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CHAPTER 7

STUDIES ON CORROSION RESISTANCE OF

STAINLESS STEEL CLADDINGS

7.1 INTRODUCTION

Corrosion is the destructive result of chemical or electrochemical

reactions between a metal or metal alloy and its surroundings. The nature of

this reaction depends not only on the chemistry of the system but also on the

structure of the metal. The grain boundaries, which are imperfect and high

energy regions, generally weaken the corrosion resistance of materials due to

the depletion of corrosion resistance alloying elements on the grain

boundaries.

There are several test methods for determining the Pitting and Inter

Granular Corrosion (IGC) of stainless steel claddings. The weight loss acid

test in which the Pitting and IGC rates are determined by measuring the

weight loss of the sample as per ASTM G-48-practice-A and ASTM A-262-

practice-C respectively for the stainless steel cladding. Another test method of

measuring the degree of sensitization to intergranular corrosion involves

electrochemical reactivation of the samples as defined in ASTM G-108. This

reactivation process is called Electrochemical Potentiokinetic Reactivation

(EPR) and has been developed in to two types: Single loop (SLEPR) and

Double loop (DLEPR). The SLEPR test is usually accounted to detect the

susceptibility of the cladding towards pitting corrosion resistance and the

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DLEPR test is usually accounted to detect the susceptibility of the cladding

towards IGC resistance.

7.2 PLAN OF INVESTIGATION

The investigations are carried out in the following sequence:

1. Conducting weight loss tests namely Total immersion ferric

chloride test and the Boiling nitric acid or Huey’s test for

detecting the susceptibility of Pitting and Intergranular

corrosion attack in stainless steel claddings as per

ASTM G-48 / Practice-A and ASTM A-262 / Practice-C

respectively, for the as cladded and liquid nitrided specimens.

2. Conducting the Single Loop (SLEPR) and Double Loop

(DLEPR) tests to detect the susceptibility of Pitting and IGC

attack in stainless steel claddings as per ASTM G-5 and

ASTM G-108 respectively, for the as cladded and nitrided

specimens.

3. Results and discussions.

7.3 WEIGHT LOSS TESTS

7.3.1 TOTAL IMMERSION FERRIC CHLORIDE TEST AS PER

ASTM G-48

7.3.1.1 Preparation of the test specimen and test solution

Four test specimens were prepared from overlay plates cladded at

low (4.10 KJ/mm), high (6.81 KJ/mm), optimum (4.61 KJ/mm) heat input

condition as well as at optimum dilution (4.61 KJ/mm) and liquidnitrided

condition for conducting the test. The top surface of the specimens were

ground flat to facilitate maximum surface exposure to the corrosive test

solution. The test solution was prepared by adding 100 gm of Ferric Chloride

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(Fecl3) to 900 ml of distilled water (6% Fecl3 by wt) as per ASTM standards.

The solution was considered appropriate because, the effect of Fecl3 was more

pronounced and also aggressive in the environments that could formulate

pitting corrosion.

7.3.1.2 Experimental procedure

The total immersion ferric chloride test was conducted as per ASTM

standards to detect the susceptibility of pitting corrosion attack in austenitic

stainless steel. Samples of size 3.7 x 1.9 x 0.5 cm3 with a surface area of

19.66 cm2

were cut from the specimens weld cladded with different heat

inputs and at optimum condition. They were polished with 120 grit abrasive

paper, washed and dried by dipping in acetone. After taking the initial

weighed of the sample in a calibrated digital balance it was placed in a glass

cradle having holes and kept inside an Erlenmeyer flask fitted with a

condenser which dissipated the heat developed during the test period.

The flask was filled with 100 ml (5ml/cm2) of the test solution to

cover the entire specimen surface. Cooling water was passed through the

condenser for dissipating the heat generated and the flask is electrically heated

and maintained at 40°C thereby keeping the test solution boiling throughout

the test period. The test period was of 72 hours duration and after the end of

the test period the specimen was rinsed with water and scrubbed with a nylon

brush under running water to remove any adhering corrosion products. Then

the specimen was dried by dipping in acetone and weighed in a calibrated

digital balance. The difference in weight is recorded for estimating the

corrosion rate.

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7.3.2 BOILING NITRIC ACID TEST (HUEY’S TEST) AS PER

ASTM A-262-C

The boiling nitric acid or Huey’s test is used for detecting the

susceptibility of stainless steel cladding towards intergranular corrosion attack

and conducted as per ASTM A-262-Practice-C. It is conducted for detecting

the susceptibility of intergranular corrosion attack in stainless steel claddings.

This procedure can also be used to check the effectiveness of the stabilising

elements and the effect of carbon content in reducing the susceptibility to

intergranular corrosion attack in Cr-Ni stainless steel.

7.3.2.1 Preparation of the test specimen and test solution

The entire lateral surfaces of the four prepared test specimens were

finely grinded to facilitate better surface exposure to the corrosive test

solution. A 65 % by weight nitric acid solution was prepared by adding

distilled water to concentrated nitric acid (HNO3) of reagent grade with

specific gravity 1.42 at the rate of 108 ml of distilled water per litre of

concentrated nitric acid as per ASTM standards. The solution was considered

appropriate because of the effect of HNO3 being more pronounced and

aggressive in the environments that could formulate intergranular corrosion..

7.3.2.2 Experimental procedure

Intergranular attack in nitric acid is associated with the intergranular

precipitation of chromium carbides. The specimen was polished with 120 grit

abrasive paper and weighed initially. It was placed in a glass cradle is

presented in Figure 7.1 (a) and kept inside the Erlenmeyer flask fitted with an

Allihn condenser with four bulbs as presented in Figure 7.1 (b) to dissipate

the heat developed during boiling of the acid.

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(a) (b)

Figure 7.1 Huey’s test setup showing: (a) the glass cradle and

(b) Erlenmeyer flask fitted with an Allihn condenser

The flask was filled with sufficient quantity of the test solution to

cover the specimen and to provide a volume of 20 ml/cm2 of the specimen

surface. Cooling water was passed through the condenser for dissipating the

heat generated and the flask is electrically heated and maintained at 60°C

thereby keeping the test solution boiling throughout the test period. The test

period was of 48 hours duration and after the end of each test period the

specimen was rinsed with water and scrubbed with a nylon brush under

running water to remove any adhering corrosion products. Then the specimen

was dried by dipping in acetone and weighed in an analytical balance. The

difference in weight is recorded for estimating the corrosion rate. This test

procedure was repeated for five consecutive boiling periods with duration of

48 hours for each period for every specimen. Fresh test solution was used

every time during the entire testing period.

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7.4 SINGLE LOOP EPR TEST AS PER ASTM G-5

7.4.1 Preparation of the test specimen and test solution

Four already prepared cladded test specimens were used for

conducting EPR tests. Figure 7.2 shows the surface of these specimens (I)

before and (II) after the EPR test.

(I) (II)

Figure 7.2 Specimen with: (a) low heat input, 4.10 KJ/mm (b) high

heat input, 6.81 KJ/mm (c) optimum heat input, 4.61

KJ/mm and (d) optimum (4.61 KJ/mm) and liquid nitrided

condition, (I) before and (II) after Single loop EPR test

The top surface of the specimen was ground flat to facilitate 1 cm2

of the surface was exposed to the corrosive test solution. It was first polished

by a 600 grit SiC paper and further wet polished with 1 µm alumina slurry on

a micro cloth mounted polishing wheel to a surface roughness of 6 m as per

ASTM E13 standards. Then they were washed with distilled water and dried

in a stream of cool air before immersing them in to the corrosive test solution.

The corrosion medium for the present investigation has been selected based

on the basis of corrosion environments as cited in the available literatures.

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A 3.56 % sodium chloride (NaCl) solution was prepared by dissolving 34 g of

sodium chloride in 920 ml of deionised water.

7.4.2 Experimental procedure

The potentiodynamic polarisation test was conducted to predict the

pitting corrosion resistance of the specimens cladded at various heat input

conditions as per the ASTM G-5 standard. The schematic and experimental

set up of ACM Gill 5500 potentiostat instrument with a flat cell in three

electrode configuration is shown Figure 7.3 (a) and (b).

(a)

(b)

Figure 7.3 EPR test set up showing: (a) Schematic diagram and

(b) Experimental set up

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The cell consists of a glass cylinder clamped horizontally between

two end plates housing the working electrode, WE (AISI 316 L stainless steel

cladded surface) and the auxiliary electrode, AE, (platinum gauze). A

saturated calomel electrode (SCE) using 0.1 M KCl was used as the reference

electrode (RE). The cell was filled with 250 ml of test solution and all the

tests were conducted at room temperature, 30 ± 2° C. All the three electrodes

are connected to corrosion measuring instrument through the leads provided

in the flat cell. Polarisation test was commenced by measuring the rest

potential after the samples were immersed for 50 minutes in non deaerated

chloride solution to allow for rest potential to settle. The potential was

anodically scanned at a rate of 60 mVmin-1

from - 400 mV to + 500 mV. The

current density was measured continuously using the data acquisition

software provided with the instrument.

7.5 DOUBLE LOOP EPR TEST AS PER ASTM G-108

Four already prepared test specimens cladded were used to conduct

the test. The double loop EPR test was done according to ASTM G-108

standard and the recommendations made by Majidi and Streicher (1984). The

standard solution was modified to suit the austenitic stainless steel and

consisted of 2M H2SO4 + 0.5MNaCl + 0.01MKSCN at 30 ± 1 C and a scan

rate of 15 V/h. The test was performed by running the sample from a potential

lower than Ecorr in the cathodic region. The potential is scanned in the anodic

direction from Ecorr to a point of 0.250 V in the middle of the passive region.

The scanning direction is then reversed and the potential is reduced back to

the cathodic region. Two loops are generated, an anodic loop and a

reactivation loop. The peak activation current (Ia) and the peak reactivation

current (Ir) were measured during the forward and backward scans,

respectively. The degree of sensitization was measured as the ratio of peak

activation current to the maximum current densities generated in the double

loop test (Majidi and Streicher, 1984).

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7.6 RESULTS AND DISCUSSION

7.6.1 Weight loss tests

7.6.1.1 Total immersion ferric acid test

The corrosion rate was measured by determining the weight loss of

the specimen after the test period and the weight loss was calculated for each

specimen. The corrosion rate was calculated by using the relation,

Corrosion rate = 7290 x W / A t, mm/month,

where W= the total weight loss of the specimen in grams,

A= the area of the specimen exposed in cm2,

= the density of the overlay material in grams / cm3

and

t= the time of exposure in hours.

The corrosion rate was calculated for each test period and the

average corrosion rate was referred against the ASTM acceptance limits for

all the four specimens cladded at different heat input conditions. The results

of the Total immersion ferric chloride test are presented in Table 7.1.

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Table 7.1 Results of total immersion ferric chloride test

No

Thermal history

of the specimen

Initial

weight

of the

specimen,

gm

Final weight

of the

specimen,

gm

Difference

in weight,

gm

Corrosion

rate,

mm/month

ASTM

acceptance

limit,

mm / month

1 Cladded at low

heat input

(4.10 KJ / mm)

30.8201 30.8111 0.009 0.0447 0.10 – 0.12

2 Cladded at high

heat input

(6.81 KJ / mm)

30.8112 30.7952 0.016 0.1142 0.10 – 0.12

3 Cladded at

optimum heat

input

(4.61 KJ / mm)

30.8224 30.7994 0.023 0.0982 0.10 – 0.12

4 Cladded at

optimum heat

input

(4.61 KJ / mm)

and liquid

nitrided

condition

30.8212 30.8102 0.011 0.0459 0.10 – 0.12

From Table 7.1 it is found that cladding produced at low heat input

and optimum conditions are having lower corrosion rates than that of other

claddings in ferric chloride solution.

7.6.1.2 Boiling nitric acid test (Huey’s test)

The results of the Huey’s test are presented in Table 7.2.

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Table 7.2 Results of Huey’s test

NoDescription

of the specimen

Total weight

loss , gms

Total

corrosion

rate,

mm / month

Average

corrosion

rate, mm /

month

ASTM

acceptance

limit,

mm /

month

1Cladded at low heat

input (4.10 KJ / mm)0.5497 0.4101 0.0921 0.14 – 0.16

2Cladded at high heat

input (6.81 KJ / mm)0.8855 0.5623 0.1182 0.14 – 0.16

3

Cladded at optimum

heat input (4.61 KJ /

mm)

0.9104 0.5912 0.1125 0.14 – 0.16

4

Cladded at optimum

heat input (4.61 KJ /

mm) and liquid

nitrided condition

0.5738 0.4902 0.0881 0.14 – 0.16

It is observed that the corrosion rate in boiling nitric acid of nitrided

claddings produced at optimum dilution condition is lower compared with all

other claddings. Also it is evident from tables that corrosion rate increases

with the increase in heat input which may be attributed to increased dilution.

The scanning electron micrograph (SEM) of the nitrided cladding

produced at optimum heat input condition (4.61 KJ/mm) and at high heat

input (6.81 KJ/mm) condition after Huey’s test are shown in Figure 7.4. A

stepped type microstructure is noticed for the nitrided specimen cladded at

optimum heat input condition. This is because of the reason that the lower

heat input promoted faster cooling rates thereby forming finer grains with

stepped structures. Finer grains with stepped structures possess excellent

corrosion resistance and tensile properties combined with good bonding

strength between adjacent grains (Aydogdu and Aydinol 2006, Mirko Gojic et

al 2008). This in turn promotes excellent ductility and toughness of the

cladding which will widen their potential applications.

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Figure 7.4 SEM photomicrograph of nitrided claddings produced

at optimum heat input (4.61 KJ/mm) condition after

Huey’s test showing a stepped structure, X500

The stepped type microstructure is presented at a higher

magnification for clearly visualising the corrosion debris after the Huey’s test,

in Figure 7.5.

Figure 7.5 SEM photomicrograph of nitrided cladding produced at

optimum heat input (4.61 KJ/mm) condition after Huey’s

test showing a stepped structure, X2000

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A ditched type microstructure noticed in the specimen cladded at

high heat input (6.81 KJ/mm) condition after Huey’s test is shown in Figure

7.6.

Figure 7.6 SEM photomicrograph of high heat input specimen

(6.81 KJ/mm) after Huey’s test showing a ditched

structure, X500

The slower cooling rates of the cladding due to higher heat input

promoted a coarser grain structure which does not have the normal

mechanical and metallurgical properties. Also, the slower cooling rates

promoted the formation of coarser grains with ditched structure. Their

bonding strength may not be evenly distributed due to the formation of

coarser grains with a lathy morphology (Arikan and Doruk 2008). The SEM

images reveal that the ditched type microstructure are highly prone to the

intergranular corrosion attack or sensitization than the stepped type structure.

The ditched type microstructure is presented at a higher

magnification for clearly visualising the corrosion debris after the Huey’s teat,

in Figure 7.7.

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Figure 7.7 SEM photomicrograph of high heat input specimen (6.81

KJ/mm) after Huey’s test showing a ditched structure,

X2000

7.6.2 Single Loop EPR test as per ASTM G-5

Graphs were plotted keeping current density in logarithmic scale

along X-axis and potential along Y-axis using the analysis software. Typical

potentiodynamic anodic polarization curves of the specimens cladded at

corresponding conditions are shown in Figure 7.8 – 7.11. For each specimen,

the test was repeated twice in different areas and the average value was

recorded for the analysis. The current density was measured continuously

using commercial data acquisition software provided with the instrument.

Corrosion behaviour was investigated using potentiodynamic polarisation

measurements in 3.5 wt. % NaCl.

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Figure 7.8 Single Loop EPR curve for a specimen cladded at low

heat input (4.10 KJ/mm) condition

Figure 7.9 Single Loop EPR curve for a specimen cladded at high

heat input (6.81 KJ/mm) condition

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Figure 7.10 Single Loop EPR curve for a specimen cladded at

optimum heat input (4.61 KJ/mm) condition

Figure 7.11 Single Loop EPR curve for a specimen nitrided and

cladded at optimum heat input (4.61 KJ/mm) condition

150

The rest potential (the potential at which the current becomes zero)

and the pitting potential are considered as a measure of the material

dissolution from the surface being tested. It has been referred that the passive

film on the surface was destroyed progressively with time and as a result

more and more of the metal (which is active) is exposed in the electrolyte.

The results are presented in Table 7.3. All potentials are vs. saturated calomel

electrode.

Table 7.3 Results of Single Loop EPR test

Sample

description

Rest

Potential,

mV

Pitting

Potential,

mV

Corrosion

current

density,

(Icorr)

A cm-2

Corrosion

rate,

mm/year

Corrosion

rate,

mils/yr

Low heat

input of

4.10 KJ/mm-180 +410 7.2X10

-60.00202 0.07945

High heat

input of

6.81 KJ/mm-120 +280 6.1X10

-6 0.04889 0.93534

Optimum

heat input

4.61 KJ/mm-240 + 260 8.3X10

-50.02378 1.92301

Optimum

heat input

4.61 KJ/mm

and nitrided

-120 +395 9.1X10-6

0.00341 0.13413

Prasad Rao et al (1986-a) investigated the pitting potential for

AISI 316L stainless steel claddings in 3.5% NaCl aqueous solution at a

controlled temperature of 30 ± 2°C were between -120 to +430 mV.

Pulino-Sagradi et al (1997) observed the same between -130 to +420 mV.

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Kamachimudali et al (2000) investigated the pitting potential between140 to

+450 mV. The results show that the pitting potential of the AISI 316L cladded

specimens in 3.5% NaCl aqueous solution at a controlled temperature of

30 ± 2°C lie between -120 to +410 mV for various heat inputs. The above

observations indicate that the values of pitting potentials obtained in the

present study are in agreement with the literature.

The optical and SEM micrographs of the nitrided cladding produced

at optimum heat input condition showing stepped structure with pits are

presented in Figure 7.12 and 7.13 respectively.

Figure 7.12 Optical micrograph of the nitrided cladding produced at

optimum heat input condition after Single Loop EPR test

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Figure 7.13 SEM micrograph of the nitrided cladding produced at

optimum heat input condition after Single Loop EPR test

The optical and SEM micrographs of the nitrided cladding produced

at high heat input condition showing ditched structure are presented in Figure

7.14 and 7.15 respectively.

Figure 7.14 Optical micrograph of the cladding produced at high

heat input condition after Single Loop EPR test

153

Figure 7.15 SEM micrograph of the cladding produced at high

heat input condition after Single Loop EPR test

A step type microstructure with pits was noticed for the specimen

cladded with optimum heat input (4.61 KJ/mm) and nitrided condition and a

ditched type microstructure with pits was noticed for the specimen cladded

with high heat input (6.81 KJ/mm). Stepped type microstructure might have

formed due to the faster cooling of the cladding at low heat input conditions.

The ditched type microstructure might have formed due to the slow cooling of

the cladding produced during higher heat input condition. It is found that the

stepped type microstructure possesses comparatively better resistance to

pitting corrosion than the ditched type microstructure.

7.6.3 Double Loop EPR test as per ASTMG -108

Polarisation graphs were plotted keeping current density in

logarithmic scale along X-axis and potential along Y-axis using the analysis

software. Typical potentiodynamic anodic polarization curves of the

claddings produced at the specified heat input conditions are presented in

Figure 7.16 – 7.19.

‘

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Figure 7.16 Double Loop EPR curve for a specimen cladded at low

heat input (4.10 KJ/mm) condition

Figure 7.17 Double Loop EPR curve for a specimen cladded at high

heat input (6.81 KJ/mm) condition

Figure 7.18 Double Loop EPR curve for a specimen cladded at

optimum heat input (4.61 KJ/mm) condition

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Figure 7.19 Double Loop EPR curve for a specimen nitrided and

cladded at optimum heat input (4.61 KJ/mm) condition

For each specimen, the test was repeated twice in different areas and

the average value was recorded for the analysis. The current density was

measured continuously using commercial data acquisition software provided

with the instrument. All potentials are vs. saturated calomel electrode. The

degree of sensitization was measured from the ratio of maximum current

densities generated in the double loop test (Majidi and Streicher 1984). The

results of the double loop EPR test is presented in Table 7.4.

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Table 7.4 Results of Double Loop EPR test

No Description

Activation

peak

potential,

Ea ,(mV)

Activation

peak

current

density,

Ia, (mA/cm2

)

Reactivation

peak

potential,

Er,(mV)

Reactivation

peak current

density,

Ir,(mA/cm2

)

Passivation

current

density,

Ipass ,(mA/cm2

)

Degree of

sensitization

( Ir /Ia

x100) %

1

Low heat

input of

4.10 KJ/mm-201.59 16.2439 -196.26 0.0096 0.0266 0.0589

2

High heat

input of

6.81 KJ/mm-214.66 24.1534 -218.19 0.8752 0.0338 3.6236

3

Optimum

heat input

4.61 KJ/mm-194.27 26.7205 -227.99 0.0089 0.0253 0.0334

4

Optimum

heat input

4.61 KJ/mm

and nitrided-212.78 23.1305 -267.33 0.0064 0.0014 0.0276

7.7 SUMMARY

From the weight loss test with ferric chloride it can be concluded

that the nitrided cladding deposited at optimum heat input condition possessed

better pitting corrosion resistance. In the Huey’s test, the claddings deposited

at low and optimum heat input conditions possessed better resistance to IGC.

Also from the single loop EPR test an increase in pitting potential is noticed

in the cladding deposited with optimum heat input condition. The positive

value of pitting potential indicates that a stable film is formed over the surface

of the cladding which confirms that the material is more nobler with increased

pitting corrosion resistance. In the double loop EPR test the ratio of the degree

of sensitisation (Ir/Ia) was found to be very lower in the cladding deposited at

157

optimum heat input condition, which reveal that the cladding possesses better

resistance to IGC.