Report Title:  Testing of Stainless Steel and High Nickel Alloys

Summary:

Immersion corrosion testing per ASTM G31 protocols was conducted on three.  Triplicate samples of flat rolled product representing each grade were tested at elevated temperatures in three different solutions that simulated cleaning/sanitizing formulations used in dairy applications.  The solutions included 10% phosphoric acid, 5% sodium hydroxide, and 400 parts per million (0.040 weight percent) sodium hypochlorite.  All exposures resulted in negligible weight changes and calculated corrosion rates below 0.1 mils per year (<2.5 microns per year), meeting the acceptance criteria (corrosion not to exceed 10 mils per year) as required by the client.  Little or no visual evidence of pitting corrosion was found.

Results and Discussion:

The present study exactly paralleled a prior study that involved three similar type stainless steels; see “Matco Report No. --- Stainless Steel Test Program” issued -----.

Materials:

The flat rolled samples submitted for triplicate immersion corrosion testing were identified as follows: 

• Alloy No. 1 – Nominal 25 Cr / 7 Ni / 4 Mo / 0.28 N
  • Heat Number ---; 9.2 mm (0.36”) thick
• Alloy No. 2 - Nominal 20 Cr / 18 Ni / 6.25 Mo / 0.75 Cu / 0.2 N
  • Heat No. ----; 6.8 mm (0.27”) thick
• Alloy No. 3 – Nominal 21 Cr / 25.5 Ni / 4.5 Mo / 1.5 Cu
  • Heat No. ---; 1.0 mm (0.04”) thick

The three simulated solutions are listed below, along with the specified test temperature: 

1. 10% phosphoric acid (H₃PO₄) at 82°C (180°F)
2. 5% sodium hydroxide (NaOH) at 82°C (180°F)
3. 400 ppm sodium hypochlorite (NaClO) at 66°C (151°F)
 

The solutions were prepared using reagent grade chemicals (85% H₃PO₄, dry NaOH, and 5% NaClO) diluted in laboratory deionized water to the specified concentrations.  

Immersion Testing: 

The testing followed the protocols of ASTM G31, “Standard Practice for Laboratory Immersion Corrosion Testing of Metals.”    The test coupons were prepared as required in the standard:  (1) coupons rough sectioned from provided panels, (2) long edges ground parallel, (3) edges and faces hand ground to 120-grit finish on water cooled polishing unit, (4) marked for identification with vibratory tool, (5) acid cleaned in 10% nitric acid for 10 minutes at 50°C, (6) lightly scrubbed with slurry of Alconox cleaner using soft bristle brush, (6) methanol rinsed and dried, and (7) weighed (to nearest 0.1 mg on electronic laboratory balance) plus accurately measured for length, width, and thickness dimensions.

Immersion exposure was performed in sealed corrosion flasks containing 1 liter of each solution.  Each flask was equipped with an Allihn condenser to prevent evaporation.  Triplicate test samples were tied using fluoropolymer filament and suspended within the flasks such that they did not contact the vessel sidewalls or each other.   The total surface coupon area per flask was controlled so that the minimum recommended solution volume to surface area ratio (0.2 mL/mm²) was met. The flasks were maintained in the static condition without agitation or aeration.  The corrosion cells were heated on laboratory hot plates to the specified temperatures and stabilized to within 1°C. The test period was measured starting from the time when the target temperature was reached and stabilized.  Each of the immersion times was longer than the specified minimum 200 hours; 258 hours for the first solution, 213.5 hours for the second solution, and 258 for the third solution.

The corrosion cells were monitored during the immersion period.  In the phosphoric acid solution, no corrosion reaction or change in coupon appearance was observed throughout the exposure cycle.  In the caustic solution, isolated dark spots indicative of corrosion reactions developed.  Non-uniformly distributed corrosion products at the surface of several of the coupons were also visible at various times during the hypochlorite exposures.  

The spotty corrosion products that developed on the coupon surfaces occurred along the faces, edges and corners of the test coupons.  The corrosion products modified or dissipated throughout the remainder of the immersion cycle.  As similarly noted in the previous study, a possible mechanism of metastable pitting followed by repassivation may explain the in-test observations.  At the end of the 200+ hour immersion cycles, cursory visual examination of the rinsed coupon surfaces showed no obvious change in appearance due to the corrosion reactions, aside from occasional light and non-uniform staining. 

All of the coupons were lightly scrubbed with Alconox slurry, and finished in methanol before final weighing and examination using a stereo microscope. Very few and isolated corrosion pits or etch spots were detected, as shown in Figure 1.

Table 1 presents the complete data for the exposure testing.  The corrosion rate for each sample in mils per year was calculated based on weight loss, initial surface area, and actual testing time.  All three alloys in all three solutions experienced either negligible or zero weight loss, and all corrosion rates were below 0.1 mil per year.  Note also that the measured weight losses (0.5 mg at most) were within or near the limit of accuracy for the electronic balance.  The weight loss data suggest that the phosphoric acid may have produced a very slightly higher level of weight loss (however negligible) compared to the other two solutions.  To maintain client confidentiality the information contained in the table has not been published.

Conclusions:

1. Immersion corrosion testing was performed in triplicate on three (3) stainless and high-nickel alloys in three different simulated solutions per ASTM G31 protocols.   The testing was requested to determine if the alloys would meet corrosion resistance standards as required by the client.     
2. Each of the three alloys experienced negligible weight loss converted to corrosion rates less than 0.1 mil (~2.5 microns) per year in 10% phosphoric acid (82°C), 5% sodium hydroxide (82°C), and 400 ppm sodium hypochlorite (66°C) in test cycles of more than 200 hours. The specified standards were therefore met.
3.  Stereo microscope examination showed occasional light staining and possible light pits or etch spots on several coupons.  In practical terms, the alloys were unaffected by immersion corrosion testing in all three solutions.

Figures 1-10.  Several stereo microscope views of corrosion test coupons after exposure in simulated solutions. To maintain client confidentiality the figures associated with this report have not been published.