TENSION CORROSION TESTS

1. Test preparation
2. Test with tension at 80ºC
3. Test with tension at 40ºC
4. Comparative tests with and without tension
5. Images of the samples results

The stresses which belleville washers are subjected to, is one of the factors that influence accelerating the corrosion process, both in stainless steel parts, as well as in those that carry an anticorrosive protection. Likewise, the increase in temperature also negatively affects the resistance of the parts to the corrosive agents.

In the following tests, we can see the resistance of the pieces to corrosion when they are under tension. We carried out two identical tests, but at different temperatures, to analyze, in addition, the effect that this has on corrosion. Following the results we show photographs of the state of the pieces after the tests.

Finally, we will use the results of these tests, to compare them with those we perform for the parts without tension. With this we can see the variation that occurs in one or another situation.

The corrosion of disc springs, when they are under tension, usually causes the formation of cracks which ones cause the fracture of the spring washer subjected to high stresses. In these tests, we measured the life of belleville washers, expressed in hours, before fracturing due to fatigue, due to the action of corrosive agents.

Test preparation

To perform this test, we placed belleville washers in stacks of 6 x 1 pieces (ie six washers stacked in series). These stacks are compressed up to 80% of their length and are immersed in the corrosive medium, inside a sealed container vessel as the one in the image. We prepared two sets of samples, using the different stainless steels and the anticorrosive coatings, as well as we explained in the page of  tests methodology. First of all we introduce, one of the sets of samples in a chamber at a stable temperature of 80ºC and then another one in another chamber at 40ºC, to later compare them.

The samples were daily examined, to observe their evolution. The corrosive solutions, where the stacks are immersed, are renewed every two weeks. The test is finished when one of the pieces fractures. Anyway, we establish a limit of 2,500 hours, through which the test is also considered concluded, understood as a limit of sufficient resistance.

This experiment was repeated several times. In each case, the worst lifetime of the stack was taken, for its evaluation.

Test with tension at 80ºC

In the following table, we can see the result of the test for the stacks, made at 80ºC de temperatura:

Test Conditions:

• Temperature: 80ºC
• The springs are mounted in a stack of 6x1 (Series) compressed to 80% of their capacity.

Tabla en PDF

Conclusions

• All stacks of disc springs showed a good resistance to sodium hydroxide 0.1 N NaOH, compared to the other corrosive solutions. Even the pieces with paint diluted in water, which got completely dissolved after two days, endured without fracturing. This is due to the oxide or protective hydroxide layers that form in solutions like this, where the PH is greater than 10.
• The stainless steels were only fractured in the 40% magnesium chloride MgCl2 solution, with the exception of 1.4568 Kolterised, which also suffered fracture in the seawater.
• The coatings in Dacromet and Geomet offered a good result in all corrosive media, except for citric acid (C₈H₈O₇) 0,1M.
• All the coated parts suffered fracture in the citric acid(C₈H₈O₇) 0,1M where once again it was demonstrated that the use of stainless steel is necessary.

Test with tension at 40ºC

After analyzing the stacks of the test done at 80 ° C, the same test is repeated but at 40 ° C this time. This second test is only carried out for those pieces that suffered a fracture in the previous one. The reason for this is that at a lower temperature, the corrosion rate decreases, improving the results. So it does not make sense to repeat the tests for those pieces, because they will always be> 2,500 hours.

We see in the following table the results of the test at 40ºC of temperature, for the pieces that failed in the previous test.

Test Conditions:

• Temperature: 40ºC
• The springs are assembled in a stack of 6x1 (Series) compressed to 80% capacity.

The test at 40 ° C is not necessary, having already demonstrated a result higher than 2,500 hours in the previous test, at 80 ° C.

Tabla en PDF

Conclusions

• The results of the test at 40 °C show a clear reduction in the number of stacks of washers that suffer fracture. We can see the influence of temperature on the pieces’ corrosion that have been compressed.

• The oiled parts, which had shown the worst corrosion result in the stress-free immersion tests, have nevertheless demonstrated a good resistance to breakage in almost all media. We can see that at 40ºC, only citric acid has been able to fracture them.

• The protection based on paint diluted in water, which had shown some resistance to corrosion in the tests without tension, is shown here to be ineffective. This coating has fractured in all media, with the exception of 0.1N NaOH sodium hydroxide.

Comparative tests with and without voltage

Once we have seen the results of the under tension tests at two different temperatures, we can establish a comparison with the results obtained previously in the tests without tension.

For this reason we compare the results of the following tests:

• On the one hand, the results obtained in the first table (except seawater), which means, results of the corrosion of the stacks, by their immersion in the different media, compressed at 80% and at a temperature of 80ºC.
• On the other hand the result of the immersion of the samples in the four different corrosive solutions, free of tension and at room temperature.

To be able to establish a homogeneous comparison between both tests, we transform the results into hours of the stress corrosion test, in a qualitative assessment according to the following scale, equivalent to the stress-free test:

• Good (G): Above 2,500 hours without fracture and with low manifestation of corrosion (points)
• Moderate (M): With fracture between 1,200 and 2,500 hours or with clear manifestations of corrosion (spots)
• Poor (P): With fracture between 300 and 1,200 hours or the pieces appear completely covered with a thin layer of corrosion.
• Very Poor (VP): With fracture below 300 hours or the pieces show a pronounced corrosion by a thick layer.

As we see, not only the hours without fracture of the pieces are taken into account. It is also important to consider the state of corrosion of the piece, which is visually appreciated. The fact that some stacks have not been broken, does not mean that their deterioration, due to corrosion, allows them to continue working in conditions. As we saw in the photos of the last section, some of the pieces, such as oiled, are in a very advanced state of corrosion.

The belleville washers evaluated as good, would allow to continue their use without any reservation. The samples evaluated as very poor should never be used. On the other hand, parts rated as moderate or poor should be carefully studied in each case.

This is the comparison between the different corrosive media:

• C-63: 63 x 31 x 1,8 (DIN 2093)
• B-80: 80 x 41 x 3,0 (DIN 2093)
• Kolsterised: Treatment that improves wear resistance in austenitic stainless steel.

Essay conditions:

• Room temperature
• Without air introduction in media

Tabla en PDF

Samples results images

It is also important to consider the visual appearance of the pieces after the test. In some cases, although the stacking has not been broken, we can see a state of deterioration in the washers, which we must consider in the analysis. Here are some photos of the different tests:

Results in seawater

The following images show the state of some of the washers, after the immersion test in seawater, under tension and at 80ºC:

Results in 40% magnesium chloride MgCl₂ solution

Result of several of the stacks after the immersion test in the 40% magnesium chloride MgCl₂solution, under tension and at a temperature of 80ºC:

Results in sodium chloride solution 3% NaCl

Images of stacks after the test of immersion under tension and at a temperature of 80ºC:

Results in citric Acids (C₈H₈O₇) 0,1M

Images of the result of the tests on several stacks in immersion in citric acid, under tension at 80ºC: