Where the
world gathers for metal finishing
Q&As since 1989


-----

Globally accepted salt spray time and conversion in years for the same

Hello all ... I want to know how many hours of salt spray test for a powder coated mild steel material is globally accepted and what's the shelf life of the same in conversion to Years.

Viipul Agarwal
own powder coating plant - India
October 3, 2009

---

Hi, Viipul. Please plug the term "salt spray real life" into the search engine to see more than three dozen threads on the subject of salt spray life vs. real life. To sum them up, the correlation is zero because the corrosion mechanism is completely different.

Unfortunately there is no globally acceptable salt spray hours or real life, but you should find specifications that you can use that will include salt spray hours. Remember that both salt spray hours and real life hours are generally more effected by the pretreatment sequence than the choice of powder. Good luck.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
October 3, 2009

Ted's analysis is correct that ASTM B117 does not bear any resemblance to natural weathering as it is a form of accelerated evaluation, either a new formulation or a competitor's material. Now ASTM G85 is a cyclic testing method whereas ASTM B117 is a continuous method and many academics are stating that the ASTM G85 test is more realistic and resembles natural weathering more than ASTM B117. Cranfield University showed me data and comparison images of ASTM G85 vs. natural weathering that seemed to support this. I do have images of powder coated mild steel and aluminium panels subjected to 1000 hours of ASTM B117 vs. ASTM G85. Unfortunately, no outside exposure testing was carried out; but the breakdown of powder films (depending upon pre-treatment) was greater and much quicker using the B117 method.

Terry Hickling
Birmingham, United Kingdom

A. Indeed Terry may be right that ASTM G85 is a better test than ASTM B117 for powder coating. The point I could have made better is that these accelerated tests are very useful as an early warning that your process, formerly under control, has gone south on you, and for comparing rather similar finishes. But when they are used to compare one finish to a rather different one they can be horribly misleading.

For example, some hot dip galvanized structures away from the sea are still fine after 75 years and more with no maintenance. In real world exposure away from the sea no other finish even compares to hot dip galvanizing, but many will outperform it in salt spray tests because there is no opportunity for the buildup of the glassy zinc carbonate corrosion products which real world exposure builds.

Luck & Regards,


Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey