316 Stainless Steel in Sodium Chloride Solution at 2500C

1.00E-09

1.00E-08

1.00E-07

1.00E-06

1.00E-05

1.00E-04

1.00E-03

-250 -200 -150 -100 -50 0 50

Applied Potential, mV (SHE)

Cra

ck V

elo

city

(m

m/s

)

Stress Corrosion Cracking of Sensitized Type 316 Stainless Steel In High Temperature Selenate Contaminated Water

Stuart Palmer

School Of Chemical Engineering And Advanced Materials, University Of Newcastle-Upon-Tyne

Aim

Increased susceptibility to stress corrosion cracking has been found in sulphate contaminated water in iron based alloys. The aim of this project was to investigate the susceptibility to SCC of sensitized 316 stainless steel in a selenate contaminated water. Selenium closely resembles sulphur in its chemical properties, which suggests that it could also have a detrimental effect on the susceptibility to SCC.

Experimental

Slow strain rate tests were performed in low-flow rate refreshed water loop autoclaves at 250oC.

The materials tested were a Chromium-Molybdenum steel (‘as received’ condition) and a sensitized type 316 stainless steel. The stainless specimens were firstly heated to 1050oC for one hour in argon and then water quenching to give the austenitic state. The specimens were then sensitized by heating for 22 hours at 650oC under argon and then air cooled.

Tests were carried out on the Cr-Mo steel in pure water doped with lithated water and sodium selenate. Sensitized type 316 stainless steel was tested in pure water doped with LiOH and Na2SeO4. The stainless steel was also tested in sodium chloride doped with sodium selenate and also pure NaCl solution.

Standard cylindrical tensile specimens with a 12.7mm gauge length were used.

Results

The tests showed that the LiOH and Na2SeO4 solution had a beneficial effect on the Cr-Mo steel (compared to work by J Congleton and E A Charles. 2001) and the sensitized type 316 stainless steel. No stress corrosion cracking occurred. The NaCl and Na2SeO4 solution however did show a detrimental effect on the stainless steel and stress corrosion cracking was present at certain applied potentials. The NaCl solution had a more detrimental effect on the 316 stainless steel than the NaCl doped with Na2SeO4 sodium selenate.

P latinum E lectrode

W ater In le t

S teel Bush

S teel N ut

PTFE Seal

Autoclave

Specim en

Pull R od

Ag/AgC lR eference E lectrode

W ater O utle t

Schematic section of a refreshed loop autoclave.

Conclusions

• The presence of selenium as an addition of selenate to Lithiated water and Sodium chloride solutions cause Cr-Mo and 316 Stainless Steel to be less susceptible to stress corrosion cracking.

• The results showed that the selenium had a beneficial effect on the two materials tested.

• Selenium reduced the crack velocity.

• The results showed a contrast to the action of sulphur in these environments.

Ductile fracture surface of 316 stainless steel specimen tested in

LiOH H2O + NA2SeO4 at 200 mV S.H.E.(2500C)

Shear dimples in the shear-lip zone of 316 stainless steel specimen tested in LiOH H2O + NA2SeO4 at 200 mV S.H.E.(2500C)

Stress corrosion fracture surface of 316 stainless steel

specimen tested in NaCl + pure water solution at 0mV S.H.E.

(2500C)

SCC of 316 stainless steel specimen tested in NaCl + pure water at 0 mV S.H.E.(2500C)

Intergranular stress corrosion cracking of 316 stainless steel specimen tested

in NaCl + pure water at -100 mV S.H.E.(2500C)

Cross section of 316 stainless steel specimen tested in Nacl and Na2SeO4 at 0mV S.H.E.(2500C). Large amounts of intergranular

stress corrosion cracking can be seen.

Figure 1: Crack velocity versus applied potential for slow strain rate test on 316 stainless steel in NaCl solution at 2500C.

316 Stainless Steel in Sodium Chloride and

Sodium Selenate Solution at 2500C

1.00E-09

1.00E-08

1.00E-07

1.00E-06

1.00E-05

1.00E-04

1.00E-03

-250 -200 -150 -100 -50 0 50

Applied Potential, mV (SHE)

Cra

ck V

elo

cit

y (

mm

/s)

Figure 2: crack velocity versus applied potential for slow strain rate test on 316 stainless steel in NaCl and Na2SeO4 solution

at 2500C.

Reference: 1. Stress Corrosion Of Iron-Based Alloys In High Temperature Water Environments. J Congleton and E A Charles. Corrosion Research Centre, University of Newcastle, Newcastle upon Tyne. (2001).