corrosion of mild steel by coal and iron ore

Corrosion of mild steel by coal and iron ore

C.P. Gardiner *, R.E. Melchers

Department of Civil, Surveying and Environmental Engineering, The University of Newcastle,

NSW 2308, Australia

Received 2 February 2001; accepted 27 February 2002

Abstract

The variation of corrosion rate of mild steel as a function of the quantity of moisture of coal

and iron ore is investigated. Two types of black coal sieved to three different size fractions up

to 2360 lm particle diameter and one type of iron ore of 600–1180 lm particle diameter were

tested. A pronounced increase in corrosion rate was observed at a moisture content between

60% and 80% of the maximum water holding capacity for all samples. The corrosion rate was

also observed to increase with decreasing particle size distribution. Parameters influencing the

corrosion rate of coal and iron ore, as identified in this study and others, are discussed.

Corrosion rates were measured inside a bulk carrier (ship) cargo hold that carried coal and

iron ore cargo on successive voyages and are shown to be similar to those measured in the

laboratory. The relevance of laboratory experiments using fine-particle samples for simulating

the corrosion of steel exposed to coal and iron ore cargo is discussed.

� 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Mild steel; Weight loss

1. Introduction

Steel structures that come into contact with coal and iron ore can undergo sig-nificant corrosion. Examples include ship’s cargo holds [1] and equipment in un-dergound mines [2], coal processing and steel-making facilities [3,4]. Althoughprotection measures are often used, they may not always be effective. For example, in

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Corrosion Science 44 (2002) 2665–2673

* Corresponding author. Address: Defence Science and Technology Organisation (DSTO), P.O. Box

4331, Melbourne, Vic. 3001, Australia. Tel.: +61-3-9626-8442; fax: +61-3-9626-8999.

E-mail address: [email protected] (C.P. Gardiner).

0010-938X/02/$ - see front matter � 2002 Elsevier Science Ltd. All rights reserved.

PII: S0010-938X(02 )00063-X

mail to: [email protected]

the lower region of a bulk carrier cargo hold protective coatings deteriorate rapidlydue to contact with cargo handling equipment. Thus, it is useful to have an un-derstanding how corrosion proceeds in the presence of coal and iron ore.

Experimental work investigating the corrosion of mild steel by coal and iron orehas focused mainly on the corrosivity of the fluid that resides within the media [3,5],or has been in contact with the media, for example, minewaters [6–8]. These studieshave shown corrosion rate to increase with increasing chloride and sulphate con-centration and decreasing electrolyte pH levels. It is known that increasing moistureincreases the rate of abrasive–corrosive wear of coal pulverising equipment [9].Furthermore, the quantity of chloride and sulphate ions removed from wet coalincreases with decreasing particle size [10]. However, the influence of the quantity ofmoisture and particle size distribution on corrosivity alone does not appear to havebeen investigated. This study investigates the effect of these two parameters for coaland iron ore.

Laboratory corrosion experiments using fine-particle black coal from two dif-ferent producers and one iron ore type with a range of particle sizes and moisturecontents are described in Section 2. Section 3 describes experiments involving thecorrosion of mild steel samples placed within a bulk carrier (ship) cargo hold andexposed to coal and iron ore cargo. Based on the results reported in this study andothers in the literature, parameters influencing the corrosion rate of coal with neutraland acidic free moisture are then discussed in Section 4. The relevance of a criticalmoisture level for maximum corrosion and also the affect of particle size are ad-dressed.

2. Laboratory experiments

2.1. Procedure

To examine the effect of particle size on the magnitude of corrosion rate the twocoal types (A and B) were each sieved to three different size fractions, 0–600, 600–1180 and 1180–2360 lm. The iron ore was sieved only to a size fraction of 600–1180lm.

Mild steel specimens were cut to 45 � 47:5 � 1:6 mm and exposed on one side.The composition of the mild steel is shown in Table 1. The underside and edges weresealed with a paraffin gel. Each mild steel specimen was prepared by pickling in asolution of hydrochloric acid and inhibitor, according to designation C3.5 of ASTMG 1–90. The specimens were then rinsed with de-ionised water and rinsed in acetone,air-dried, and weighed before use. The underside and edges of each specimen were

Table 1

Composition of the mild steel specimens

C P Mn Si S

0.23 0.04 max 0.60–0.90 0.35 max 0.04 max

2666 C.P. Gardiner, R.E. Melchers / Corrosion Science 44 (2002) 2665–2673

then sealed with a paraffin gel and then placed in the bottom of a rectangular plasticcontainer with base dimensions 50 � 50 mm.

The samples of porous media were prepared by dividing each sieved sample typeinto seven portions. Each portion was then oven dried at 60 �C. Dryness was checkedby re-weighing until there was no further weight loss. Each of the seven subsampleswere then set to the appropriate moisture content by adding de-ionised water. Theappropriate true moisture value was calculated after determining the maximumwater holding capacity of each specimen gravimetrically.

The steel specimens were then covered with a porous media sample to a depth of18.0 mm and placed in a 50 l insulated cooler, which was subsequently closed. Theinsulated coolers were then subsequently placed in a constant temperature room(maintained at 15 �C) for six weeks. As a control, plastic containers holding a sampleof each type of coal and iron ore without steel specimens were also placed in eachinsulated cooler. These were used to periodically measure the weight loss of theporous media that occurred due to a decrease of moisture as each sample ap-proached equilibrium with the water vapour pressure in each insulated cooler.Furthermore, two mild steel specimens without a porous sample were exposed to testthe ‘‘atmospheric corrosivity’’ of the exposure environment. After six weeks thespecimens were removed and cleaned. The weight loss was then measured and theaverage corrosion rate for the six week exposure period was calculated.

2.2. Results

Measurement of the mass loss of the porous samples without a steel specimenrevealed that the moisture loss for all sample types was in the range of 0.052–0.085wt% per day. Therefore, the porous sample evaporated 2–3 wt% of moisture over thesix week testing period. This introduces a degree of uncertainty when estimatingvalues of critical moisture.

Exposure of the two specimens without a covering of porous media revealed theenvironment to be uncorrosive. The mass loss was negligible and corrosion was notvisible. However the test samples covered with porous media were observed tocorrode relatively evenly and pitting corrosion was not evident. A thin corrosionproduct layer was observed on all specimens.

Corrosion rate results obtained for coal A are shown in Fig. 1. The corrosion ratesfor the 0–600 lm size fraction were not obtained due to an error in retrieving thesamples. The moisture is expressed as a fraction of the maximum water holdingcapacity determined on a wet-mass basis:

M ðwt%Þ ¼ 100Mf

Mf þMs

ð1Þ

M ðwt%Þ ¼ 100qfVf

qfVf þqsVsð1 � eÞ

e

ð2Þ

where V is the volume, M is the mass, q is the density, e is the porosity, and thesubscripts, f and s, refer to fluid and solid, respectively.

C.P. Gardiner, R.E. Melchers / Corrosion Science 44 (2002) 2665–2673 2667

The theoretical maximum water holding capacity (MWHC) for a packing ar-rangement represented by a given value of porosity is obtained by setting the fluidvolume equal to the void volume in Eq. (2). The moisture expressed as a percentageof the maximum water holding capacity is then given by Eq. (3):

M ð% MWHCÞ ¼ 100M ðwt%ÞMWHC

ð3Þ

It is evident from Fig. 1 that the pattern of corrosion that has been observed for steelin sand and soil also occurs for fine-particle coal. The maximum corrosion rateoccurs in the range of 60–80% MWHC. This is comparable with the results observedin the literature for soil (65% [11]) and sand (�75% [12]). The magnitude of thecorrosion rate was higher for the finer grained coal sample, which also agrees withthe soil and sand experiments of Gupta and Gupta [11] and Wang [12]. Note that asmall amount of corrosion was measured for the sample that did not have moistureadded. This is most likely due to the inherent moisture of the coal samples.

Increasing corrosion rate with decreasing particle size is also clearly evident forcoal B, as shown in Fig. 2. The corrosion rate increased only slightly from zero to60% MWHC. It is unclear why the corrosion rate for coal A increased more thancoal B over the lower range of moisture. Nevertheless, the relatively small change incorrosion rate over this range is expected, as discussed in Section 4. The maximumcorrosion rate for coal B is observed to occur in the range 70–80% MWHC.

Results for the iron ore sample are shown in Fig. 3. The maximum corrosion ratealso occurs at a moisture content in the range 70–80% MWHC.

It is noted that the moisture value corresponding to the maximum corrosion rateshown in Figs. 1–3 is not necessarily the point of critical moisture. This is due to theuncertainty caused by the lack of experimental data points and the 2–3 wt% moistureloss over the duration of the experiment. However, despite these limitations, it isevident from the measured data that a characteristic critical moisture does exist forfine-particle coal and iron ore.

Fig. 1. Measured corrosion rate as a function of moisture for coal A.


3. Shipboard experiments

Corrosion rate measurements were undertaken in the lower region of cargo hold 6in an Australian registered cape-size bulk carrier. Mild steel samples were cut fromthe same plate that was used for the laboratory experiments. The samples were fixedon the 13/8/97 in Newcastle, NSW, Australia, prior to loading coal cargo. The vesselthen carried coal to Japan, and sailed in ballast to Western Australia before carryingiron ore to Port Kembla, NSW, Australia, where the samples were removed on the12/10/97.

Samples were fixed in a vertical position on the inner surface of the access ladderlocated on the forward bulkhead stool of cargo hold 6. It was necessary to isolate thesamples from the access ladder to avoid galvanic effects. This was achieved by fixing

Fig. 2. Measured corrosion rate as a function of moisture for coal B.

Fig. 3. Measured corrosion rate as a function of moisture for iron ore.


the samples to pvc backing plates which were themselves placed on the surface of theaccess ladder. Three samples were attached to each main backing plate in two stages.Firstly, a sample was glued to a slightly larger piece of pvc plate. Each pvc plate, withattached sample, then slid into the main backing plate. This assisted the removal ofthe specimens at the conclusion of the experiment.

Each coupon was 100 � 47:5 � 1:6 mm and was prepared and cleaned using theprocedure described for the laboratory results. Twelve samples, placed on fourbacking plates were fixed to the lower stool. Eleven samples were retrieved at theconclusion of the test and one sample was lost from the pvc fixture. The corrosionrate measured for each sample is given in Table 2. The mean and standard deviationof corrosion rate was 0.10 and 0.014 mm/yr, respectively.

4. Discussion

It is clearly evident that the corrosion rate of mild steel exposed to fine-particlecoal is dependent on the particle size (Figs. 1 and 2). The corrosion rate increases asthe mean particle size is reduced. Additionally, the corrosion rate is dependent on themoisture content of the coal and iron ore (Figs. 1–3). The materials tested had pH-neutral free moisture. However, the free moisture in some types of coal can be acidic.Factors influencing the pH level of free moisture in coal are now outlined. Then,based on the measurements reported herein and in the literature, a description of thecorrosion process for coal with pH neutral and acidic free moisture is given.

The pH level of free moisture in coal is dependent on the relative rates of both theacid and alkali forming reactions, given by Eqs. (4) and (5), respectively [13]:

FeS2 þ 72O2 þ H2O ! Feþþ þ 2SO2�

4 þ 2Hþ ð4Þ

CaCO3 þ Hþ ! HCO�3 þ Caþþ ð5Þ

Evidently, the pH of the free moisture is a function of the relative supply of pyrite(FeS2) and carbonates (mostly CaCO3) from the coal.

When the pH is neutral the reduction of oxygen becomes the cathodic reaction(Eq. (6)) in the corrosion process.

O2 þ 2H2O þ 4e� ! 4OH� ð6ÞCorrosion studies of pH-neutral underground minewaters did not reveal a cor-

relation between corrosion rate and chloride and sulphate concentration [7,8]. Thiswas to be expected as the mild steel samples were immersed in stagnant solutionssuch that the corrosion rate was low and controlled by the diffusion of oxygen.

Table 2

Measured corrosion rates for shipboard exposure in the cargo hold of a bulk carrier

Specimen 1 2 3 4 5 6 7 8 9 10 11 12

Corrosion rate

(mm/yr)

0.10 0.10 0.13 0.10 0.09 0.10 0.10 0.12 0.09 0.12 0.09 n/a


Immersion experiments in aerated minewaters showed the corrosion rate of mildsteel to increase with increasing chloride and sulphate concentration before de-creasing as the concentration increased further [14]. The peak was attributed to theincreasing salt concentration reducing the solubility of oxygen in the solution. Thesefindings for stagnant and aerated immersion corrosion conditions have been welldocumented in the corrosion science literature. They are also relevant for the cor-rosion of mild steel covered by coal (or other porous media) but in this case the levelof aeration is determined by the quantity of moisture in the coal sample. This isexplained as follows.

The existence of a maximum corrosion rate at a critical moisture content suggeststhat there must be a ratio of water to air in the media at which corrosion reactionsare maximised [11]. This has been explained in terms of the continuity of the fluidand air phases and their subsequent influence on the magnitude of the electricalconductivity and oxygen diffusion coefficient [15]. At low levels of moisture the airphase is continuous within a coal (or iron ore) sample and the underlying steel isefficiently aerated such that the diffusion of oxygen to the corroding surface does notlimit the rate of corrosion. The influence of the ohmic overpotential then dominatesand the corrosion rate is a function of the electrical conductivity of the sample[11,15]. This is in turn related to the concentration of chloride and sulphate ionswithin the free moisture. Alternatively, at high levels of moisture a continuous waterphase is established and the corrosion rate becomes limited by the cathodic reductionof oxygen [12,15]. The concentration of chloride and sulphate ions then have aminimal influence. In summary, for coal or iron ore with approximately neutral pH(usually the case for iron ore), the corrosion rate is a function of the quantity ofmoisture, particle size, and at low moisture levels the concentration of chlorides andsulphate anions also affect the corrosion rate.

When the free moisture is acidic the cathodic reaction is the hydrogen evolutionreaction:

2Hþ þ 2e� ! H2 ð7Þ

Due to the consumption of hydrogen ions in the cathodic reaction it may be expectedthat the rate of corrosion is dependant on the level of pH. Spero and Flitt [5]measured the corrosion rate of mild steel immersed in de-aerated acidic coalleachates and found the following relationship between corrosion rate, I and pH:

logðIÞ ¼ a� bpH ð8Þ

where, a and b are constants.The corrosion rate increased with increasing chloride and sulphate concentration

that could be represented with a bi-logarithmic equation.Despite these observations on the affects of pH level, chloride and sulphate con-

centration, the influence of the quantity of moisture and particle size in coal sampleswith acidic free moisture does not appear to have been investigated. However, theinfluence of the quantity of moisture is postulated as follows. Firstly, it has beenshown experimentally [11] and theoretically [15] that for porous media withhigh values of moisture, greater than approximately 60% MWHC, the electrical


conductivity remains approximately constant with increases in moisture. The diffu-sivity of oxygen does vary over this moisture range, however this is not relevant forcorrosion in an acidic solution. Alternatively, as noted above, the electrical con-ductivity is strongly dependant on the quantity of moisture at lower levels ofmoisture (less than approximately 60% MWHC). Assuming the corrosion processin acidic conditions to be at least partially under anodic control, the contribution ofthe ohmic overpotential will be important. Hence, the electrical conductivity, andtherefore the quantity of moisture will affect the rate of corrosion. Therefore, as afirst approximation, for mild steel covered by coal (or iron ore) containing acidic freemoisture the rate of corrosion will increase with increasing quantity of moisture upto a certain point (approximately 60% MWHC) and then remain reasonably con-stant with further increases of moisture.

The experimental evidence from this study and others [11,12] suggests that thecorrosion rate is maximised at a range of moisture common to all samples, that is60–80% MWHC. Therefore, to assess the corrosivity of fine-particle coal or iron ore(or soil and sand) it is beneficial to express the moisture as a percentage of itsmaximum water holding capacity as opposed to the traditional scale of volumetric ormass percentage. The maximum water holding capacity is dependent on the inherentmoisture content defined by the moisture bound within the particles, and also theporosity of the media. The maximum water holding capacity increases in proportionto the porosity. It may therefore be expected that the maximum corrosion rate ofloosely packed, highly porous samples of fine-particle coal, iron ore, sand or soil willoccur at a higher value of moisture (expressed with the usual mass or volumetricpercentage scale) compared to a more tightly packed sample.

The above comments refer to fine-particle porous media. However, the corrosionof mild steel exposed to coal and iron ore cargo with particles ranging up to �50 mmdiameter is of interest for understanding corrosion processes occurring in ships andother cargo handling equipment. It is therefore worthwhile to compare the corrosionrates measured within the bulk carrier cargo hold (Section 3) to those measured inthe laboratory (Section 2). The specimens within the cargo hold were covered withcoal cargo for approximately one-third of the duration, then were not covered whenthe ship sailed a voyage unloaded, and were then covered with iron ore for ap-proximately the final one-third of the voyage. Comparsion with the fine-particlelaboratory experiments is therefore approximate. Nevertheless, the mean corrosionrate of the eleven shipboard samples, 0.10 mm/yr, is comparable to the corrosionrates measured in the 60–80% MWHC range for the fine-particle coal and iron ore(Figs. 1–3). This suggests that the laboratory experiments with fine-particle materialare representative of the corrosion conditions of large-particle cargo. Such a result isexpected considering that large particles of coal and iron ore are porous and arecovered by fine-grained material which usually deposits on adjacent surfaces. It isproposed that the corrosion of mild steel covered by large-particle coal and iron oreis dominated by the fine-particle that is deposited and to which the steel substrate hasa greater area of exposure compared to the contact points of the large particles.Theoretical calculations based on the relevant physical processes support this prop-osition [15].


5. Conclusion

The corrosion rate of mild steel covered by pH-neutral coal and iron ore wasfound to be highly dependent on the quantity of moisture. The corrosion rate of thecoal samples increased with smaller particle size distribution. A critical moisturecorresponding to the maximum corrosion rate was observed between 60% and 80%MWHC for all samples (coal and iron ore). This is consistent with experimentsconducted in soil and sand [11,12]. It is suggested that the quantity of moisture is theprimary factor influencing the corrosion rate of coal and iron ore. The influence ofchloride and sulphate concentration, and to a lesser extent particle size, is dependanton the quantity of moisture.

The laboratory experiments yielded similar corrosion rates to those measuredwithin a bulk carrier (ship) cargo hold. Based on this, and the porous texture anddustiness of coal and iron ore cargo, it is proposed that the corrosion rate of mildsteel in contact with coal or iron ore cargo is dominated by the fine-particle materialthat is deposited from the larger particles.

Acknowledgements

This work was undertaken with the support of the Australian Maritime Engi-neering Cooperative Research Centre (AME CRC).

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corrosion of mild steel by coal and iron ore

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