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Referencia 9026 |
| Título: |
¿Por qué se oxidan los tornillos de acero inoxidable? |
| Título original: |
Why Do Stainless Steel Screws Rust? |
| Resumen: |
A case study involving research and analysis into the cause of a rust condition on the heads of stainless steel screws.
A Taiwanese fastener supplier received claim from one of its customers about a rust condition that was observed on the heads of stainless steel screws after a 24-hour acid and salt spray test. The salt spray test conducted by the supplier before the shipment of the screws indicated no evidence of any rust condition.
Therefore, the supplier asked Q-LAB Inc. to perform an RCA (Root-Cause Analysis) on the stainless steel screws and to provide the company with a suggestion for a possible remedy to the problem. The supplier agreed to allow Q-LAB Inc. to test the stainless steel screw parts with any suitable international test method.
Test Samples Two groups of samples were provided by supplier and customer. The supplier provided only passivated parts (no nonpassivated parts were available). The customer provided two groups of rusted samples including two sample pieces from after the acid salt spray test and two sample pieces from after the salt spray test only.
The fastener samples tested were 302 stainless steel passivated, Phillips Ty-I truss head machine screws, sized #8-32 UNC-2A x 2-1/4" Test Limitations As Q-LAB does not have the equipment to conduct the acid salt spray test, only neutral salt spray tests were conducted.
Also, because all of the samples were from after the passivation process, Q-LAB could not compare and test the plain parts (from before the passivation). Only four failed parts were observed. Insufficient quantities may not represent the condition of the whole group, which could result in an incorrect conclusion.
Analysis Method Document Review: The passivation process document that was issued by the supplier indicated the treatment solution, volume percentage, temperature and time duration of immersion.
Q-LAB Inc. simply compared these parameters with ASTM A380. See Table 1 for the results of this review.
Salt Spray Test: Q-LAB randomly selected two sample pieces from the supplier to perform the salt spray test. There was not any stain or rust observed on any surface of the parts after the 24-hour salt spray test. However, these two screws were not from the same lot as the failed screws provided by the customer.
Macroscopic Method: A 10x power microscope was used to observe the head surface of the failed screws provided by the customer. Refer to Figure 1, Figure 2, Figure 3 and Figure 4 for macroscopic views of the samples.
Reference Standards The reference standards utilized in conducting these tests included the following:
• ASTM B117 Standard Practice for Operating Salt Spray (Fog) Apparatus.
• ASTM A380 Standard Practice for Cleaning, Descaling, and Passivation of Stainless Steel Parts, Equipment and Systems.
• ASTM A967 Standard Specification for Chemical Passivation Treatment for Stainless Steel Parts.
Test Results The acid cleaning solution that the supplier used was close to the Code F of Table A2.1 of the ASTM A380 standard.
Therefore, Q-LAB compared the condition of the treatment of the Code F with all the parameters from the supplier’s passivation certification (see Table 1).
From the document review, both the solution and volume percentage lie in the range of ASTM Code F, but not the temperature and time. However, the practice of Code F shows decreased time duration of immersion for a higher temperature of solution. Both the temperature of the solution and time of immersion used by supplier were higher than the guidelines of Code F. Macroscopic observation of the surface condition of rusted parts provided by customer showed that only small stains appeared on the surface of the head, but not in the other areas of the screw. These stains looked like rust by the naked eye, but they do not expand over time. Evidence shows that such stains probably come from other sources or the heading processes during fabrication rather than the material itself, because the stains cannot be found in any other areas except the head.
Therefore, we call these stains contaminants.
Q-LAB also found that the contaminants only appeared in the small opening rather than in the large crack (see Figure 1 and Figure 2). Therefore, if the contaminants existed in both large crack and small opening before the passivation process, during the passivation process, the contaminants are much more easily brought out in the large crack rather than in the small openings. The contaminants could be a result of handling or from wire drawing prior the passivation process.
In any event, due to the insufficient evidence, it is possible That the contaminants could also occur during the passivation process.
Findings The stain in question is an external contaminant, which may come from any of the production or treatment processes rather than raw material itself, as the stain only appeared on the small opening on the surface of the head rather than on other areas of the screw. Therefore, the heading process during cold forming is the most likely process to cause the contaminants.
However, according to our analysis/test in this case, the stainless screw will not get rust after a 24-hour salt spray test, but the contaminants on the head of the screw will rust.
Contaminants that existed in the small opening are harder to be brought out during the passivation process than the ones in the large crack. Another source of contaminants may possibly come from the passivation process.
Not every lot or every part will have the contaminants observed after 24 hours of acid and salt spray testing. The sampling quantity should be increased as much as possible.
Suggested Solution A temporary solution would be to redo the passivation process per the standard process of ASTM A380, which may result in the elimination of all the contaminants.
A long-term solution would be to find out the root cause of the problem and then correct it relative to the directions for further analysis given below.
Directions for Further Analysis An analysis of the element components of the contaminant should be undertaken. This could result in the determination of the contaminant’s original source.
The microscope should be used to inspect the heads of screws after each process to find the most possible cause of the small opening. Also, the salt spray test should be performed on each process to check if the stain appeared on the head in each process.
A copper sulfate test should be conducted after the passivation process per Clause 7.2.5.3 of ASTM A380, in order to verify the results of passivation.
Also, increasing the number of test samples in the above tests would avoid any sampling errors.
Finally, after the stain appears during the salt spray test, the stain should be observed to see if it increases over an extended time period. If the stain remains in the same area without expanding, it is the result of the contaminant rather than the rust.
Conclusion This case finally had been solved by conducting the passivation process per ASTM A380, and such problem never happened again. Generally speaking, stainless steel parts should be able to resist 24-hour salt spray without problem.
Any rust that appeared on the parts for such a short period of the salt spray test could be considered as being caused by foreign materials. Www.qlab.com.tw |
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| Idioma: |
Inglés |
País: |
Taiwan |
Año: |
2009 |
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| Tipo: |
Artículo |
| Área: |
Materiales y Materias primas > Aceros resistentes a la corrosión y la temperatura
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| Subsectores de aplicación: |
· Maquinaria y bienes de equipo
· Automoción e industria auxiliar
· Fabricación de productos metálicos, excepto maquinaria y equipo
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| Entidad: |
Q-lab |
| Fuente: |
Fastener Technology International |
| Localización en fuente: |
October/November 2009 |
| Descriptores: |
rust, screw, stainless steel, salt spray test |
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