Inconel 625 is an
outstanding heat resistant alloy with good mechanical characteristics at the
high temperatures and supreme corrosion resistance. These characteristics make
it significant for use as a structural material in steam engines, nuclear
plants and aircraft engines. It offers supreme welding properties and as a
result it is used for weld overlay in the carbon steel pipes. Therefore it can
be used instead high corrosion resistant steels for example duplex stainless
steel. It also develops synergistic effect by weld overlay with carbon steel
materials to supplement the yield strength of Inconel alloy and small corrosion
resistance of carbon steel so it can be utilized as structural materials in the
severe media like for example in crude oil. Moreover the heat expansion
coefficients of the to metals are identical that decreases the chances of
cracks due to thermal stress under the high temperature media. Although the
Inconel alloy comprises of nickel and chromium, that may develop carbides and
secondary phases, on the base of specific temperature and exposure time. Such
carbides and secondary phases have a significant role in affecting the
corrosion resistance and physical characteristics of alloy and causing the crack
development.
With the passage of time different carbides and secondary phases
are developed that precipitate in the temperature limits from 600oC to 950oC. The
development of these carbides can be a contributing aspect to the reduction of
corrosion resistance. Intergranular regions of carbides are thermodynamically
more inconsistent and highly active than other types of intergranulars. Intergranular
regions increase owing to development of carbides. Additionally the carbides
have higher chromium content than that present in the base metal. So when
carbides are developed in the intergranular regions, the chromium content
reduces around them resulting in to areas with low chromium content along the
intergranualr regions. In this mechanism, chromium lacking areas are more prone
to intergranular attack than other regions, so causing corrosion, is named as
sensitization.
Electroslag welding
method is implemented with alloy EQNiCrMo-3 utilized as a filler metal.
Aging
Aging heat processing
was performed to find the intergranular resistance at 500A to 620A samples.
Chromium carbides were developed through age heat processing at constant
temperature for 100 hours in a vertical furnace at 850oC.
Corrosion tests
A single and double
loop electrochemical reactivation test was performed as an electrochemical
method to estimate the corrosion resistance. This test shows more significant
results than with other chemicals. The sensitivity of intergranular attack on
Inconel meals is assessed. The outcomes are assessed from non-uniform attack in
the intergranular region and the outcomes should be evaluated by the current
ratio. On the other hand, mistakes due to surface conditions are nominal and
the test values can be easily received and compared with the single loop
analysis. In high concentration of sulphuric acid, intergranular and other
types of corrosion increases whilst in the low content, the corrosion is not
noticed. So depending on the test condition factors, the tests are performed on
nickel based alloy.
In the chemical
analysis to evaluate the intergranular attack on stainless steel or super alloy Inconel 625 plate, the tests included sulphate- sulphuric acid and nitric acid were
performed. The ferric sulphate sulphuric acid test in the ASTM G28 method was
conducted to assess the corrosion sensitivity of super alloys.
In the test of ferric
sulphate sulphuric acid, H2SO4 acid solution comprising of 400 ml water and
236ml sulphuric acid where 25g dissolved Fe2(SO4)3 was utilized. It is warmed on
a hot plate, the alloy sample was plunged for around 120 hours and the material
loss was evaluated. To avoid evaporation of the solution beyond 120 hours, the
boiling stone was kept in the solution and the vapour was condensed through
flowing water condenser. The test results were received by substituting the
noticed material loss of the samples.
In the another
chemical test, the nitric acid test was performed that was aimed on evaluating
the austenitic stainless steel and therefore was organized to fit the nickel
based alloys in this analysis. Nickel super alloys have higher PREN than
austenitic steel grades; the plunging period was increased up to 120 hours. In
this test nitric acid is taken in 65% content.
No corrosion sign was
found by intergranular attack on Inconel 625 due to its small carbon
concentration and sufficiently high niobium magnitude. With small carbon
concentration, chromium carbides, the main reason of chromium lacking regions,
were not precipitated during aging heat processing, and with niobium effect,
the niobium carbides were accumulated, hence preventing the development of
chromium carbide. So the alloy was stabilized by precipitation along the grain.
It is found that increased weight loss occurs with high heat supply to the
sample. The weight loss in material is a crucial factor that shows the speed of
corrosion as it results into the material degradation in the component.
Various corrosion
rates are based on the type of attack. To determine this, microstructures of
the corrosive surfaces were noticed after these chemical tests. The samples
initially experienced corrosion at their intergranular regions. Initially thin
and lengthy corrosion shape was seen however later the corrosion area increased
and then at the maximum value of current the corrosion area becomes round. Alike
outcomes were observed in the both tests.
Generally, the bigger
weld heat supply, high dilution occurs in the weld metal and the base metal.
With increase in weld heat supply, it results into melting a part of the main
metal sample, therefore the metal’s atoms are diluted in the weld metal. It
causes to increased diluted iron content in the weld metal hence decreasing the
breaking potential. This iron dilution mechanism is noticed in the melted part
and can also be seen in the fusion line zones by energy dispersive X ray
spectroscopy. Increased niobium and molybdenum contents resulted into cracking.
Therefore with increase in heat supply, the dilution effect is enhanced hence
increasing iron concentration in the fusion line. As a result, large magnitude
of iron was coagulated, initially in the dendritic regions, through weld
solidification, niobium and molybdenum were emitted into the interdendritic
regions. This method created microcrakcing in the dendritic and interdendritic
regions, hence showing a variation in the corrosion resistance offered in these
areas, and an unlike corrosion shape.
Outline
The samples were not
found to be sensitive towards intergranular attack. Irrespective of nitrogen
aging heat processing, precipitation of chromium carbide didn’t occur and
niobium carbide stabilized the sample.
In the ferric sulfate
sulphuric and nitric acid tests, heat supply improved with increase in material
loss, hence corrosion rate is accelerated.
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