Corrosion Testing Results

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A 3-month corrosion test was performed in a simulated marine environment to evaluate the seawater corrosion resistance of Inconel 718. Three test samples were utilized which namely were first, a machined cylindrical (8-inch diameter by 4.75 inch thick) part made out of Inconel 718, and secondly a welded assembly composed of a DMLS fabricated Inconel 718 tiedown welded to a 1-inch thick annealed AISI 4130 steel plate that utilized Inconel 718 welding rod via the TIG welding method, and thirdly, a simple rod of AISI 4130 annealed steel. All samples were thoroughly cleaned prior to the test by sanding followed by being wiped down with a clean cotton cloth wetted with acetone. All samples were placed in separate containers in a plastic clear greenhouse tentlike structure. A small child’s pool was filled with water and placed in the center of the greenhouse and an inflatable boat was floated on top of the small pool. The Inconel 718 tiedown and welded steel plate assembly was placed inside the boat simulating a flight deck on a destroyer. Once the greenhouse was closed, the relative humidity rose fast and remained very high all throughout the 3-month test thus simulating a marine environment. Occasionally, the water in the pool was replenished to compensate for evaporation. Attached to the pool was a humidity and temperature remote sensor that sent constant signals to a control station which readings were recorded daily. A measured amount of genuine pacific seawater was daily splashed upon all three test samples. This resulted in all samples being constantly wet with seawater and partially immersed in seawater due to the accumulation of daily splashing. Relative humidity and temperature values were recorded daily. 

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The object of this corrosion test was to determine the seawater corrosion resistance of the two Inconel 718 parts (machined and DMLS fabricated) and if they differed in resistance and also evaluated was the galvanic compatibility of AISI 4130 steel plate welded to a part made of Inconel 718. Lastly, a sample of unprotected AISI 4130 steel rod was utilized to show the contrast between noncorrosive resistant material versus the high corrosive resistance of Inconel parts. The AISI 4130 steel plate, at the beginning of the test, to prevent its corrosion had two coats of primer paint applied followed by two coats of enamel paint applied. A few days later it was observed that some areas of the steel plate were corroding because of the severe corrosive environment and due to defects in the coating which did not cover all areas of the steel so several more coats of paint (primer and enamel) needed to be applied to the AISI 4130 steel. This was done to avoid contamination of the Inconel 718 samples. Any defects in the steel plate paint coating showed up promptly. The results of the Inconel 718 parts indicated that no corrosion ever was present. The Inconel parts after 3 months were bright, shiny, and there was no evidence at all of corrosion and were completely unreactive to the constant exposure to seawater in a very high humidity simulated marine environment. The Inconel 718 DMLS fabricated tiedown and the machined cylinder of Inconel 718 material were both completely unreactive to daily splashing of seawater and continuous partial immersion in seawater. There was no difference in the corrosion resistance of either Inconel 718 test sample which proves that a DMLS fabricated Inconel 718 part has equal seawater corrosion to a machined part made of Inconel 718. Both were completely impervious to seawater corrosion. As expected, the AISI 4130 control sample immediately began corroding within minutes of exposure to seawater and after 3 months was heavily corroded. 

 

Pictures of the results of this corrosion test and the test samples and setup are shown below.

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Figure 1. Day 1 of the Corrosion Test. Pictured here is the AISI 4130 annealed steel rod just prior to exposure to seawater on day 1.

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Figure 2. Day 1 of the Corrosion Test. Pictured here is the Inconel 718 tiedown welded to the painted welded steel plate (upper and lower views). Note that the tiedown was sanded to remove the purple oxide coating deposited during solution annealing and precipitation heat treatment.

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Figure 3. Day 12 of the Corrosion Test.

 

 

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Figure 4. Day 12 of the Corrosion Test. Pictured here is the 4130 steel corroding. Like all samples it remained wetted with seawater and partially immersed in seawater.

 

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Figure 5. Day 92 of the corrosion test. Pictured here is the simulated marine environment setup. The greenhouse constantly remained sealed, retaining humidity, except when daily it was opened to splash seawater on all test samples. A measuring cup, as shown, was used to splash a measured amount of seawater daily on all samples.

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Figure 6. Day 92 of the Corrosion Test. Pictured here is a closeup showing all three test samples. The orange-colored liquid is the 4130 rod corroding. The Inconel 718 tiedown and steel plate assembly is resting inside the boat on top of a pool of water.

 

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Figure 7. Day 92 of the Corrosion Test. Pictured here is a closeup of the 3 test samples.

 

 

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Figure 8. Day 92 of the Corrosion Test. The Inconel 718 tiedown was completely free of corrosion and was as shiny and new looking as day 1. No evidence of any galvanic corrosion between the steel plate and the Inconel 718 existed.

 

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Figure 9. Day 92 of the Corrosion Test. All three test samples were taken out of the greenhouse and photographed. As one can see, both test samples made of Inconel 718 were bright, shiny, and clean with no evidence of any reaction to the seawater whatsoever nor any corrosion at all. The welded Inconel 718 tiedown and 4130 plate assembly looked like new with no corrosion of any kind present nor any galvanic corrosion to the painted steel plate either. Only the unprotected 4130 steel rod was corroded – as expected.  Inconel 718 is completely impervious to seawater corrosion and will last 40 years in a marine environment with no required maintenance. After 40 years exposure to seawater these Inconel 718 tiedowns will be clean, bright and shiny and look like new – just like one can see here in these photos.

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Figure 10. Day 92 of the Corrosion Test. Pictured here is another photo of the 3 test samples immediately after removing them from the greenhouse. As one can see, the Inconel parts are completely corrosion free. They look as new as day 1.