This is so, even for a matching nut and bolt
Thanks to Luis Fernandes Silva for providing the sample.
It is not possible to talk sensibly of corrosion resistance without reference to the specific environment that is to be resisted.
Whilst it is true we might not see performance differences between commercial samples of 304 SS in terms of resistance to say chloride pitting, the world of difference can appear if we add sulphate, and so invite sulphate reducing bacteria to our party.
The above picture is a section through the nut/bolt part of a 10mm expansion bolt taken from the sulphate sea cliffs of Praia da Ursa. The bolt snapped at a point below the rock surface when struck with a hammer, and thus left the undisturbed nut/bolt assembly for me to play with. I embedded it in epoxy resin, and then ground and polished a section that runs pretty much through the centre-line of the bolt. Metallurgical differences in composition have been highlighted by very brief anodic etching in 50% nitric acid.
We have looked at this bolt in an earlier post where it was shown that the small weep from the nut/bolt junction was sulphide positive.
In order to understand the significance of what we are seeing here then it is necessary to get across the rather extensive backstory. The elevator pitch goes something like –
- low nickel steels such as 304 have a tendency to transform from their initial austenitic structure to a martensitic structure when deformed during the production process.
- the martensite so formed is localised to the slip planes of the crystal lattice that facilitate the deformation of the metal.
- such bands of martensite open ‘highways’ for atomic hydrogen penetration into the bulk metal, and thereby facilitate the various destructive modes of hydrogen attack.
- sulphate reducing bacteria (SRB) produce hydrogen sulphide which acts to enhance the uptake of atomic hydrogen by the metal surface, and thus sponsors hydrogen attack.
- depending upon its martensitic composition, there is a large difference in the vulnerability of a steel to SRB-induced hydrogen attack.
- products formed from 304 can vary greatly in amount of martensite acquired during processing, and thus will vary greatly in their resistance to SRB mediated hydrogen attack.
Before the bolt was sectioned, I used a magnetic balance technique to establish a rough estimate of the martensite levels within the bolt and nut assembly. It is a fairly precise measurement, but, in its current form, suffers from poor calibration. Even so it was easy enough to establish that whilst the martensite composition varied between 30% and 50% along the length of the bolt, the nut was much less magnetic – maybe about 5%. This difference in martensite composition would be sufficient to engender a 20-fold difference in the atomic hydrogen diffusion capability of the two materials. We would anticipate the bolt to have a life expectancy of 5 years, compared with 100 years for the nut fitted to it. The basis for this calculation can be found in this earlier post.
| item | martensite % | expected service age (years) |
|---|---|---|
| bolt | 30 – 50 | 3 – 5 |
| nut | 5 | >100 |
Furthermore it was very obvious that the bulk of the magnetism was associated with the rolled threads of the bolt. The process of rolling, rather than cutting, threads confers robustness by imparting high levels of work hardening to the thread surfaces. However, for 304 bar stock that has a nickel composition close to the 8.0% specification limit, it is likely that such work hardening will be accompanied by high levels of martensitic conversion unless the process temperature is maintained somewhere in the region of 80ºC to 100ºC. As evidenced by the specimen we have here, it is likely that the forming temperature is not considered critical by the manufacturer, and consequently we will see significant variation in the martensitic content of the resultant product.
If we take a look at the metal toward the centre-line of the bolt we see clearly defined grain boundaries with an abundance of strain-induced features, something not altogether unexpected for a part that has been cold-worked to improve its tensile strength.
We can ignore for now the delta-ferrite stringers which originate from a foundry process upstream of bolt manufacture proper. The main items to note are the twinning features that partition individual crystals, and the multitude of slip bands that populate any one twin partition.
As we move our attention from the centre-line of the bolt out toward the rolled thread edges, the orderly grain structure gives way to the more chaotic pattern of the material comprising the highly, work-hardened threads. The criss-crossing of a multitude of slip bands are visible, though faint.
There is nothing particularly remarkable to be observed here until we come to examine a section of thread that presumably has been impacted by the action of SRB. The difference is striking.
We need to clear about what we are looking at here. When a chemical contrast treatment is applied, as is the case for this image, and for the ones above, what remains on view is the result of a chemistry experiment. We can say for sure that the contrast differences we observe are spatial differences in the composition of the metal matrix. We can’t be so sure what those differences mean.
However, we can draw two conclusions that are likely to be sound. Firstly a chemical agent is diffusing into the solid metal from the outside, and secondly, that agent is materially changing the metallurgy at the slip planes.
The literature abounds with evidence supporting the migration of atomic hydrogen along the transformed martensite of the slip planes of the crystal lattice. However, it is not so replete with evidence for mechanisms by which the permanent disordering of this material might occur. I’ll point once again to this earlier post for the very extensive background information that underlies what I am saying here.
So what is so bad about the penetration of the metal by hydrogen? Well, if you look at the photo above, it can been seen that, at certain points, there is some sort of coalescence occurring along the lines of intersecting slip planes. The photo below identifies points where such intersections seem to be acting as sites for the the initiation of stress cracks.
Looking elsewhere on the specimen, it is easy to locate points on the thread surface where the cohesion of the metal matrix has been completely lost. Note that this loss of cohesion is not at the inter-granular boundaries, but is on a much finer scale, with multiple zig-zag cracks traversing any one metal grain.
It isn’t that the material of the nut is impervious to corrosion, it most definitely is, as can be seen in the photo below. However any such corrosion is superficial, and we don’t observe evidence for a diffusive process running into the metal ahead of the bulk corrosion process.
I have a large post in preparation that will look at the issue of brittle fracture at Cabo da Roca in greater detail.
This short report is little more than a foretaste of this complex but fascinating phenomenon.