Corrosion Resistance of Heat-Treated NST 37-2 Steel in Hydrochloric Acid Solution

doi:10.4236/jmmce.2013.11001

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Journal of Minerals and Materials Characterization and Engineering, 2013, 1, 1-7

http://dx.doi.org/10.4236/jmmce.2013.11001 Published Online January 2013 (http://www.scirp.org/journal/jmmce)

Corrosion Resistance of Heat-Treated NST 37-2 Steel in

Hydrochloric Acid Solution

David Abimbola Fadare*, Taiwo Gbolarumi Fadara

Mechanical Engineering Department, University of Ibadan, P.M.B 1, Ibadan, Nigeria

Email: *[email protected]

Received July 20, 2012; revised August 27, 2012; accepted September 10, 2012

ABSTRACT

Corrosion of metal components constitutes a major challenge in many engineering systems, with appropriate design,

proper material selection, and heat treatment as commonly used control strategies. In this study, the corrosion behaviour

of heat-treated (annealed, normalised, hardened, and tempered) NST 37-2 steel in three concentrations (1.0, 1.5 and 2.0

M) of hydrochloric acid solution was investigated using weight loss and electrode-potential methods. Results showed

that corrosion rate increased with increase in acid concentration. The decreasing order of corrosion resistance was

Tempered > Annealed > Normalised > Hardened > Untreated. The surface pictures of the heat-treated and untreated

samples showed uniform and pitting corrosion with the latter becoming more pronounced as concentration increased.

Keywords: Heat Treatment; Corrosion Resistance; Hydrochloric Acid; NST 37-2 Steel

1. Introduction

Corrosion of metal components has been recognized as a

major problem in many engineering applications. Failure

of engineering systems due to corrosion is as common as

failure due to mechanical causes such as brittle fracture

and fatigue. The annual cost of corrosion for the US and

UK has been estimated at about $70 billion accounting

for about 4% of the national Gross Domestic Product

(GDP) [1]. This estimate may well be higher in some less

developed countries, although it is probably less in the

least developed countries [2]. About 15% - 25% of the

annual global steel production is estimated to be used for

repair or replacement of damages due to corrosion [3].

Corrosion failures are minimized by appropriate design,

proper material selection, control of metallurgical struc-

ture through heat treatment and use of inhibitors. Of all

these control strategies, heat treatment is most commonly

used due to its cost effectiveness.

Annealing, normalising, hardening and tempering are

the most commonly used heat treatment of carbon steels.

Annealing is most frequently applied in order to soften

carbon steel materials and refines its grains due to ferrite-

pearlite microstructure [4,5]. In normalising, the material

is heated to the austenitic temperature range and this is

followed by air cooling to obtain a mainly pearlite matrix,

which results into increase in strength and hardness [6].

In hardening, the material is heated to a temperature high

enough to promote the formation of austenite, held at that

temperature until the desired amount of carbon has been

dissolved and then quench in oil or water at a suitable

rate to obtain 100% martensite with maximum yield

strength, but it is very brittle and thus quenched steels are

used for very few engineering applications. By tempering,

the properties of quenched steel are modified to decrease

hardness, increase ductility and impact strength mode-

rately, resulting microstructures are bainite or carbide

precipitate in a matrix of ferrite depending on the tem-

pering temperature.

Investigations on the effects of heat treatment on the

corrosion behavior of different carbon steel materials

have been reported by many researchers. In particular,

the effects of heat treatment on corrosion behaviour of

AISI 403 martensitic stainless steel [7], AISI 420 mar-

tensitic stainless steel [8], AISI 52100 steel [9], 304 L

stainless steel [10], 14Cr-3Mo martensitic stainless steel

[11] have been reported. Corrosion behavior of different

carbon steel materials in different media typical of the

in-service environments has also been investigated by

many researchers. Acidic environment are generally en-

countered in many industrial processes. Acid solutions

are used especially for the removal of undesirable scales

and rust from carbon steel materials. Particularly, hydro-

chloric acid are widely used for the picking processes of

metals [12]. Corrosion behavior of numerous grades of

carbon steels in hydrochloric acid solutions has been

widely studied by many researchers [13-18]. However,

there are numerous studies on the effect of heat treat-

ment of carbon steel grades on the corrosion behaviour in

*Corresponding author.

D. A. FADARE, T. G. FADARA

hydrochloric acid solutions. Strobel et al. [3] reported the

effect of the heat treatment on the corrosion resistance of

a martinsitic stainless steel CA6NM in HCl solutions,

while Al-Quran and Al-Itawi [19] reported an increase in

the corrosion resistance of chromium-nickel alloy steel in

0.1 M HCl solution due to intermediate spheroidal an-

nealing.

NST 37-2 steel is a commercially available carbon

steel grade locally produced by the Delta Steel Company

(DSC) located at Ovwian-Aladja, Warri, Delta State,

Nigeria. DSC is the only operational integrated steel plant

producing steel from basic raw in the south of the Sahara

and West Africa [20]. Its steel production capacity is

expected to hit 2.4 Million Tonnes Per Annum (MTPA)

by 2015 from the current 1.0 MTPA [20]. About 320,000

tonnes per annum of the steel produced is rolled into

light sections such as: flats, channels, angles, I-Beams

and square bars, and also as plain and ribbed bars of

different dimensions [20]. NST 37-2 steel is widely used

as construction materials in building, road, and bridge

constructions, and as well as in fabrication of machinery

and manufacturing of machine components [21]. NST

37-2 steel constitutes one of the mostly used steel in the

Nigerian construction industry for reasons of its ver-

satility, strength, toughness, low cost and wide avail-

ability [22,23].

Although, investigations on the heat treat ability and

mechanical property enhancement of NST 37-2 steel are

few in literature. The effect of heat treatment on the mi-

crostructure and mechanical properties has been reported

by Fadare et al. [24], while the effect of heat treatment

on the fatigue behaviour has been reported by Malomo et

al. [25]. Studies on the machinability of NST 37-2 steel

have also been reported by Fadare and Ashafa [26,27].

Previous studies on NST 37-2 steel were focused

mainly on the microstructure and mechanical property

improvement. However, the effect of heat treatment on

the corrosion behavior has not been reported. Hence, the

objective of this study is to investigate the effect of heat

treatment (annealing, normalising, hardening, and temper-

ing) on the corrosion resistance of NST 37-2 steel in 1.0,

1.5 and 2.0 M HCl solutions.

2. Materials and Method

2.1. Sample Preparation

Samples of hot-rolled, 16 mm diameter, NST 37-2 steel

bars were purchased from a local market in Lagos, south-

western Nigeria. The chemical composition of the steel

sample as determined by optical emission spectropho-

tometer is given in Table 1, while the mechanical pro-

perties and microstructure of the as-received sample are

given in Table 2 and Figure 1, respectively. Cylindrical

coupons (Q16 × 25) mm dimensions were machined

from the sample and subjected to four commonly used

industrial heat treatment processes for carbon steels: an-

nealing, normalising, hardening, and tempering in ac-

cordance to American Society of Materials (ASM) In-

ternational Standards [28]. The heat treatment conditions

applied are listed in Table 3. The heat treated and the

as-received (untreated) samples were washed in HCl so-

lution and rinsed in distilled water to remove scale for-

mulation on the samples prior to the corrosion test.

Table 1. Chemical composition of the as-received NST 37-2

steel.

C (%)Si (%)S (%)P (%)Mn (%) Ni (%) Cr (%)Mo (%)

0.34220.20200.01080.00490.7374 0.0067 0.01040.0011

Zn (%)As (%)Sn (%)Al (%)Fe (%) Cu (%) V (%)

0.00130.00050.00220.001398.6824 0.0033 0.0006

Table 2. Mechanical properties of the as-received NST 37-2

steel.

Properties Average Value

Yield Strength (MN/m2) 245.41

Tensile Strength (MN/m2) 342.33

Elongation (%) 18.48

Reduction in Area (%) 15.05

Young Modulus (GPa) 198.50

Hardness (BHN) 48.50

Density (g/cm3) 8.15

Table 3. Heat treatment conditions.

Condition AnnealedNormalized HardenedTempered

Temperature (˚C)910 910 910 450

Holding Time (min)90 90 40 90

Cooling MediumFurnaceAir Water Air

Figure 1. Microstructure of the as-received NST 37-2 steel

(400×) (Source: Fadare et al. [24]).

D. A. FADARE, T. G. FADARA 3

2.2. Corrosion Test

Electrolyte solutions consisting of 1.0, 1.5, and 2.0 M

HCl were prepared with distilled water. The heat-treated

and untreated (control) coupons were weighed using a

digital balance with accuracy of ±0.0001 g. Copper wire

(electrical conductor) was brazed on the surface of each

coupon and used as working electrode, while Ag/AgCl(s)/

KCl saturated (aq) half-cell was used as the reference

electrode. Complete immersion test was carried out using

mass loss and electrode potential methods for 120 hours

(five days) duration on a corrosion rig consisting of nine

(9) corrosion cells. Each corrosion cell consisted of a

beaker containing 400 mL of the electrolyte solution, test

coupons with conducting wire, reference electrode, glass

tube for air bubbles circulation and a plastic lid (Figure

2). The test solution temperature was maintained at room

temperature (28˚C - 32˚C). The electrode potential of the

cell was taken at 6 hours interval using a digital multi-

meter at a sweep rate of 20 mV, while mass loss of the

coupon was measured daily. Compressed air was bubbled

through the electrolyte via glass tube suspended at the

centre of each cell for purposes of oxygen circulation and

agitation of the medium. The heat-treated and untreated

coupons were placed together in random order in each of

the three (3) cells containing difference concentrations

(1.0, 1.5 and 2.0 M) of the test solution.

The experiment was replicated in triplicate given rise

to nine (9) cells. After each exposure time the coupons

were removed from the cells, properly cleaned in dis-

tilled water, dried with cotton wool and then reweighed

to determine the mass loss. The average mass loss was

determined and the corrosion rate in millimetre penetra-

tion per year (mm/y) was calculated using the following

relationship [18]:

87.6 loss

corrosionrate









 



(1)

where Wloss = mass loss (mg), a = total exposed area of

coupon (cm2), t = duration of immersion (hours) and ρ =

material density (g/cm3). Density for NST 37-2 steel (ρ =

8.15 g/cm3) as given in Table 2 was used in the compu-

tation.

Figure 2. Experimental set-up.

3. Results and Discussion

The electrode potential (mV) and corrosion rate (mm/y)

of heat-treated and untreated NST 37-2 steel coupons

immersed in 1.0, 1.5 and 2.0 M HCl solutions are shown

in Figure 3 and Table 4, respectively. Generally, the

electrode potential of both heat-treated and untreated

samples increased linearly with increase in acid concen-

tration and time of immersion in test solution, while the

corrosion rate increased linearly with increase in concen-

tration but varied nonlinearly with time of immersion in

test solution. The observed linear increase in both elec-

trode potential and corrosion rate with increasing acid

concentration of the test solution can be attributed to in-

crease in ion exchange capacity of acid at high concen-

trations.

(a)

(b)

(c)

Figure 3. Variation of electrode potential (mV) of heat-

treated and untreated NST 37-2 steel in 1.0 M (a), 1.5 M (b)

and 2.0 M (c) HCl solutions with exposure time.

D. A. FADARE, T. G. FADARA

Table 4. Corrosion rate (mm/y) of untreated and heat-treated NST 37-2 steel immersed in different concentrations of HCl

solution.

Corrosion Rate (mm/y)

Concentration Time (Hours)

Untreated Hardened Normalized Annealed Tempered

0 0.00 0.00 0.00 0.00 0.00

24 27.90 25.20 20.70 19.80 17.10

1.0 48 22.05 18.45 15.30 13.95 11.70

72 20.40 18.00 15.30 10.50 9.90

96 18.22 16.65 14.62 10.57 7.65

120 16.92 14.22 12.96 10.26 6.66

0 0.00 0.00 0.00 0.00 0.00

24 49.49 39.59 33.29 30.60 27.90

1.5 48 31.05 26.10 23.85 20.25 17.55

72 24.90 21.60 19.80 17.70 14.40

96 21.82 18.67 16.42 14.85 12.15

120 20.16 18.54 16.38 14.04 12.78

0 0.00 0.00 0.00 0.00 0.00

24 68.39 61.19 48.59 43.19 39.59

2.0 48 44.99 37.34 31.94 25.65 22.95

72 35.99 28.50 26.40 21.30 18.90

96 31.94 25.42 22.72 19.57 17.77

120 29.34 27.00 25.20 19.08 17.64

The electrode potential tended to be more positive (an-

odic) with increase in concentration of test solution and

time of immersion. The increased positivity led to corre-

sponding increase in the electromotive force (corrosion

current) between the working (anode) and reference (ca-

thode) electrodes and hence accelerated corrosion of the

anode (test material). It can be observed that the elec-

trode potential of the untreated sample tended to be more

positive consistently in all the three acid concentrations

of the test solution followed by the heat-treated samples

in decreasing order: hardened, normalized, annealed and

tempered. On the other hand, the corrosion rate of the

heat-treated and untreated NST 37-2 steel coupons in dif-

ferent concentrations of HCl solution are shown in Ta-

ble 4.

Both heat-treated and untreated samples varied non-

linearly with time of immersion in the test solution. The

corrosion rates increased rapidly during the first 24 hours

(1 day) of immersion, after which it began to decline

progressively with time. This nonlinear trend in the cor-

rosion curve may be attributed to the surface passivation

of the carbon steel, in which the formation of rust on the

attacked region of the steel tends to form a protective

layer on the surface, thus protecting the parent material

from further attack. Since rust formation is known to be

water and oxygen permeable, the passivity of the surface

tends to breakdown progressively, hence resulting in re-

duction in corrosion rate of the material. Similarly, non-

linear trends in corrosion-time curves of 18/8 stainless

steel and nickel-plated low carbon steel in cassava fluid

[29], and mild steel and SS 304L in presence of dissolved

copper [30] have been reported by other researchers. In

contrast, linear relationship has been reported for chro-

mium-nickel alloy steel in 0.1 M HCl solution [19]. The

discrepancy in these observations may be attributed to

disparity in the elemental composition of the parent alloy

materials.

Similarly, in same order with the observed electrode

potential trends, the untreated sample showed the highest

corrosion rate in all the three acid concentrations of the

test solution, followed by the heat-treated samples in

decreasing order: hardened, normalized, annealed and

tempered, thus indicating that the heat treatment pro-

cesses tend to improve the corrosion resistance of NST

37-2 steel in HCl solutions. Hence, the microstructural

evolution of the material during the heat treatment pro-

cesses played a fundamental role on the corrosion be-

havour of the material. The effects of these heat treat-

ment processes (annealing, normalising, hardening, and

tempering) on the microstructural evolution and me-

chanical properties of the material have been reported

earlier [24]. The ferritic + pearlitic matrix microstructure

of the as-received (untreated) sample tends to increase

the dissimilar metal composition of the material, leading

to galvanic corrosion and hence, the accelerated corro-

sion rate observed in the untreated sample. The marten-

sitic matrix of the hardened sample tends to be more cor-

rosion resistant than the dual-phase matrix of the un-

treated sample. Similarly, Keleştemur and Yıldız [31] has

reported that intercritical annealing heat treatments of

D. A. FADARE, T. G. FADARA 5

dualphase steel embedded in concrete has a good corro-

sion resistance which increased with increase amount of

martensite microstructure in the steel. The single-phase

pearlitic and ferritic matrix of the normalised and an-

nealed samples respectively tend to be more corrosion

resistance compared to the dual-phase of the untreated

sample, while the dual-phase martensitic and ferritic ma-

trix of the tempered sample tends to be more corrosion

resistance than both the single-phase martensite and fer-

rite microstructure of the hardened and annealed samples

respectively. Lucio-Garcia et al. [32] has shown that the

steel with a martensitic microstructure had the highest

corrosion rate; up to one order of magnitude higher than

the corrosion rate for steels with a ferritic and bainitic

microstructure, whereas the steel with the ferritic micro-

structure showed the lowest corrosion rate.

The surface scan of the corroded surface (Figure 4)

revealed that both uniform and pitting corrosion occurred

Concentration of HCl

Heat Treatment 1.0 M 1.5 M 2.0 M

Untreated

Hardened

Normalised

Annealed

Tempered

Figure 4. Surface scan of the corroded heat-treated and untreated NST 37-2 steel coupons different concentrations of hydro-

chloric acid solution.

D. A. FADARE, T. G. FADARA

on the samples.

4. Conclusion

The effect of heat treatment on corrosion resistance of

NST 37-2 steel in HCl solutions has been investigated.

The analysis showed that both corrosion rates and elec-

trode potentials of the untreated sample had the highest

corrosion rate and shifted more to the positive values as

the concentration of acid was increased. For the heat

treated samples, the corrosion rate in the test solutions

ranked in decreasing order: hardened, normalized, an-

nealed and tempered.

5. Acknowledgements

We are grateful to an anonymous referee for helpful

comments. We also wish to thank Kim Humphreys for

English editing. All errors are ours.

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