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.
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.
In the following table, we can see the result of the test for the stacks, made at 80ºC de temperatura:
|Variants||Different Aggressive Solutions|
|Spring + Material + Manufacturing Method||Sea water||40% MgCl2||3% NaCl||0,1 n NaOH||0,1 m acid|
|C-63 - 1.4310 - Stamping - Rectified||> 2.500 hs||356 hs||> 2.500 hs||> 2.500 hs||> 2.500 hs|
|C-63 - 1.4310 - Stamping - Rectified - Shot Peneed||> 2.500 hs||429 hs||> 2.500 hs||> 2.500 hs||> 2.500 hs|
|B-80 - 1.4310 - Stamping - Rectified||> 2.500 hs||1.968 hs||> 2.500 hs||> 2.500 hs||> 2.500 hs|
|C-63 - 1.4568 - Stamping - Rectified||> 2.500 hs||140 hs||> 2.500 hs||> 2.500 hs||> 2.500 hs|
|C-63 - 1.4568 - Stamping - Rectified - Shot Peneed||> 2.500 hs||140 hs||> 2.500 hs||> 2.500 hs||> 2.500 hs|
|C-63 - 1.4568 - Stamping - Rectified - Shot Peneed - Kolsterised||284 hs||2177 hs||> 2.500 hs||> 2.500 hs||> 2.500 hs|
|Yellow Galvanized||912 hs||> 2.500 hs||> 2.500 hs||> 2.500 hs||68 hs|
|Transparent Galvanized||1.129 hs||> 2.500 hs||> 2.500 hs||> 2.500 hs||68 hs|
|Dacromet||> 2.500 hs||> 2.500 hs||> 2.500 hs||> 2.500 hs||891 hs|
|Geomet||> 2.500 hs||> 2.500 hs||> 2.500 hs||> 2.500 hs||891 hs|
|Delta Tone + Delta Seal||620 hs||> 2.500 hs||738 hs||> 2.500 hs||526 hs|
|Paint dissolved in water||1.057 hs||837 hs||45 hs||> 2.500 hs||380 hs|
|Oiled||837 hs||> 2.500 hs||360 hs||> 2.500 hs||262 hs|
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.
|Variants||Different Aggressive Solutions|
|Spring + Material + Manufacturing method||Sea water||40% MgCl2||3% NaCl||0,1 n NaOH||0,1 m acid|
|C-63 - 1.4310 - Stamping - Rectified||●||> 2.500 hs||●||●||●|
|C-63 - 1.4310 - Stamping - Rectified - Shot Peneed||●||> 2.500 hs||●||●||●|
|B-80 - 1.4310 - Stamping - Rectified||●||> 2.500 hs||●||●||●|
|C-63 - 1.4568 - Stamping - Rectified||●||> 2.500 hs||●||●||●|
|C-63 - 1.4568 - Stamping - Rectified - Shot Peneed||●||> 2.500 hs||●||●||●|
|C-63 - 1.4568 - Stamping - Rectified - Shot Peneed - Kolsterised||> 2.500 hs||> 2.500 hs||●||●||●|
|Yellow Galvanized||> 2.500 hs||●||●||●||45 hs|
|Transparent Galvanized||> 2.500 hs||●||●||●||284 hs|
|Dacromet||●||●||●||●||> 2.500 hs|
|Geomet||●||●||●||●||> 2.500 hs|
|Delta Tone + Delta Seal||> 2.500 hs||●||> 2.500 hs||●||> 2.500 hs|
|Paint dissolved in water||834 hs||694 hs||116 hs||●||1.917 hs|
|Oiled||> 2.500 hs||●||> 2.500 hs||●||356 hs|
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.
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.
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:
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:
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:
|Variants||Different aggressive solutions|
|Spring + Material + Manufacturing method||40% MgCl2||3% NaCl||0,1 n NaOH||0,1 m acid|
|C-63 - 1.4310 - Stamping - Rectified||M||P||G||G||G||G||G||G|
|C-63 - 1.4310 - Stamping - Rectified - Shot Peneed||M||P||G||G||G||G||G||G|
|B-80 - 1.4310 - Stamping - Rectified||M||M||G||G||G||G||G||G|
|C-63 - 1.4568 - Stamping - Rectified||M||VP||G||G||G||G||G||G|
|C-63 - 1.4568 - Stamping - Rectified - Shot Peneed||P||VP||M||G||G||G||G||G|
|C-63 - 1.4568 - Stamping - Rectified - Shot Peneed - Kolsterised||VP||VP||P||M||G||G||G||P|
|Delta Tone + Delta Seal||G||M||M||P||G||P||P||P|
|Paint dissolved in water||G||P||G||P||M||M||P||VP|
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:
The following images show the state of some of the washers, after the immersion test in seawater, under tension and at 80ºC:
Result of several of the stacks after the immersion test in the 40% magnesium chloride MgCl2solution, under tension and at a temperature of 80ºC:
Images of stacks after the test of immersion under tension and at a temperature of 80ºC:
Images of the result of the tests on several stacks in immersion in citric acid, under tension at 80ºC: