SUS304 stainless steel is safe for food contact and is one of the most widely used grades in cookware, water bottles, kitchen appliances, and food processing equipment worldwide. SUS304 is simply the Japanese Industrial Standard (JIS) name for what’s known internationally as 304 or 18/8 stainless steel, referring to its composition of roughly 18% chromium and 8% nickel. It’s the same material you’ll find in most reputable kitchen brands. That said, “safe” comes with some nuance, especially around acidic foods, long cooking times, and nickel sensitivity.
How the Protective Layer Works
The reason 304 stainless steel resists corrosion is a self-forming oxide layer on its surface, only a few nanometers thick. Chromium in the steel reacts with oxygen to create this barrier, which is heavily enriched with a stable form of chromium (Cr III). This layer is continuous and self-repairing: if you scratch it, it reforms within hours as long as oxygen is present. Iron in the alloy dissolves faster than chromium during this process, which is actually what concentrates the protective chromium at the surface.
This passive layer is what keeps metals from freely leaching into your food or water under normal conditions. It doesn’t make leaching zero, but it reduces it to trace levels that are generally well within safety limits for everyday use.
What Actually Leaches Into Food
The most important research on this topic comes from controlled cooking studies using tomato sauce, one of the most acidic common foods. Tomato sauce cooked without any stainless steel contact contained about 0.13 mg/kg of nickel and 0.20 mg/kg of chromium. When the same sauce was cooked in 304-grade stainless steel for six hours, nickel levels jumped to around 5.9 mg/kg and chromium to 5.75 mg/kg, roughly a 26-fold and 30-fold increase respectively.
At 20 hours of continuous cooking, nickel reached 7.63 mg/kg (a 95-fold increase over baseline) and chromium hit 7.06 mg/kg. These are extreme cooking durations that don’t reflect how most people use their kitchens, but they illustrate that the protective layer has limits when exposed to prolonged acid contact.
Here’s the reassuring part: leaching drops significantly as cookware is used repeatedly. By the tenth cooking cycle with the same pan, a single 126-gram serving of tomato sauce picked up about 88 micrograms of nickel and 86 micrograms of chromium. A well-seasoned, frequently used stainless steel pan leaches far less than a brand-new one.
How Leaching Compares to Safety Limits
The European Food Safety Authority set a tolerable daily intake for nickel at 13 micrograms per kilogram of body weight per day. For a 70-kg (154-lb) adult, that works out to 910 micrograms of nickel daily. The 88 micrograms from a serving of acidic food cooked in a well-used stainless steel pan represents less than 10% of that limit, and most meals aren’t nearly as acidic as tomato sauce.
For chromium, the picture is even less concerning. The chromium that leaches from stainless steel is primarily Cr(III), the same form found naturally in many foods and considered an essential trace nutrient. It’s Cr(VI), a different oxidation state produced in industrial processes, that poses serious health risks. Normal cooking does not generate Cr(VI).
Nickel Sensitivity Is the Main Concern
If you have a known nickel allergy, 304 stainless steel deserves more caution. Nickel allergy is one of the most common contact allergies, affecting roughly 10-15% of women and 1-3% of men. Some people with nickel contact dermatitis also experience systemic reactions when they ingest nickel, including flare-ups of eczema, digestive discomfort, or headaches.
For these individuals, the 88 micrograms of nickel per serving from a well-used pan, or the much higher amounts from a new pan cooking acidic food, could be enough to trigger symptoms. If you react to nickel jewelry, consider using ceramic, glass, or nickel-free cookware for acidic dishes. For dry or low-acid cooking like boiling pasta water or searing meat, even nickel-sensitive individuals typically have no issues with 304 stainless steel.
Salt, Chlorides, and Pitting
The biggest environmental threat to 304 stainless steel’s protective layer is chloride exposure, primarily from salt. Pitting corrosion occurs when chloride ions break through the passive layer at tiny weak points, often at microscopic sulfide inclusions in the steel’s surface. Research on 304 stainless steel found that pitting progresses when chloride concentration exceeds about 6 molar, a level far beyond what you’d encounter in normal cooking but possible if salt water is left sitting in a pan and allowed to evaporate and concentrate.
In practical terms, this means you should avoid leaving salty water or brine sitting in stainless steel cookware for extended periods. Adding salt to already-boiling water (rather than cold water that sits) and washing promptly after cooking salty foods will prevent pitting. Once pitting starts, those small holes compromise the protective layer locally and can increase metal leaching from those spots.
Do Scratches Make It Unsafe?
Surface scratches are a common worry, but the research is more reassuring than you might expect. Studies on stainless steel surfaces with varying roughness levels found no significant difference in bacterial adhesion regardless of scratch depth. Bacteria counts at the same initial concentration (one million colony-forming units per milliliter) were comparable across smooth and rougher surfaces. What matters more is proper cleaning, since microorganisms can nest in deep crevices and survive inadequate sanitizing.
For metal leaching, minor surface scratches aren’t a meaningful concern because the chromium oxide layer reforms quickly. Deep gouges, visible pitting, or obvious rust spots are different. If your cookware shows actual corrosion rather than cosmetic scratching, it’s time to replace it. A pan with light scratches from normal use with metal utensils remains safe.
Practical Tips for Safe Use
- Limit acid exposure time. Cook tomato-based sauces, citrus reductions, and vinegar dishes for shorter periods when possible. An hour of simmering is very different from six hours in terms of metal leaching.
- Season new cookware. The first several uses release the most nickel and chromium. Some people boil water with a splash of vinegar in a new pan a few times before cooking food in it, essentially “pre-leaching” the surface to stabilize it faster.
- Wash promptly after salty foods. Don’t let salt water dry in the pan. Rinse soon after cooking to prevent chloride concentration from building up.
- Replace visibly corroded pieces. Light scratches are fine. Rust spots, deep pits, or a rough, discolored interior mean the protective layer is compromised.
- Consider alternatives for high-acid storage. Don’t store leftover tomato sauce or citrus marinades in stainless steel containers overnight. Transfer to glass or food-safe plastic.
SUS304 vs. 316 Stainless Steel
You may see 316 stainless steel marketed as a premium upgrade. It contains about 2% molybdenum, which improves resistance to chloride corrosion and makes it preferred in marine environments and some food processing facilities. However, for home cooking, the difference is modest. Interestingly, in the tomato sauce cooking studies, 316-grade steel actually released the most chromium of any grade tested: 890 micrograms per serving after 20 hours, compared to lower amounts from 304. Both grades are considered food-safe, and neither has a clear advantage for typical kitchen use. The choice between them matters more in industrial settings with constant salt or chemical exposure than in a home kitchen.