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Ni-Cr-Mo Alloys as Corrosion Barrier for the Rad-Waste Containers

 

D.C. Agarwal

Krupp VDM Technologies

 11210 Steeplecrest Drive # 120

Houston, TX 77065-4939

 

Richard A. Corbett

Corrosion Testing Laboratories

60 Blue Hen Drive

Newark, DE 19713

Copyright by NACE International

 

Abstract 

Currently the Ni-Cr-Mo alloys are the leading candidates for the latest design of rad-waste containers for the Yucca Mountain Project. Alloy 22 is the leading candidate whereas alloy 59, an advanced Ni-Cr-Mo alloy, is under consideration as an equal or better alternative. The Nuclear Waste Policy Act of 1982 established an objective of Nuclear Waste disposal in a deep geological repository. This act was later amended in 1987, and established Nevada as the only site to be characterized. In 1994 a technical decision was made for a multipurpose container consisting of an outer barrier of carbon steel, alloy 400 or Cu-Ni 70/30 and an inner barrier of alloy 825. This concept was later modified to require a more corrosion resistant alloy for the inner barrier i.e., an alloy of the Ni-Cr-Mo family, alloy 22 (UNS N06022), titanium or a titanium alloy. Since then many papers (1-6) have been written comparing the corrosion resistant characteristics of various alloys such as alloy 825, 625, C-276 and alloy 22. The design of the waste package underwent several iterations with one of the latest designs called "Enhanced Design Alternative" (EDA) which will consist of 20 mm thick alloy 22 as the outer container barrier. This will be shrunk fit to a 50mm thick inner barrier fabricated of type 316 nuclear grade or standard 316L SS. This waste package was then to be enclosed by a self-supported 20 mm thick Ti-grade 7 mailbox shaped drip shield. This design may be further modified as more comprehensive corrosion characteristics of uniform corrosion, localized corrosion, stress corrosion cracking, thermal stability, microbiological corrosion, galvanic corrosion, intergranular corrosion for both the base metal and more importantly, the weld joints in these waste containers under realistic repository environments are obtained. This paper presents data on a new but well established corrosion resistant alloy 59 (UNS N06059) of the Ni-Cr-Mo family. The alloy appears to have better corrosion resistance, both uniform and localized, better thermal stability and better weldability than alloy 22 as measured in standard ASTM laboratory tests and Modified Varestraint tests for measuring susceptibility to hot cracking. Data from some of these laboratory tests on alloy 59 and 22 along with the various interactions with Lawrence Livermore National Laboratories and the TRW Environmental Safety Systems, Management and Operating Contractor for the waste package design, are discussed. 

Introduction

The design for containment of spent-fuel and high-level nuclear waste at the proposed geological repository at the Yucca Mountain, Nevada was a two-layer canister. In the previous designs, the inner barrier was to be alloy 825 later changed to alloy 22 with outer barrier of carbon steel, Ni-Cu alloy 400 or Cu-Ni alloy 70/30. This design concept has now been again modified to "Enhanced Design Alternative" (EDA), which will consist of 20 mm thick alloy 22 as the outer container barrier. This will be shrunk fit to a 50mm thick inner barrier fabricated of type 316 nuclear grade or standard 316L SS. This waste package will then be enclosed by a self-supported 20 mm thick Ti-grade 7 mailbox shaped drip shield. It was assumed that the slow uniform corrosion of the 20 mm thick alloy 22 will accomplish containment of the nuclear waste, without degradation of the container, to well beyond 10,000 years. In this latest design, assumption has been made that alloy 22 will be immune to localized crevice corrosion, an assumption which is not valid as shown by some of the tests recently conducted in ASTM G48 and other tests as described later in this paper. The occurrence of localized corrosion and its rate of propagation can not be fully determined but tests on alloy 22 in the presence of ferric chloride has shown its susceptibility to crevice attack at temperatures below 60 C on base metal alone (weldments will be even lower). The objective of this paper is to present data on a nearly pure ternary alloy of the Ni-Cr-Mo family, alloy 59, with typical iron content of less than 0.5% (not intentionally added) and having superior uniform corrosion resistance in a variety of corrosive media, superior crevice corrosion resistance and thermal stability in comparison to alloy 22. 

Metallurgy & Corrosion Resistance of "Ni - Cr Mo" Alloy 59 

Table 1 gives the basic chemical composition of the various alloys of this family developed in the 20th century. As is evident, the major alloying elements are nickel, chromium and molybdenum with some alloys containing either tungsten or copper, as an intentional alloying addition, whereas others are of pure Ni-Cr-Mo ternary alloy, such as alloy 59. The alloys developed during the 1960's and later, had very low carbon content due to the improved AOD / VOD melting technology. This overcame the often serious intergranular corrosion attack in heat-affected zone (HAZ) of weldments of the first alloy of this family, "Alloy C", UNS N10002, that was developed in the 1930's. Greater details on the physical metallurgy and development of the "C" family of alloys are well documented in the open literature. (7-10)

The next few sections briefly describe the corrosion resistance of alloy 59 in comparison to alloy 22. The dialogue and various interactions and corrosion data generated on the electrochemical testing of alloy 59 and alloy 22 with Lawrence Livermore National Laboratory (LLNL) are also presented. The results of these interactions with LLNL and the TRW Environmental Safety Systems, Management and Operating Contractor for the Waste Package Design, finally led to the inclusion of alloy 59 in the test matrix where a parallel evaluation will be done in comparison to alloy 22.

 

Corrosion Resistance and Weldability of Alloy 59

Uniform Corrosion:

Table 2 gives the uniform corrosion rate of the alloys 59 and 22 in some standard and non-standard boiling corrosive media. As is evident, overall alloy 59 appears to have the lowest corrosion rate. The lower iron content of alloy 59 also contributes to its excellent corrosion resistance in these severe corrosive media.

Localized Corrosion Resistance:

Table 3A gives the localized corrosion resistance in Green Death, a highly chloridic low pH oxidizing solution. The higher the critical pitting and crevice corrosion temperature (CPT and CCT), the better is the localized corrosion resistance. As is evident, alloy 59 was superior to alloy 22. This is easily explained by the fact that the PREN (pitting resistance equivalent number) due to the higher molybdenum and chromium content in alloy 59 is significantly greater than for alloy 22. 

Table 3B presents the localized corrosion resistance behavior of alloy 59 and alloy 22 as measured by the CPT (critical pitting temperature) and CCT (critical crevice temperature) in the ASTM G 48 test solution (10% FeCl3). As is evident the lower molybdenum-containing alloy 22 had significantly lower CCT, than the 16% molybdenum Ni-Cr-Mo alloy 59. It has been postulated that ferric chloride may become active in the repository environment with passage of time. Hence crevice corrosion in this environment takes on an added importance. The ASTM G48 Committee has conducted round robin tests (6 laboratories) on alloy 22 where the critical pitting temperature was > 85 deg. C but the critical crevice corrosion temperature was significantly lower and varied between 50 and 60 degrees C, with only one laboratory reporting at 67 deg. C and one laboratory abstaining from providing the CCT data. This data is currently shown in ASTM Vol. 03.02 under G48 (11) specification and is presented in Table 3B. 

Thermal Stability:

This is an important feature of any alloy system in overlay welding and welding of thick sections, where multiple passes will be required. This will be the case in welding the waste containers, which will be 20 mm thick and also in the closure welds which will be even thicker. Table 4 presents the thermal stability data as measured by aging at 1600F (871 C) followed by corrosion tests in ASTM 28A and 28B test solution. As is clearly evident, the non-tungsten and non-copper containing alloy 59 was the only alloy free of any localized (inter-granular) attack. Alloy 22 suffers deep pitting and inter-granular attack due to precipitation of detrimental inter-metallic phases during the aging process. Figure 1 shows the extent of the severe pitting attack on the tungsten-containing alloy 22 with no attack on alloy 59. This same phenomenon could occur when welding thick sections requiring multiple weld passes, leading to undesirable phase precipitation in the heat-affected zone and thus becoming susceptible to pitting attack in the corrosive media of the Yucca Mountain Repository environment. 

Fabricability and Welding Characteristics of Alloy 59 Fabricability 

For alloy 59 the hot working processes like forging, rolling and extrusion, and all cold forming operations like bending, stretching, and drawing, follow the same procedure and experiences established over many years for the well established alloy C-276 and is very similar to that of alloy 22. The same is true for sawing, machining, drilling and chemical milling. The data established for alloy C-276 serves as an equivalent guideline in establishing the optimum parameters for the manufacturing of alloy 59 into various shapes. Heat-treating follows the established rules for the other Ni-Cr-Mo alloys. Solution annealing of alloy 59 is done at 2050F (1120C), similar to alloy C-276 / 22. Alloy 59, due to its improved thermal stability, is easier to handle i.e. more forgiving than other alloys of the Ni-Cr-Mo "C" family when cooling down from the temperature of solution annealing, followed by water quenching or fast air cooling. Depending on the hot forming operation, which is generally done in the temperature range of 1175 to 900C (2150 to 1650F), the material must be solution annealed followed by water quenching or fast air cooling. A solution anneal is also required after any cold forming operation, when the strain in the outer fiber is equal to or exceeds 15%. Some cases may require a solution anneal even after 10% strain. 

Weldability & Corrosion Resistance of Weldments:

Welding of alloy 59 follows the same general rules established for welding of high-alloyed nickel base materials, where cleanliness is very important and critical. Heat input should be kept low with inter-pass temperature not exceeding 150C, preferably 120C. The use of a matching filler metal is recommended (AWS A5.11 and A5.14, ENiCrMo-13, ERNiCrMo-13). Preheating is not required except to bring the material to room temperature when stored outside in cold weather. Details on welding parameters are given in alloy 59 data sheet from the supplier. (12) In comparison to other Ni-Cr-Mo and some other alloys, the sensitivity to hot cracking, as measured by the Modified Varestraint Test (MVT), alloy 59 exhibits superior behavior. In this test a specimen is melted with a GTAW (Gas Tungsten Arc Welding) torch under defined conditions over a specific length and mechanically bent over a defined radius. The total length of the cracks visible on the surface at a magnification of 25X is measured as a function of the applied bending strain. This measures the sensitivity to hot cracking resistance. Figure 2 clearly shows alloy 59 to be better than many alloys, including alloy 22. Other tungsten containing alloys such as alloy C-276 behaved similar to alloy 22. The only material, slightly better than alloy 59, was another tungsten free alloy C-4 in this test. The corrosion resistance of alloy 59 weldments is essentially similar to that of the base metal without any degradation as shown in Table 5. Corrosion resistance of various Ni-Cr-Mo alloy weldments welded with matching filler metal is shown in Table 6. As is evident, alloy 59 gave the best performance amongst all the Ni-Cr-Mo alloys tested. Both alloy C-276 and 22 not only had significantly higher corrosion rates than alloy 59 but also suffered crevice corrosion attack. 

Chronology of Various Interactions with NiDI, LLNL & Waste- package Materials Dept. of TRW Environmental
Safety Systems on Alloy 59 

NiDI Workshops On Rad-waste Containers

         NiDI (Nickel Development Institute) sponsored a forum / workshop on Rad-waste containers, Feb.25, 1995, Tucson, AZ (13)

       NiDI sponsored another forum on Phase Stability in Nickel Alloys for Rad-waste containers, March 19-20, San Diego, CA(14)

       October 6, 1998: Letter from Waste Package Materials Department of TRW Environmental Safety Systems (15) (TRW) indicating that alloy 59 would be tested in this program and that alloy 22 will not be referred to as Hastelloy alloy C-22, since this is a registered trade mark of a particular company but will be referred as either alloy 22 or by its UNS number N06022. They also initiated with ASME the request for a nuclear code case for alloy 59. ASME nuclear code case N-625 for alloy 59 was approved on May 7, 1999.

         NiDI sponsored another forum on Fabrication and Welding of Nickel and other materials for the Rad-waste containers, Oct. 27-28, 1998, Las Vegas, NV (16)

         NiDI sponsored Fourth Workshop Forum on Design / Fabrication of the Yucca Mountain Waste package for the Rad-waste containers Oct. 17-18, 2000, Las Vegas, NV (17)

Data on alloy 59 was presented at all the forums / workshops showing the superior corrosion resistance of alloy 59 and its weldments in comparison to alloy 22. 

Electrochemical Testing Comparing Alloy 22 and 59:

After discussions with LLNL and TRW personnel, alloy 59 and 22 were tested per ASTM G 61 Cyclic Potentiodynamic Polarization Testing in solution chemistries supplied by LLNL (SCW Simulated Concentrated Water and SAW Simulated Acidic Concentrated Water test solutions). The chemistry of these solutions is shown in Table 7. The test parameters were as follows: 

Temperature

90.0 deg. C

Gas Sparge

Air (150 cm3 / min.)

Initial Potential

-0.100 V from open circuit

Scan Rate

0.17 mV per second

The key-point voltage and current data are summarized below:

Polarization Scan

EOC

ECORR

EPIT

EREPASS

ICORR

Hysterisis

Alloy 59 in SAW

51

59

617

401

0.09

Yes

Alloy 22 in SAW

69

84

577

310

0.10

Yes

Alloy 59 in SCW

-247

-246

143

-201

0.25

Yes

Alloy 22 in SCW

-240

-232

145

-203            

0.24

Yes

* Note: E-values are in millivolts and I-values are in micro-amps (10-6

As is evident both alloys behaved similarly except that in one of the test solutions (SAW), alloy 59 exhibited a nobler (positive) repassivation potential (+ 401 mv) than alloy 22 (+ 310mv). Also no pits or crevice attack could be seen on either alloy when examined at 40x magnification 

Corrosion rates and localized attack propensity from this report (17) is presented below:

Polarization Scan

Corrosion rate (mpy)

Pitting & Crevice Attack Propensity

Alloy 59 in SAW Soln.

0.04

Possible

Alloy 22 in SAW Soln.

0.04

Possible

Alloy 59 in SCW Soln.

0.10

Possible

Alloy 22 in SCW Soln.

0.09

Possible

 Based on these results it is clear that alloy 59 and alloy 22 perform similarly. The report dated June 3, 1999 was forwarded to the LLNL and TRW personnel. (18) After reviewing the results, LLNL provided a new solution recipe on July 22, 1999 for the Modified SAW solution (Table 7) and suggested new tests be done on alloy 59 and 22 as well as on platinum. These were completed and the report (19) dated August 9, 1999 was forwarded to LLNL and TRW personnel.  The key-point voltage and current data are presented below along with the calculated corrosion rates.

Polarization

Scan

EOC

ECORR

EPIT

EREPASS

ICORR

Corrosion

Rate, mpy

Hysteresis

Present

Platinum

+414

+404

+620

N/A

0.080

0.047

No

Alloy 22

-30

-24

+605

+454

0.035

0.013

Yes

Alloy 59

-133

-130

+610                 

+373                 

0.031

0.013

Yes

These results were again in total agreement between the two laboratories of LLNL and Corrosion Testing Laboratories. 

Based on these results a decision was made to bring the Corrosion Testing Laboratories equipment to LLNL in Livermore, California to conduct a round robin side-by-side test on the two machines. These tests were run on September 20 through 22, 1999. The results obtained on the two machines on alloy 59 and alloy 22 were identical. After the extensive testing over the last few months and analysis of all the data, TRW issued a letter on October 14, 1999 (20) indicating that alloy 59 will be evaluated in parallel with the on going evaluation of alloy 22, which was the primary corrosion resistant barrier alloy of the waste package. Several actions were initiated:

         Thermal aging treatments will be conducted in parallel to those being performed on alloy 22 for phase analyses (identification of phases and volume fraction estimate) and production of aged test specimens for corrosion testing in simulated repository test solutions.

         Further electrochemical tests on alloy 59 in comparison to alloy 22.

The necessary samples of alloy 59 were sent to LLNL during end of 1999. KVDM has initiated its own long term aging studies on alloy 59 (both welded and unwelded specimens) up to 20,000 hrs. at 200, 300, and 427 deg C. Results of this program will be available during the last quarter of 2001 and first quarter of 2002. 

Discussions 

Alloy 59 in the various tests has clearly proven to be an equal if not a better candidate material of construction of the rad-waste containers. Its superior crevice corrosion resistance and thermal stability characteristics in comparison to alloy 22 cannot be ignored. The extensive side-by-side electrochemical tests done at LLNL and Corrosion Testing Laboratories show that the alloys behave similarly. A simple question needs to be raised: Is the program of "Rad-waste Containers" requiring long term reliability in the extremely harsh and unpredictable conditions over a period of 10,000 years, served better by using alloy 22, the current material of choice or alloy 59 which has shown to be better in many of the tests conducted or a combination of both alloys. This is a vital question needing deep scientific thought with suitable data for both the base metal and weldments. 

Recently, a study by Professor Aaron Barkatt of Catholic University (21) and Dr. Jeffery Gorman of Dominion Engineering entitled "Tests to Explore Specific Aspects of the Corrosion Resistance of Alloy 22", dated August 1, 2000, performed for the state of Nevada, clearly shows failure of alloy 22 when life expectancies of 10,000 years was considered. Alloy 22 corroded after only 30 days of exposure to water samples from the Yucca Mountain, Nevada containing lead and mercury. Fissure as deep as 0.25" were detected on alloy 22. It is recommended that alloy 59 be tested under similar conditions. 

Conclusions

            Even though alloy 22 is the current material of choice for the rad-waste containers for the Yucca Mountain project, data generated on alloy 59 proves that it is superior material of construction.

            Even though alloy 59 behaves similarly to alloy 22 in electrochemical tests, its localized crevice corrosion resistance is clearly superior to alloy 22 when measured per ASTM G48 test.

            Alloy 59, in a variety of laboratory and industrial environments, has shown better uniform corrosion resistance than alloy 22.

            Alloy 59 which is a pure ternary alloy of the Ni-Cr-Mo family shows superior thermal stability than the tungsten containing alloy 22.

            Alloy 59 shows better weldability characteristics than alloy 22.

 

 

References 

1.        R. Daniel McCright and Willis L. Clarke, Corrosion/98 paper # 159, NACE International, Houston, TX 1998

2.        J.C. Farmer and R.D.McCright, Corrosion/98 paper # 160, NACE International, Houston, TX 1998

3.        D.S. Dunn et al., Corrosion/2000 paper # 206, NACE International, Houston, TX 2000

4.        A.Roy et al., Corrosion/98 paper # 157, NACE International, Houston, TX 1998

5.        N.Sridhar et al., Experimental Investigation of Failure Processes of High Level Nuclear Waste Container Material, CNWRA 95-010, San Antonio, TX: Center for Nuclear Waste Repository Analysis, 1995

6.        B.A. Kehler et al., Corrosion/2000 paper # 182, NACE International, Houston, TX 2000

7.        D.C. Agarwal et al., The "C" family of Ni-Cr-Mo alloys partnership with the chemical process industry: The Last 70 years, Materials and Corrosion 48, 552-548, 1997

8.        D.C. Agarwal et al., Case Histories on solving severe corrosion problems in the CPI and other industries by an advanced Ni-Cr-Mo alloy 59 UNS N06059" , Corrosion/2000 , paper # 501, NACE International, Houston, TX, 2000

9.        D.C. Agarwal et al., Alloying Elements and Innovations in Nickel Base Alloys for Combating Aqueous Corrosion, VDM Report No. 23, 1995, Krupp VDM, P.O. Box 1820, Werdohl, D 58778, Germany

10.     D.C. Agarwal et al., Reliability / Corrosion Problems of FGD Industry: Cost Effective Solutions by Ni-Cr-Mo alloys & an advanced 6Mo alloy 31, Corrosion / 2000 paper # 574, NACE International, Houston, TX 2000

11.     ASTM G48, Standard Test Methods for Pitting and Crevice Corrosion Resistance of Stainless Steels and Related Alloys by use of Ferric Chloride Solution, Vol.03.02, Annual Book of ASTM Standards

12.     Krupp VDM, Alloy 59 Data Sheet, latest edition, Krupp VDM, Werdohl, Germany

13.     Forum for the use of Nickel Alloys for Radwaste Containers, The Nickel Development Institute, Held at Tucson,

AZ. Feb.22-23, 1995. Proceedings available from Nickel Development Institute, Toronto, Canada

14.     Workshop on Phase Stability in Nickel Alloys for Radioactive Waste Containers, Held at San Diego. CA, March 19-20, 1998. Proceedings available from Nickel Development Institute, Toronto, Canada

15.     Private Communication

16.     Workshop on the Fabrication and Welding of Nickel Alloys and Other Materials for Radioactive Waste Containers, Las Vegas, NV, October 27-28, 1998. Proceedings available from Nickel Development Institute, Toronto, Canada

17.     Fourth workshop on the Design & Fabrication of Yucca Mountain Waste Package for Radioactive Waste Containers, Las Vegas, NV, October 17-18, 2000. Proceedings available from Nickel Development Institute, Toronto, Canada

18.     Rick Corbett, Alloys 59 and C-22 in Yucca Mountain Project Water Formulations ASTM G61 Cyclic Potentiodynamic Polarization Testing, June, 3,1999, Available from Krupp VDM, Houston, TX

19.     Rick Corbett, Alloys 59 and C-22 in Yucca Mountain Project Water Formulations ASTM G61 Cyclic Potentiodynamic Polarization Testing Supplemental Report No. 1, August 9,1999, Available from Krupp VDM, Houston, TX

20.     Private Communication

21.     Aaron Barkatt and J.A. Gorman, Tests to Explore Specific Aspects of the Corrosion Resistance of C-22, Aug. l, 2000, NWTRB meeting, Carson City, NV

 

Table 1.
Typical Chemical Composition of "C" Family Alloys

Alloy (UNS #)

Decade Introduced

Ni

Cr

Mo

W

Cu

Fe

C (N10002)

1930's

Bal

16

16

4

-

6

C-276 (N10276)

1960's

Bal

16

16

4

-

5

C-4 (N06455)

1970's

Bal

16

16

-

-

2

22 (N06022)

Mid 1980's

Bal

21

13

3

-

3

59 (N06059)

Early 1990's

Bal

23

16

-

-

<1

686 (N06686)

Early 1990's

Bal

21

16

4

-

2

UNS N06200

Mid 1990's

Bal

23

16

-

1.6

2

 

Table 2.

Typical Corrosion Rate of Ni-Cr-Mo Alloys in Boiling Corrosive Environments (MPY)

 

Alloy

Alloy

Alloy

Media

C-276

22

59

ASTM 28A

240

36

24

ASTM 28B

55

7

4

Green Death

26

4

5

10% HNO3

19

2

2

65%HNO3

750

52

40

10% H2 SO4

23

18

8

50% H2SO4

240

308

176

1.5% HC1

11

14

3

2% HC1

51

61

3

10% HC1

239

392

179

10% H2 SO4+ 1 % HC1

87

354

70

10% H2 SO4+ 1% HC1(90C)

41

92

3

 

Table 3A.

Localized Corrosion Resistance in "Green Death' Solution

(11.4% H2SO4 + 1.2% HC1 + 1% FeCl3 + 1% CuCl2)

Alloy

PREN*

CPT (oC)

CCT (oC)

22

65

120

105

59

76

> 120

110

* PREN = Pitting Resistance Equivalent Number = %Cr + 3.3 (%Mo) + 30N

** Above 120C the Green Death Solution chemically breaks down.

  

Table 3B.

Localized Corrosion resistance in 10% FeCl3 Solution (ASTM G-48)

 

 

Alloy

PREN*

CPT (oC)

CCT (oC)

 

 

 

22

65

>85*

58**

 

 

 

59

76

>85

>85

 

* Above 85oC, the 10% FeCl3 solution chemically breaks down

** Average of the ASTM round robin tests conducted on alloy 22 at the 6 laboratories which is presented below

Laboratory

1

2

3

4

5

6

CPT

> 85oC

> 85oC

> 85oC

> 85oC

> 85oC

> 85oC

CCT

50, 50

50, 55, 55

55, 60, 60

67, 67

----

55, 55

  

Table 4.

Thermal Stability per ASTM G-28A and G-28B after Aging for 1 hr at 1600F (871 C)

 

Corrosion Rate (mpy)

Media

22*

59**

ASTM G-28A

> 500*

40**

ASTM G-28B

339*

4**

Pitting Attack

Severe

No attack

Intergranular Attack

Severe

No attack

* Alloy 22 -- Heavy pitting attack with grains falling out due to deep inter-granular attack.

** Alloy 59 -- No Attack

  

Table 5.

Corrosion Resistance of Alloy 59 Base Metal vs. Weldment

 

Media

Unwelded

GTAW*

GPAW**

H2SO4

0.003 mm/y

0.007 mm/y

0.003 mm/y

70,000 ppm Cc-

No pitting

No pitting

No pitting

pH 1, Boiling, 21 days

 

 

 

* GTAW Gas Tungsten Arc Welding

** GPAW Gas Plasma Arc Welding

  

Table 6.

Corrosion Resistance of Various Ni-Cr-Mo alloy Weldments

In High Chloride, low pH Media *

Sample

Base Metal / Filler

Corrosion Rate (mm/yr)

Pitting Corrosion

Crevice Corrosion

625 / 625

1.15

No**

No**

C-4 / C-4

0.58

No**

No**

C-276 / C-276

0.32

No

Yes

22 / 22

0.44

No

Yes

59 / 59

0.007

No

No

* 70,000 ppm C1', pH 1, Temperature 105C, 21 days

** High corrosion rate masks any localized attack

 

Table 7.
Chemistry of SCW and SAW and Modified SAW Test Solutions

 

Chemical (grams/liter)

SCW*

SAW**

Modified SAW

Ca(NO3)2*4H2O

12.17

5.89

0.30

CaC12.2H2O

7.60

---

--‑

CaCO3

37.12

---

 

H2SO4

0.0768

0.44

pH = 2.7

HC1

0.07

 

 

KCl

6.28

6.48

6.61

KHCO3

0.19

---

-‑-

MgSO4.7H2O

21.39

10.14

0.52

Na2SiO3.5H2O

0.37

0.37

0.38

Na2SO4

12.25

50.60

57.27

NaCl

---

34.89

35.59

NaF

3.18

 

 

NaHCO3

128.30

---

 

NaNO3

---

27.29

31.94

Measured pH

8.4

3.0

2.7

* SCW = Simulated Concentrated Water Solution

** SAW = Simulated Concentrated Acid Water Solution

 

Figure 1: Influence of Aging on Thermal Stability of Alloy 22 and Alloy 59 as measured after aging at 1600 F for 1 hour and testing in ASTM G-28B Test Solution

Figure 2: Hot Cracking Susceptibility of various alloys as measured per Modified Varestraint Test

 

 

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