully, partially, or intermit-
specimens were cut from plates parallel to and per- tently immersed. A large program of work between a
pendicular to the rolling direction. There was little number of research laboratories in Europe was car-
difference in general corrosion performance, although ried out over an extended period to study the influ-
pitting was somewhat worse on the plate cut parallel ence of alloying elements on corrosion of low alloy
11
to rolling. steel and the main findings, which are summarized
For low alloy steels generally under immersed in Table 3, are still relevant.
conditions, alloying additions of at least 3% (e.g., of From a consideration of Table 3, steel containing
chromium, nickel, etc.) are necessary to obtain any copper and phosphorus might be chosen for its resis-
marked improvement in the corrosion-resistance. tance to corrosion in the critical tidal and splash zone.
400 μm 200 μm
(a) (b)
Figure 4 Directionality in microstructure after mechanical deformation. (a) 0.2% C steel after hot rolling showing banded
carbidemicrostructure. (b) 0.6%C steel after cold wire drawing showing highly deformed grain structure Reproduced by kind
permission of Cochrane, R. F. University of Leeds and the DoITPoMS Micrograph Library (www.doitpoms.ac.uk).
Table 2 Corrosion rates of mild steels in seawater, total immersion for 203 days at Plymouth
a 1
Type of steel Analysis (%) Average general penetration (mmyear )
C Mn P S
Basic Bessemer, rimming ordinary 0.05 0.64 0.06 0.02 0.143
High phosphorus 0.03 0.31 0.14 0.04 0.143
High phosphorus and sulfur 0.03 0.30 0.10 0.07 0.148
Open-hearth, rimming ordinary 0.13 0.33 0.03 0.03 0.143
From haematite pig 0.06 0.32 0.01 0.03 0.140
Open-hearth, killed ordinary 0.10 0.35 0.03 0.02 0.140
From haematite pig 0.11 0.34 0.01 0.03 0.136
Open-hearth, killed ordinary 0.22 0.71 0.03 0.03 0.143
From haematite pig 0.21 0.58 0.02 0.03 0.158
a
The copper contents of the steels, which were supplied through the courtesy of l’Office Technique pour l’Utilisation de l’Acier (France),
varied from 0.03 to 0.11%. The killed steels contained 0.04% AI and 0.1% Si.
Source: After Hudson, J.C. J. Iron Steel Inst. 1950, 166, 123.