SUS304 stainless steel, also known internationally as AISI 304 or 18/8, is safe for food contact under normal cooking conditions. It does leach trace amounts of nickel and chromium into food, but in quantities that fall well below levels considered harmful for the general population. The picture changes somewhat for people with nickel sensitivity, and the amount that leaches depends heavily on what you cook, how long you cook it, and how new the cookware is.
What SUS304 Is and Why It Resists Corrosion
SUS304 is a designation from the Japanese Industrial Standard (JIS) system, but the steel itself is identical to what the rest of the world calls AISI 304 or EN 1.4301. It is an austenitic stainless steel containing roughly 18% chromium and 8% nickel, which is why you sometimes see it labeled “18/8” on cookware packaging. These alloying elements give the steel its hallmark corrosion resistance. The chromium reacts with oxygen in the air and forms an invisible, self-healing oxide layer on the surface, often called the passive film, that shields the underlying metal from corrosive substances like water, salt, and acid.
This passive film is remarkably resilient. When it gets scratched, it rebuilds itself within hours in the presence of oxygen.1PubMed Central. Reconstruction of the Passive Layer of AISI 304 and 316 Steel After Scratching That self-repair ability is a large part of why stainless steel has become the default material in food service, medical equipment, and chemical processing. But “resistant to corrosion” is not the same as “immune to corrosion,” and certain cooking environments can chip away at that protective barrier.
What Leaches and How Much
The most detailed study on stainless steel leaching into food used tomato sauce as a test medium, because its acidity represents a realistic worst case for home cooking. After six hours of simmering in stainless steel, nickel concentrations in the sauce increased up to 26-fold and chromium concentrations up to 7-fold compared to sauce cooked without any stainless steel contact. After twenty hours of cooking, nickel concentrations climbed to about 7.6 mg/kg (roughly a 95-fold increase) and chromium reached about 7.1 mg/kg.2PubMed Central. Stainless Steel Leaches Nickel and Chromium into Foods During Cooking Those numbers sound alarming in isolation, but context matters. A twenty-hour tomato sauce simmer is an extreme scenario. Under more typical cooking durations of one to two hours, leaching is substantially lower.
The fold-increase numbers also mask the fact that the baseline concentrations are tiny. The study found that by the tenth cooking cycle, each 126-gram serving of tomato sauce picked up an average of 88 micrograms of nickel and 86 micrograms of chromium from well-seasoned stainless steel.2PubMed Central. Stainless Steel Leaches Nickel and Chromium into Foods During Cooking To put that in perspective, the tolerable daily intake for nickel set by the European Food Safety Authority is around 13 micrograms per kilogram of body weight, which for a 70-kilogram adult works out to roughly 900 micrograms per day. A single serving of tomato sauce from a seasoned pot contributes less than a tenth of that.
Why New Cookware Leaches More
One of the most practical findings from leaching research is that brand-new stainless steel cookware releases significantly more metal than cookware that has been used several times. The passive film on a fresh piece of 304 steel has never been tested by real cooking conditions. The first few rounds of acidic food strip away some of the surface layer and any residual manufacturing compounds, leading to a burst of leaching that is considerably higher than what you see later.
That initial burst stabilizes. Metal leaching decreases with each sequential cooking cycle and levels off after about the sixth use.2PubMed Central. Stainless Steel Leaches Nickel and Chromium into Foods During Cooking After that point, the passive film has been effectively conditioned, and leaching drops to a lower steady state. If you have just bought a new 304 stainless steel pot, boiling water or a mild vinegar solution in it a few times before cooking acidic food is a sensible way to get through that high-leaching break-in period without any food involved.
Acidic Foods, Organic Acids, and Cooking Time
Acidity is the single largest driver of metal leaching. The organic acids in foods like tomatoes, citrus, and vinegar do not simply dissolve the metal mechanically. The acid anions form complexes with nickel and chromium ions on the steel surface, which is what pulls them into solution. Research on various organic acids confirms that the leaching rate is largely a function of how many free acid anions are available to bind with metal ions.3PubMed. Studies on leaching of Cr and Ni from stainless steel utensils in certain acids and in some Indian drinks Citric acid, which is abundant in tomatoes and citrus fruits, is a particularly effective complexing agent.
Cooking duration amplifies the effect. The same tomato sauce that showed a 26-fold nickel increase after six hours climbed to a 95-fold increase after twenty hours.2PubMed Central. Stainless Steel Leaches Nickel and Chromium into Foods During Cooking The practical takeaway is straightforward: quick cooking of mildly acidic foods in 304 stainless steel is fine. Slow-simmering highly acidic sauces for hours pushes leaching into a range that, while still low for most people, starts to matter for those with metal sensitivities.
Temperature also plays a role, though it is less studied in isolation. Higher temperatures increase the kinetic energy of the reaction between acids and the metal surface, so a rolling boil extracts more than a gentle simmer. In practice, though, cooking temperature and cooking duration tend to go hand in hand, so the advice is the same: keep highly acidic cooking sessions reasonably short.
Nickel Sensitivity and Who Needs to Be Careful
For the majority of the population, the nickel and chromium that leaches from 304 stainless steel cookware has no measurable health effect. But nickel allergy is surprisingly common, affecting an estimated 10 to 20 percent of women and 1 to 3 percent of men. For those individuals, even small dietary doses of nickel can trigger flare-ups of allergic contact dermatitis, which typically shows up as an itchy, blistering rash.
Research on 18/10 grade stainless steel (which is actually 316, a slightly different alloy with more nickel) found that under common cooking conditions, the amounts of nickel and chromium released were below known allergy-triggering thresholds. The study concluded that stainless steel pots are safe for the majority of people with nickel or chromium allergies.4PubMed. Release of nickel and chromium in common foods during cooking in 18/10 (grade 316) stainless steel pots The catch is the phrase “majority.” Highly sensitive patients sit at one end of a spectrum, and for them the total nickel load in a meal, combining what is naturally in the food with what the cookware adds, can exceed their individual tolerance.
Toxicological data suggests that single oral doses of nickel as low as 3,000 micrograms can provoke dermatitis in sensitized individuals.5Oregon State University. Nickel Beyond Environmental Exposure: Stainless Steel Cookware’s Contribution to Nickel Exposure from Cooked Foods A single serving of acidic food from a well-seasoned pot contributes under 100 micrograms, so the cookware alone is unlikely to push you over that threshold. But foods like chocolate, nuts, legumes, and oats are naturally high in nickel, and when you add the cookware contribution on top of an already nickel-rich meal, the combined dose can become relevant. Earlier research recommended that nickel-sensitive patients consider switching to a material other than stainless steel altogether.6PubMed. Stainless steel cookware as a significant source of nickel, chromium, and iron
There is an interesting wrinkle in the allergy story. Some immunologists have noted that repeated low-dose oral exposure to nickel could contribute to immunological tolerance rather than sensitization. The same study that found stainless steel pots safe for most allergic patients also raised this possibility: that small, consistent doses of nickel from cookware might actually help desensitize some individuals over time.4PubMed. Release of nickel and chromium in common foods during cooking in 18/10 (grade 316) stainless steel pots This is far from settled science, but it is a reminder that the relationship between nickel exposure and nickel allergy is not as simple as “less is always better.”
How Salt Affects 304 Stainless Steel
Salt is the other environmental factor that gives 304 stainless steel trouble. Chloride ions are small and aggressive enough to penetrate the passive film and initiate pitting, a form of localized corrosion that creates tiny holes in the surface. Research on stainless steels in sodium chloride brine solutions found that increasing chloride concentration and decreasing pH both made the environment more corrosive. Oxygen content also mattered: aerated conditions were the most corrosive. Standard austenitic grades like 304 (and its close relative 316L) were the least resistant among the grades tested, suffering crevice corrosion or pitting in all tested environments.7ResearchGate. Corrosion of stainless steel in sodium chloride brine solutions
In practical kitchen terms, this means you should avoid leaving salty water or brines sitting in a 304 stainless steel pot for extended periods. The classic mistake is adding salt to cold water before bringing it to a boil: the salt granules sink to the bottom and create concentrated pockets of chloride solution against the steel surface, which can leave pitting marks. Adding salt after the water is already boiling disperses it immediately and avoids that problem. If you pickle or brine foods, transferring the brine to a glass or ceramic container for the long soaking step is a worthwhile precaution.
Care Practices That Protect the Surface
The passive film is your best defense against leaching, and maintaining it is mostly about avoiding the things that damage it unnecessarily. Acidic residues left on the surface after cooking can continue to attack the film, so rinsing your pot or pan promptly after use helps. Abrasive steel-wool pads scratch the surface and temporarily remove the passive layer, though it does regenerate. Non-abrasive scrubbers or the textured side of a kitchen sponge handle most cleaning jobs without surface damage.
Bleach and chlorine-based cleaners deserve special attention. Hospital research has shown that bleach disinfectant wipes cause severe corrosion on stainless steel, measured at rates above 5 mils per year (where one mil is a thousandth of an inch).8PubMed. Novel colour additive for bleach disinfectant wipes reduces corrosive damage on stainless steel While that study focused on hospital-grade bleach wipes rather than kitchen concentrations, the lesson translates: chlorine attacks the chromium in the passive film. If you use a dilute bleach solution to sanitize your kitchen, rinse stainless steel surfaces thoroughly afterward and dry them to prevent chloride residues from sitting on the surface.
A few simple habits make a real difference:
- Dry after washing: Standing water, especially if it contains dissolved salts or cleaning agents, can cause water spots and pitting over time.
- Season new cookware: Boil water or a dilute vinegar solution in a new pot several times before cooking acidic food in it, to get through the high-leaching break-in phase.
- Avoid prolonged acidic storage: Cook your tomato sauce in stainless steel if you like, but transfer leftovers to glass or plastic containers rather than letting them sit in the pot overnight.
- Skip the steel wool: Use baking soda paste for stubborn spots instead of abrasive pads.
How 304 Compares to 316 and Other Grades
When people ask whether 304 is safe, they often want to know if they should have bought 316 instead. Grade 316 (sometimes labeled 18/10 or SUS316 in Japan) contains about 2 to 3 percent molybdenum in addition to the chromium and nickel, which gives it better resistance to chloride-induced pitting. It is the standard choice for marine environments and chemical processing, and it shows up in high-end cookware marketed as “surgical steel.”
For everyday kitchen use, the practical difference between 304 and 316 is modest. Both leach nickel and chromium under acidic conditions. The leaching study that tested multiple grades of stainless steel found that the amount of metal released varied by grade, but all grades contributed measurable amounts of nickel and chromium to food during six-hour cooking tests.2PubMed Central. Stainless Steel Leaches Nickel and Chromium into Foods During Cooking The 316 grade does perform slightly better against salty or chloride-rich environments, so if you do a lot of brining, pickling, or seawater cooking, it is the better choice. For sautéing vegetables or boiling pasta, either grade works fine.
Higher-alloy grades like super-austenitic and duplex steels offer much better corrosion resistance in harsh industrial brines, but these are not available as consumer cookware and would be prohibitively expensive if they were. At the other end of the spectrum, cheaper stainless steels with lower chromium and nickel content (sometimes labeled “200 series”) do exist on the market, particularly in budget cookware from manufacturers with inconsistent quality control. These grades offer weaker passive films and may leach more metal under the same conditions. If the grade is not clearly stated on the packaging, that is a reason for caution.
Chromium in Food Versus Hexavalent Chromium
Chromium from stainless steel cookware is trivalent chromium (Cr III), the same form found naturally in many foods. Trivalent chromium is an essential trace nutrient involved in insulin function and is present in broccoli, grape juice, and whole grains. It has very low toxicity and is the form sold in dietary supplements.
Hexavalent chromium (Cr VI) is a different story altogether. It is a known carcinogen, associated with lung cancer in industrial workers exposed through inhalation. The chromium that leaches from stainless steel during cooking is not hexavalent. The conditions inside a cooking pot, moderate temperatures, presence of organic matter, and a reducing chemical environment, strongly favor the trivalent form. Confusing the two is one of the most common errors people make when reading about chromium leaching from cookware, and it leads to unnecessary alarm.
When Stainless Steel Is Not the Best Choice
For most cooking scenarios, 304 stainless steel remains one of the safer cookware materials available, and overviews of cookware safety consistently rank stainless steel and cast iron among materials with lower potential for metal leaching compared to alternatives like uncoated aluminum or certain ceramics.9Kashmir Journal of Science. Cookware as source of toxic metals: An overview There are situations, though, where a different material genuinely makes more sense.
If you have confirmed nickel sensitivity and eat a diet already high in nickel-rich foods, glass, enameled cast iron, or high-quality ceramic cookware eliminates the cookware contribution entirely. For long-simmered acidic dishes like all-day chili or overnight bone broth with vinegar, an enameled Dutch oven avoids the extended acid exposure that drives higher leaching from bare stainless steel. For fermentation and pickling, where salt and acid combine in a worst-case scenario for 304, glass jars or food-grade plastic are standard for good reason.
Nonstick coatings (PTFE-based or ceramic) sidestep the metal-leaching issue entirely because food never contacts the steel substrate, but they introduce their own set of questions about coating integrity and chemical exposure at high temperatures. There is no universally perfect cookware material, just trade-offs that depend on what you cook and how sensitive you are to the specific risks each material presents.