The best way to verify the integrity of the sub-surface structure of a casting is by x-ray. By taking x-ray shots along the length of the casting to ASTM E94 it is possible to view images of the structure. Therefore, generally defining the overall integrity of the casting. X-ray acceptance standards ASTM E186 and ASTM E446 list defects from severity level 1 (the best level) down to level 5 (the worst level).
Many companies are now specifying that anodes should be supplied that pass an x-ray test to level 1, level 2 or level 3 ASTM E186. Although it is not economically viable to carry out an x-ray test on every anode, it is possible to demand that anodes supplied should pass an x-ray criteria level such as level 2 or better and that a percentage of anodes should be tested. An x-ray test might cost approximately $100 per test so it is common for specifications to require 1 in 50 anodes to undergo testing, hence adding $2 to the cost of each anode.
What is the significance of defect levels?
By examining the quality of levels 1 to 5 that are displayed by anodes you are able to understand how the theoretical life design of the anode system will be affected.
The table below represents the most up to date summary for assessing the design life against the x-ray level acceptance criteria. Some companies, such as the ENI Group simply state their refusal to accept any anodes that are below level 3 and other companies have based their design life on their own experience and installations.
|X-Ray Level||Design Life|
If the x-ray testing determines the casting flaws are level 1 then it is highly unlikely that the life of the anode will be cut short.
At level 2 for the acceptance criteria, there are some specific flaws that will influence the overall life of the anode. However, the size and frequency of these flaws are sufficiently small so their existence does not immediately require the need for the casting to be scrapped. The x-ray acceptance criteria should (and does) take in to account both the size, frequency and type of defect present.
At acceptance criteria level 3 the casting may not perform correctly. Even though the silicon iron material retains gas porosity, if the anodes are produced to level 3 then there are additional flaws in the manufacturing procedure which are quantifiable and ideally should not be accepted.
Level 4 and 5
This is the same for level 4 and 5 as these flaws are increasing and the design life will be dramatically shortened.
OUR TESTING AND RESULTS
We set out to evaluate the silicon iron castings and the quality and properties for die-cast versus centrifugally-cast anodes through controlled accelerated corrosion tests. These are our key findings.
In the top x-ray (of the die-cast anode) dark circular shadows depicting sub-surface shrinkage can be seen around the area of the pouring cup, i.e. where the metal has been poured into the anode. These shadows are depicting an area of shrinkage and it is clear from the image that the area around the pouring cup also contains additional fragmented pockets of shrinkage seen as ‘dark patches’ scattered away and around the pouring cup position. In the image, this area has been de ned as a level 3 defect. If an accelerated corrosion test was carried out on the whole anode, then this area would be consumed at a faster rate than the remainder of the anode. Furthermore, these areas around the pouring cup and riser have historically been the source of fissuring and cracking of the anode causing premature failure.
Likewise, if a mechanical test bar was machined out of this section of the casting, the results would fall far short of those exhibited by any section of the centrifugally-cast anode. We have retained these pieces for further testing by any interested party, if so desired.
This change in section is an inherent weakness in the design of a die-cast anode because the change in wall thickness means the thicker part of the casting cools more slowly than the narrow section. This slower cooling rate not only causes an inferior grain structure that is almost as poor as sand-cast anodes, but also necessitates an additional riser to help feed the shrinkage that is catalysed from the change in section. The x-ray image of the die-cast anode above shows a serious casting aw visible as a dark shadow in the connection area and is a real-life sample con rming the di culties encountered with this design.
By comparison, any impurities in a centrifugal casting are thrown to the bore of the casting by application of Stoke’s law and are thus not detrimental to the life of the anode. This is a unique feature of the Centrifugal Casting method and is highly advantageous to the integrity of the casting.
It should be noted that the type of defect seen on the x-ray and its position in the casting will further influence the design life. For instance; a linear shrinkage defect in the wall of the casting may well cause a more dramatic reduction in life if it causes the part of the anode to ssure or tear along its axial length when in use.
Additionally, the centrifugally cast anode utilises a straight-walled anode design that has no change in cross section or thickness at the centre of the anode and no complex arrangement of sand cores (die cast anodes require up to 10 sand cores per anode) which are susceptible to individual discrepancies between every casting.
Specialist Castings have a manufacturing plant in the UK and a state of the art $2m manufacturing facility at Ningbo in China, where both tubular anodes and solid anodes are made. We have anodes stocked in 2 locations in the USA, utilising purpose-made connectors that have been used in over 1 million anodes over the last 35 years (see g 4 for TA2 and TA5A connectors). We have previously been the only manufacturer exclusively supplying tubular anodes to the Durichlor 51 Anode company. The only tooling that is required to make these connections are 2 torque wrenches (1.2 metres long) that tighten the bolt heads at the end of the connectors, to a pressure of 40lbs (ft/lbs) or 54Nm. These connections can be made in the factory or in the eld.
We have 3 XRF chemical analysis machines (in both the UK and China) and our pricing re ects the signi cant disparity between production costs when comparing foundry production costs in China to those in North America. This chart shows a comparison in sizes between Specialist Castings Anodes and other common anode sizes.
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