Mechanics of Corrosion of Trip-Assisted Steels in Different Nacl Solutions

Animesh Talapatra¹, Jayti Datta^2* and N.R. Bandyopadhyay³

¹MCKVIE, Department of Automobile (Under WBUT) Liluah, Howrah - 711 204, India. ²Faculty, Department of Chemistry, BESU, Shibpur, Howrah - 711 103, India. ³School of Materials Science and Engineering BESU, Shibpur, Howrah - 711 103, India

Mechanics of corrosion of two TRIP-assisted steels designated as A (having no Cr and Cu content) and B (having higher Ni, Cr and Cu content) heat treated at different condition to alter micro-structure phases. After two TRIP –assisted steels have been studied under the simulated condition of sea atmosphere in the laboratory scale in different level of salinity under the influence of neutral pH as well as exposing them in real sea water condition by using electrochemical test. Micro-structural characterization of Pre & post corrosive samples and quantitative phase analysis were carried out to arrive at heat treatment-structure-properties co-relation to get knowledge about mechanics of TRIP-assisted steels in different NaCl solutions.

Corrosion; TRIP-assisted Steel; Electro-chemical test; Phase analysis; Heat treatment

Introduction

From the history of steel development for vehicle, we can see that there were many kinds steels, which had been developed and used. From the 1950’s to the 1960’s, the rimmed steel was researched and used. From the 1960’s to 1970’s, many killed steels were developed and employed. From the 1980’s to the 1990’s, to highly improve the formability of sheet steel , the research of auto steel was mainly centralized in interstitial-free(IF) steel. By the 21^st century, most projects of auto steel focused on developing new kinds of HSS and new style steels to meet the needs of the plan of ultra light steel auto body (ULSAB).These steels include double-phase (DP)steel, TRIP-steel and TWIP steel.

The  former steels also  known  as  high  tensile  steels  (HT S)  have   moderate strength  (350MPa)  and  increased  strength  through  pearlite  strengthening by the addition of carbon upto 0.2%. In 1960’s quenched  and tempered  (Q and T) steels  were developed  to improve  the  performance  of  industrial  applications  [5]-[7]. Due   to   higher   carbon   contents   these suffer   from poor weld ability and for this reason new classes of steels   having low carbon content and higher alloy elements were developed. Steels having a fully austenitic microstructure are called TRIP steels (Zackay et al., 1967). These steels tend to be rich in nickel and other expensive austenite stabilising elements. By contrast, austenite is only a minor phase in the overall microstructures of TRIP-assisted steels (Matsumura et al., 1987; Takechi et al.,1987).

Development of  newer  types   of  low  alloy  high  strength corros ion   res istance   s teel   has   been   attempted   by   many  res earchers all over the world for indus trial application in s ea atmos phere [1]-[6].

The major factors those effects the corros ion rate of HSLA steel are

Chemical composition

Composition of sea atmosphere

Type of e xpos ures

In moderate-velocity and high velocity sea atmos phere, Ni bas e alloy is frequently used for pumping. It has excellent resistance to cavitations, eros ion   and   exhib its   corros ion   rates   of less   than   0.025 mm/year.  Other  Ni-bas e alloys  containing  Cr and  Mo  offer increased  res is tance  to  localized  corros ion  in  s tagnant  s ea atmos phere.  Sea  water  is  a  highly  conductive  environ ment with  3.4%  s alt  (NaCl)  concentration  [3]-[6].Approximately 91.1% of the   dissolved s alts are chlorides.  There are other commonly occurring   cons tituent,   dissolved   gases, living organis ms and various other materials found in sea water. Keeping in v iew of the above s tudy on corros ion behaviours to moderately lo w ca rbon s teel having Si, Mn, Ni, V, Nb, Mo, Cu and Cr des igned and develop for industrial application in s ea atmos phere  has been attempted in this s tudy. No data base being available in literature on the systematic study on the effect of   corros ionon multi-phas e   micros tructure. It is essential to do more study in corrosion behaviour and performance of the material in order to evaluate and improve the design, cost effectiveness and reliability of each material used. Khoshnaw et al. (2007) have studied fatigue strength of low alloy steels in chloride solution. Turnbull et al. (2008) have studied stress corrosion cracking of stainless steel in chloride solutions. Chen et al. (2005) have studied mechanical properties of low-alloy steels in atmosphere containing chloride in tension test.

Exp Erimental P Rocedure

Materials

Here Two moderately lo w Carbon high strength low alloy steels for sea atmosphere application  is designated as A and B having compos ition  given  in Table I  were supplied  by  DM RL, Hyderabad.

Comosition of Materials

Composition of Met

Steels were heat treated as per s schedules as shown in below in

Table II to develop diffe rent micros tructure.

Heat Treatment of Steel 

Optical Micrography

The micro-structural characterizations of the steels were carried out us ing  an OLYMPUS CK4OM-CP optical micros cope.The optical metallographies of all these samples were carried out in the usual way. Samples were cut from plates of theses steels.These samples   were polis hed on polis hing wheel after 1 to 6 (rough to fine) e mery papers. The polis hed s urface appeared like mirror having no s cratches and the etchant was used 2% nital. The was hed and dried s amples

were observed  carefully   in  Micros cope  at  different magnification   and   some   s elected   photomicrographs   were taken.

Phase Analysis

The  phase  analysis  have  been  done  by  using  Olysia  m3 software.  Samples were prepared as like as  preparation of sample for optical micros cope observation. Then images were taken in optical microscope. The acquired images of a multi- phase object were taken for analysis. Phase analysis will be conducted on a gray-value image. The image is s elected and threshold is set to define the gray value ranges for the s separate phases. The OLYSIA software created a meas ure ment s heet s howing the absolute area and proportional area (in %) of all the phases. The measured values are taken. The phase analyses ASTM E 566  1245     have been done by using Olysia m3 software. Samples were prepared as like as preparation of sample for optical microscope observation. Then images were taken in optical microscope. The acquired images of a multiphase object were taken for analysis. Phase analysis will be conducted on a gray-value image. The image is selected and threshold is set to define the gray value ranges for the separate phases. The OLYSIA software created a measurement sheet showing the absolute area and proportional area (in %) of all the phases. The measured values are taken.

Electro-chemical study

Samples and Solution preparation:   Samples were cut from plates of these steels. These samples  were polis hed on belt  followed  by  polis hing  on  polis hing  wheel  with  1 to  6 (rough to fine) eme ry papers . The polis hed surface appeared like mirror having no   scratches. They  were  then  degreas ed with  acetone  before  e xpos ing  to  the  electro-chemical  tes t. Solution  of  0%,  0.1%,  1%  and  3%  NaCl  at  pH  6.5  were prepared with triple distilled water. Electrochemical test set up has s hown in Fig-1.

Figure 1: Electro-chemical test set up. Click here to View Figure

Polarization Study:  Samples   were taken in an area of

0.204 square cm size coupons for performing potentiodynamic polarization studies in de-aerated condition in cells with three electrode configuration  and using aqueous  saturated  calomel SCE  (W)  as  the  reference  electrode  and  Pt  foil  as  counter electrode. Linear s weep voltammetry was preformed with the help  of  AUTOLA B  12  PGSTAT ,  Eco   Chemic   B.V  (the Netherlands ) at 0.5 mV/s s can rate within the potential range of -1500  mV to  the  cathodic  potential of 650  mV vs .SCE. Potential s cans  were  conducted  in  de-aerated  conditions  by purging  the s olution  with  nitrogen  for 10min.Tafe l analys is was perfomed to determine the corros ion para meters .

Electro-chemical  Impedance Spectroscopy:  EIS at  the res pective   OCP   value   were   recorded   with   the   help   of AUTO-LA B 12 PG STAT, Eco Chemie B.V (the Netherlands ) combined  with  frequency  res pons e analyser (FRA) module.

 Result and Discussion

A. Micro structural Characteristic

Pre-corros ion   microstructure   for steel A1    and  B1    reveals polygonal ferrite plus te mpered bainite and blocky polygonal ferrite plus tempe red bainite (as s hown in Fig.2 and Fig.3).

Figure 2: Opt ical microstruct ure A1 as per schedule 1 Click here to View Figure

Figure 3: Opt ical microst ruct ure B1 as per schedule 1. Click here to View Figure

A2  and B2  reveals  granular ferrite  plus  bainite  and granular ferrite  plus  tempered  bainite.  A3   and   B3   reveals accicular ferrite   plus bainite and accicular ferrite.  A4   and B4  s hows ferrite and   martens ite. Finally A5   and B5   reveals ferrite and pearlite.  Post corrosion microstructures, taken for a few samples for both steels, reveal adequate corrosion both in the form of grain boundary attack and pitting. However steel having high percentage of Cu and Cr, in case of steel B, this corrosion attack is comparatively less in all heat treatment schedules seemingly due to formation of protective oxide layer. Pos t   corros ion   micros tructures    reveal   adequate corros ion both in the form o f gra in boundary attack and pitting (as shown in Fig.4.and Fig.5 ).

Figure 4: Opt ical microst ruct ure A1 in 0.1% NaCl ( 500X). Click here to View Figure

However, s teels having high percentage of Cu and Cr (Steel B) face corros ion attack comparatively less in all heat treatment due to format ion of protective oxide layer.

Figure 5: Opt ical microstructure B1 in 0.1% NaCl ( 500X  ). Click here to View Figure

All   the   above   mic ros tructure   when   s ubjected   to   image analys ing system (as s hown in Fig.6.and Fig.7).

Figure 6: Corresponding threshold image A1 as per schedule 1.   Click here to View Figure

Figure 7: Corresponding threshold image B1 as per schedule 1. Click here to View figure

It is s een that high % of martens ite and/ or pearlite in ferrite matrix during water and a ir cooling

B. Corrosion Characteristic

Corros ion res ults have been s hown in Table III, Table IV& V

 TABLE III

Corrosion Rate ( C R )

TABLE IV

Polaristion Resistance

Figure 8: Polarisation plot for corrosion st udies in 0.1%. Click here to View Figure

Corrosion Current

A2 and B2 in 0.1% Na Cl shows (as s hown in Fig.8 ) that B2   is characterized with a narrow but distinct passive region while in A2 there is no s uch region (as shown in Fig.9.A samples are more resistant than B samples Rp (A) > Rp (B) as shown in Table no-5. It may be predicted that matrix / grain boundary of A is more reactive than that of B in neutral conditions and that may be due to formation of thicker and stern passive layer (oxide film) in alloy B. This gets disrupted immediately in contact with Cl-. In most of A and B samples 10-fold decrease is observed when exposed to 0.1% NaCl. However this decrease is much restricted in case of B2 and B3 samples. With further increase in Cl- ion Rp decrease as usual.A more or less similar behavior is reflected with polarization studies.

Figure 9: Polarisat ion plot for St eel A2 and B2  in 0.1% NaCl. Click here to View figure

In neutral pH when  EIS repres ents dual character A s a mples are more res istant than B samples .With further increase in Cl ion res is tance decreas e usual.Corros ion current /corros ion rate are  much  higher  for  A  s amples  then  B ones .Interes tingly enough, in cas e of B2  and B3  s amp les corros ion rates are not that  s ignificantly  accelerated  with  Cl  ion  as  in  cas e  of  A s amples .EIS meas urement revea ls the Nyquis t plot (as s hown in Fig.10 and Fig.11) where ha lf circles diameter represent the circuit resistance of the material.