Pitting corrosion in my iron pan?
Is it possible to recreate pitting corrosion?
We Put It to the (Cooking) Test
On September 14, 2016, we received a phone call from a customer who lives in Japan's Aichi prefecture asking "Do the small pits in my frying pan mean it's defective?" We asked them to send us the frying pan to verify the cause of these pits, and received it on September 18.
The Frying Pan's Condition
Overall, the frying pan we received was very clean, but there were a lot of small pits on the inner surface. For a frying pan that's meant to be used with oil to cook food, there was almost no oil on the surface, and it was clearly dry to the touch.
Section With Holes, Enlarged
The customer felt that the frying pan was too hard to use because too much food kept sticking to it when cooking or stir-frying. In addition, she said that she uses it to sweet simmer often, and that she didn't apply oil to the frying pan once they were done. She also told us that she repeatedly used the frying pan to store finished food. When we received the frying pan, we could tell that these were the reasons why there was little oil on it. Her exact words were "I bought this because it had great reviews, but food sticks to it all the time, and it's even getting small pits in it now. Is my frying pan just defective?"
We told the customer that it was important to take the food out of her iron pan and wash it right after she was done cooking, and the reasons why finished food should not be stored in the pan are explained in the pan's instruction manual.
As a result, we received permission from our customer to use photos of her frying pan on our website, along with the results of tests we conducted to demonstrate how and why this phenomenon occurs. To thank her for granting permission, we gave her a new frying pan, along with the same usage method and advice detailed on our website designed to prevent food sticking in the future. Although our recreation test was a little slow to begin, in December, we used a new pan to conduct cooking tests. How do iron pans change? Do they really develop pits? We will be revealing our test results about once per month.
Conducting Cooking Tests
For our tests, we used a brand-new pan, and performed the following steps to care for it both before and after use.
- Before using the pan, we followed the instructions in the manual to acclimate it to oil.
- Every time we cooked, we made sure to coat the pan in oil before cooking.
- Normally after cooking, you need to move the food from the frying pan to a dish or storage container, but for the purposes of this experiment, we will leave the finished food in the pan for a short while. *The amount of time food is left in the pan differs each time, but in general, it ranges from at least 30 minutes to an entire day and night.
- After leaving food in the frying pan for a while, we take it out and wash and wipe the pan.
Cooking Test 12/6/2016
For December, we simply took turns stir-frying cabbage and Chinese cabbage. For seasoning, we used standard amounts of salt and vinegar.
Results: No Problems
Cooking Test 1/10/2017
It has been about 1 month since I started this experiment, and as of now the frying pan's surface remains unchanged. Today (1/10/2017) the CEO said he'll do the test, and brought curry to the office that he had made at home the night before.
Heating up Curry in an Iron Pan
- I placed the frying pan being used in these experiments on the stove, then heated it on low heat until the oil on its surface became soft and runny.
- Once the oil had become runny enough, I turned off the heat and used paper towels to cleanly wipe off all the oil from the frying pan's surface. This is an important step that ensures the oil that became oxidized through exposure to the air is removed.
- I then put the frying pan back on the stove and set it on medium heat to warm it up.
- Once the frying pan was warm, I added about 1 Chinese ladleful of cooking oil to the pan and slowly rotated the oil around until it reached 120–130°C (about 250–270°F), letting the frying pan's surface properly acclimate to it.
- Once it has been acclimated, the oil is returned to an oil pot.
- Now that the pan has acclimated to the oil, it's finally time to put last night's curry into the pan and let it simmer. Since this curry is already cooked, once it's heated up it's ready to eat.
- We turn off the heat, put a lid on the frying pan, and let it sit for a few hours.
- 5 hours later, during the 3PM break, we transfer the curry to a storage container.
- Wash the inside of the frying pan cleanly with hot water from a water heater.
- Wipe away the remaining water, dry it on the stove, and check the frying pan's surface. As we suspected, there are still no problems occurring with the pan.
- Polish the frying pan's surface with a thin layer of oil, and put it away.
- Next time, I will conduct an experiment with the same steps.
Cooking Experiment 2/10/2017
Heating up Curry in an Iron Pan, Take 2
Like we did last time, I tried heating up curry I'd made at home in the iron pan. Although these pictures may look very similar to those of the last experiment, since this is a test that is conducted frequently, it was very convenient to use leftover curry again.
- As usual, first we spread some oil around the pan, make sure there's enough oil in the pan for it to acclimate, then wipe it off and add the curry.
- Next we put a lid on the frying pan, let it sit there from morning to night, then transfer the curry to a storage container.
- Wash the inside of the frying pan cleanly with hot water from a water heater.
- Wipe away the remaining water, dry it on the stove, and polish the frying pan's surface with a thin layer of oil.
- Check the frying pan's surface. No changes.
Even if you love curry, you can't help but get tired of it if you eat it frequently. Next time, I think we'll try a different menu.
Cooking Experiment 3/17/2017
Making Mapo Tofu in an Iron Pan
For this dish, we'll be making mapo tofu using retort-packed sauce in order to finish it quickly. When it was done, we ended up keeping the mapo tofu in the pan for nearly 20 hours.
- As usual, first we spread some oil around the pan, make sure there's enough oil in the pan for it to acclimate, then begin cooking.
- Add ingredients in order, and efficiently finish the dish.
- When it's ready, we put a lid on the pan and let it sit there for the weekend.
Keep it in the frying pan from Saturday afternoon to Monday morning
- Once we heat up the cold mapo tofu, we transfer it to a dish.
- Since we left food in the pan for about 20 hours, I made sure to wash it even more carefully than usual.
- Rinse the pan off, let it dry, then check its condition.
As you can see, there was nothing wrong with the frying pan this time either.
- Finally, we acclimate it with lots of oil even more thoroughly than usual to prepare it for next time.
Result: No Problems
When not taking pictures, we used the frying pan to make fried rice for our box lunches, stir-fry vegetables, make soup, and more. The temperature is starting to improve, and sign of spring are finally starting to arrive. We're currently thinking about what to make for next time.
Cooking Experiment 7/20/2017
Cooking Experiment: Making Ratatouille With an Iron Pan
For this dish, we tried making ratatouille, which uses summer vegetables. Ratatouille is a southern French country dish from Nice and the La Provence region that consists of stewed summer vegetables. It's a popular dish even in many Japanese homes. Since it's a stew dish that takes advantage of the acidity in tomatoes, normally it would be served immediately once it was ready.
This time, we left a ratatouille that was finished around 7PM sit in the frying pan until about 11AM the next morning, a period of about 16 hours, keeping the lid on the pan the whole time. We then reheated it and had it for lunch that day, and could slightly smell the frying pan coming from the food.
It just went to show that the best way to enjoy food is to have it as soon as it's ready to eat.
- As usual, we start by acclimating the pan to oil, and begin cooking once it's sufficiently acclimated.
- Since you'll be stir-frying lots of vegetables, please make sure to ensure the pan is thoroughly acclimated to the oil.
- When it's ready, we put on a lid and let it sit till the next day.
Put on lid again, then let sit from 7PM to 11AM
- After reheating the food, we served it and washed the pan.
- After rinsing the pan, wipe it off and heat it on medium heat to dry it.
Once again, there was not a single problem with the pan.
- Finally, we acclimate the pan to oil again in preparation for the next time.
Result: No Problems
The tests we began conducting last December to verify whether or not iron pans develop corrosive pitting lasted for a period of about 7 months. Naturally, we used the same stir-fry pan every time to perform the tests. We kept using it; sometimes we took the pan home and used it for our own cooking, and other times we used it at work to cook our lunch and for the cooking shown in demonstrative photos.
However, despite all our cooking with the pan, and caring for it, we were completely unable to reproduce the corrosive pitting phenomenon that our customer experienced. The pre-cooking oil acclimation for the pan and post-cooking caring (oil acclimation after using it for stews or simmered dishes) were conducted entirely according to basics. You can see everything about how we used it just by looking at the pictures. We made sure to use the pan improperly by storing food in it for considerable stretches of time, but there were still never any signs of pitting corrosion. So how in the world would one have to use this ultimate frying pan in order to have it develop holes?
Every kind of ingredient has its own unique properties.
As long as you understand your ingredients' properties, and use your pan in a way to take advantage of them, you're sure to see good results. As you can see by the pictures, as long as you follow the steps needed to properly care for your pan both before and after cooking, your pan will never be subject to corrosive pitting. Please make sure you fully understand your ultimate frying pan's characteristics before using it to cook.
That said, the iron nitride layer underneath the ultimate frying pan's surface can develop cracks if struck with something hard (for example, a Chinese ladle). If salt or vinegar or other such ingredients were to seep into these cracks, it will result in corrosive pitting, so please do be careful not to strike your pan with hard objects.
Please make sure you fully understand your ultimate frying pan's characteristics before using it to cook.