Corrosion Behavior of Selected Stainless and Nickel Alloys, and Anti-Galling Coatings in a Sour Environment

Introduction

The use of stainless steels and nickel alloys for production tubulars and other oilfield components has led to much testing and evaluation of these materials for resistance to corrosion and environmentally induced embrittlement. Another important aspect of these materials is their inherently poor resistance to galling as compared to conventional steels. Consequently, the galling resistance of these steels is an important consideration, particularly in threaded connections. Extensive testing has been carried out on the effects of various antigalling coatings on the "make-break" behavior of threaded connections. By comparison, little work had been conducted on the corrosion resistance of antigalling coatings and their effects on the corrosion of substrate materials. This report summarizes an experimental investigation conducted on the behavior of various antigalling coatings in sour environments. Laboratory tests were conducted to examine the corrosion resistance of antigalling coatings and their effect on the corrosion behavior of selected stainless steels and nickel alloys.

Experimental Procedure

Corrosion coupons machined from stainless steel and nickel alloy tubulars were exposed to NACE Solution in both the uncoated and coated condition. These samples were evaluated following exposure on the basis of (1) nature of corrosion, (2) extest of general and localized (pitting and crevice) corrosion, and (3) condition of coating.

Materials

Four alloys were chosen as the substrate materials for this study: (1) 13 Cr, (2) SAF-2205, (3) Sanicro 28, and (4) Hastelloy alloy C-276. The nominal compositions of these materials is shown in Table 1. In all cases, the starting materials were mill-processed tubing. A total of eight antigalling coatings were examined: (1) Electroless Ni, (2) Zinc Silicide, (3) Sn Electroplate, (4) Zn Electroplate, (5) Cu Electroplate, (6) Oxylate Coating, (7) Al/Cu ion Implantation coating and (8 Cr/Au Ion Implantation Coating. The Electroless Ni, Zn and Cu coatings were employed with a thin under layer of Ni electroplate.

Specimens

The specimen used in this study was a creviced corrosion coupon of dimensions 5/8-inch x 4 inches x 1/8-inch. A ¼ -inch diameter hole was located at each end which was used to crevice the samples as well as to attach them to the specimen holder. The coupons were creviced with slotted teflon washers which were ½ -inch diameter x 1/8-inch thick with four 1/32-inch wide slots on the side facing the metal surface. Appendix I shows the condition and configuration of the coupons as they were tested.

Environment

The coupons were separated into eight groups according to the nature of the coatings and substrate material. The groupings used in this study are given in Table 2. In all cases, NACE Solution was used as the test environment. This media consists of an aqueous solution containing 5 percent NaCl and 0.5 percent acetic acid saturated with H2S at 15 psia. The solution was mixed as a master batch which was subsequently transferred into the one-gallon test containers. These tests were conducted at 75 F for a period of 30 days.

Results

Visual Examination

Following exposure, the coupons were removed from the test solution, rinsed with distilled water, ethanol and acetone, and dried in air. After this treatment, the specimens were examined. A majority of the specimens were then cleaned in an inhibited 15 percent hydrochloric acid solution to remove sulfide corrosion products. The only samples not cleaned in this manner were the coupons coated with the electrolytic Cu coating and the ion implantation coatings on the materials other than 13 Cr. Half of the Cu coated samples were cleaned in a 5 percent potassium cyanide solution to remove the Cu sulfide corrosion products. It was found that the condition of the coatings was not significantly changed as a results of the cleaning operations. The main result of the cleaning operations was to remove the sulfide corrosion products from the corroded materials. Consequently, a majority of observations noted in this section were made on the cleaned specimens. These observations are given below:

Uncoated Materials

Specimen Material Condition 00 Sanicro 28 slight pitting attack under the crevice of 01 Sanicro 28 depth 0.001-0.003 inch; no attach elsewhere 02 13 Cr severe general and pitting corrosion; pit 03 13 Cr depth 0.018-0.023 inch; no crevice corrosion 04 SAF-2205 slight crevice attack of depth 0.001-0.007 05 SAF-2205 inch. No attack elsewhere 06 C-276 No attack 07 C-276

Cr/Au Ion Implant Coating

Specimen Material Condition 07 Sanicro 28 coating generally good with some small areas 08 Sanicro 28 of poor adhesion under crevice; slight pitting under crevice of depth 0.002-0.005 inch in poor adhesion areas 37 13 Cr coating remaining only under creviced areas; 38 13 Cr pitting and general corrosion with pit depth of 0.005-0.025 inch; no crevice corrosion 47 SAF-2205 coating generally in good condition; crevice 48 SAF-2205 corrosion on bare side of coupon of depth approx. 0.002 inch 77 C-276 bad coating adhesion on specimen 77; no 78 C-276 corrosive attack on specimens

A1/Cu Ion Implant Coating

Specimen Material Condition 17 Sanicro 28 poor coating adhesion ; 18 Sanicro 28 evidence of intergranular corrosion of coating under crevice; no corrosive attack of specimens 27 13 Cr little evidence of coating present on corroded 28 13 Cr specimens; pitting depth on specimens of 0.005-0.012 inch 57 SAF-2205 little evidence of coating on specimens; 58 SAF-2205 etching of entire surface of specimens; severe crevice corrosion of depth 0.002-0.005 inch 67 C-276 poor coating adhesion; no corrosive attack 68 C-276 on specimens

Electrolytic Cu Coating

Specimen Material Condition 05 Sanicro 28 removal of Cu coating by sulfide corrosion 15 Sanicro 28 except under crevice; nickel undercoating remained; some blistering; no corrosive attack on specimens. 25 13 Cr severe sulfide corrosion of Cu coating; 35 13 Cr severe general and pitting attack of specimen in uncoated region with pit depths of 0.005- 0.008 inch 45 SAF-2205 sulfide corrosion and poor adhesion of Cu 55 SAF-2205 coating; some blistering of coating; etching of exposed surfaces of base material; no crevice corrosion 65 C-276 sulfide corrosion, blistering and poor 75 C-276 adhesions of coating; no corrosion attach on specimen

Electrolytic Sn Coating

Specimen Material Condition 03 Sanicro 28 coating in excellent conditions; no corrosive 13 Sanicro 28 attack on specimens 23 13 Cr coating in excellent conditions; general and 33 13 Cr pitting corrosion of bare regions of coupon with pit depths of 0.005-0.010 inch 43 SAF-2205 coating in excellent condition; etching of 53 SAF-2205 bare region of coupon 63 C-276 coating in excellent condition; no corrosive 73 C-276 attack on base material

Electroless Ni Coating

Specimen Material Condition 01 Sanicro 28 coating tarnished; no corrosive attack of 11 Sanicro 28 base material 21 13 Cr coating tarnished and badly blistered; general 31 13 Cr and pitting corrosion of bare regions of specimen with pit depths of 0.008-0.010 inch 41 SAF-2205 coating tarnished; no corrosive attack of 51 SAF-2205 base material 61 C-276 coating tarnished; no corrosive attack of 71 C-276 base material

Zinc Silicide Coating

Specimen Material Condition 02 Sanicro 28 coating mostly removed during exposure, 12 Sanicro 28 but left deposit under crevice; no corrosive attack on base material 22 13 Cr coating removed during exposure; general 32 13 Cr corrosion with no evidence of crevice corrosion 42 SAF-2205 coating removed during exposure; etching 52 SAF-2205 of surface of coupons; severe crevice corrosion of depth 0.002-0.005 inch 62 C-276 coating removed during exposure except for 72 C-276 deposit under crevice; no corrosive attack on base material

Electrolytic Zn Coating

Specimen Material Condition

04 Sanicro 28 Zn removed, only Ni undercoating remains; 14 Sanicro 28 no corrosive attack on base material 24 13 Cr Zn removed, only Ni undercoating remains; 34 13 Cr general corrosion with no pitting of bare regions of specimens 44 SAF-2205 Zn removed, only Ni undercoating remains; 54 SAF-2205 slight etching of bare region of specimen with no crevice corrosion 64 C-276 Zn removed, only Ni undercoating remains; 74 C-276 no corrosive attack of base material

Oxylation Coating

Specimen Material Condition 06 Sanicro 28 coating removed during exposure; no 16 Sanicro 28 corrosive attack on base material 26 13 Cr coating removed during exposure; severe 36 13 Cr general and pitting corrosion of base material with pit depths of 0.012-0.024 inch 46 SAF-2205 coating removed during exposure; slight 56 SAF-2205 etching and severe crevice corrosion with pit depths of 0.002-0.005 inch

A summary of the localized corrosion behavior exhibited by the various materials is given in Table 2. The corrosion depth were determined after the one month exposure and normalized over one year to get mpy for pitting. For the 13 Cr, these depths refer to pitting which occurred on all exposed surfaces of the coupon in combination with general corrosion. Essentially no crevice corrosion was observed for 13 Cr in these experiments. The pit depths measured for Sanicro 28 and SAF 2205 refer to localized corrosion and pitting which occurred under the crevices. Table 2 indicates that the application of different antigalling coatings can effect the corrosion behavior of the stainless steels. This behavior waried from that exhibited by SAF 2205 where large variations existed to HASTELLOY Alloy C-276 which was essentially unaffected. The behavior of SAF 2205 was such that the Al/Cu, Cu and oxylate coatings accelerated crevice corrosion. By comparison, the Sn, Ni an Zn coatings appeared to retard in the tests, the application of the Zn and Zn silicide coatings appeared to retard pitting. The behavior of the duplex stainless steel (SAF 2205) is most likely due to the marginal passivity of this material in the relatively low pH, H2S and Cl- containing environment. Under such conditions the galvanic effects of the various coatings can be most readily observed.

TABLE 1

Groupings for Exposure Tests Vessel Material Coatings 1 13 Cr Ni, Sn, cu 2 13 Cr Zn Silicide, Zn, uncoated 3 13Cr Cr/Au, Al/Cu 4 SAF 2205 Oxylation SANICRO 28 Oxylation 5 SAF 2205 Ni, Sn, Cu SANICRO 28 Ni, Sn, Cu C-276 Ni, Sn, Cu 6 SAF 2205 Zn Silicide, Zn SANICRO 28 Zn Silicide, Zn C-276 Zn Silicide, Zn 7 SAF 2205 Cr/Au, Al/Cu, uncoated SANICRO 28 Cr/Au, Al/Cu, uncoated C-276 Cr/Au, Al/Cu, uncoated 8 13 Cr Oxylation

TABLE 2 Summary of Observations for Localized Corrosion Behavior of Corrosion

Coupons Material Coating Pit Depth (inch) Pitting Rate (mpy)

SANICRO 28 -- 0.001-0.003 12.36 Cr/Au 0.002-0.005 24-60 Al/Cu <0.001* 0 Cu <0.001* 0 Sn <0.001* 0 Ni <0.001* 0 Zn Silicide <0.001* 0 Zn <0.001* 0 Oxylation <0.001* 0 13 Cr -- 0.018-0.023 216-276 Cr/Au 0.005-0.025 60-300 Al/Cu 0.005-0.012 60-144 Cu 0.005-0.008 60-96 Sn 0.005-0.010 60-120 Ni 0.005-0.010 96-120 Zn Silicide <0.001** 0 Zn <0.001** 0 Oxylation 0.012-0.024 60-288 SAF 2205 -- 0.001-0.007 12-84 Cr/Au 0.001-0.002 12-24 Al/Cu 0.005-0.012 60-144 Cu 0.005-0.008 60-96 Sn <0.001* 0 Ni <0.001* 0 Zn Silicide 0.002-0.005 24-60 Zn <0.001* 0 Oxylation 0.002-0.005 24-60

C-276 -- <0.001* 0 Cr/Au <0.001* 0 Al/Cu <0.001* 0 Cu <0.001* 0 Sn <0.001* 0 Ni <0.001* 0 Zn Silicide <0.001* 0 Zn <0.001* 0 Oxylation not tested --

* - No corrosive attack ** - Uniform corrosion without pitting