Strongbox

Strongboxes are items that appear in. They are the method of obtaining Weapons and Armor, rather than buying weapons like in previous games. You can find strongboxes in various locations throughout the game, but they are primarily located in secret rooms and can be obtained from killing bosses. Multiple bosses do not give multiple strongboxes, and the amount of strongboxes obtained from a boss at the end of a mission is limited to one. Additionally, objects like vases in the VIP Mission can contain loot drops, including strongboxes. Killing zombies also has a small chance of dropping strongboxes, and collecting a Daily Reward may also award strongboxes.

Tiers of Strongboxes
There are nine tiers of Strongboxes you can find. In order from lowest to highest tier, they are:
 * 1) Steel
 * 2) Titanium
 * 3) Molybdenum
 * 4) Iridium
 * 5) Neodymium
 * 6) Promethium
 * 7) Thulium
 * 8) Nantonium
 * Black: Black Strongboxes do not have a designated number for rating and need a key to be opened.

Properties

 * Each Strongbox opened contains exactly one weapon or armor piece.
 * The higher level the strongbox, the better items you'll get. However, it is strictly off chance, therefore there are still chance to unboxing great gear off low tier and undesirable gear off high tier, although there are less chance for those to happen
 * The higher level the strongbox, the more "flash" you'll see from collecting the box in a mission. Nantonium and Black strongboxes give off a lightning blast as well from the sprite which notifies the strongbox collected.
 * When obtaining an augment grade 8+ piece of gear from a strongbox, the interface will flash lightning twice in rapid succession and display the text "HIGH LEVEL GEAR!" in the upper-right-hand corner. However, this does not necessarily mean the gear is "good", as it could be bad gear with many augment slots and a high augment grade.

Black Strongbox
A ninth Strongbox was added into the game in the February 3rd update of 2015, which can only be opened with a Black Key. You must be over level 25 in order to open a Black strongbox. There is a small chance (5%) of obtaining a Black Key via a Nantonium Strongbox in Nightmare Mode, although they can also be obtained via rewards or be bought in the store for 20 Kreds/NK coins. Additionally, there is a chance of getting one as a reward for completing Events; if the player is in the Diamond tier, they'll have a 10% chance, Platinum a 5% chance, and Gold a 0.5% chance.

Black strongboxes contain a Black weapon/armor.

Also, unlike other strongboxes where players can just accumulate as many as they want, the player is only allowed to have a maximum of 5 Black strongboxes and any more must be discarded if they cannot be opened at the end of the mission in which they are collected.

Black strongboxes are not tier 9 strongboxes, and a Nantonium strongbox dropped when using High Roller is not guaranteed to turn into a Black strongbox. When a strongbox drops, there is a small chance it will be dropped as a Black strongbox, and the loot that Black strongbox will give depends on the tier strongbox it was. So a Black strongbox can be an any tier strongbox.

Related Achievements

 * For Me?
 * Boxer
 * Most Toys Wins
 * BLACK is the new RED

Trivia

 * The Steel, Black, and possibly Promethium, Thulium and Nantonium strongboxes are the only ones colored like the material they are made from.
 * Titanium strongboxes are green when they are actually silver-colored, Molybdenum strongboxes are blue when it is really gray, Iridium strongboxes are blue when it's actually silver-colored, Neodymium and Promethium strongboxes are purple when NeodyMium is actually silver and Promethium's color is a silvery-white, though in some forms, it's a blue-green glow, and Nantonium strongboxes are bright yellow, but its true color is unknown due to Nantonium being fictional.
 * All element-metals are silver/grey in color apart from gold and copper. The Strongboxes made from a pure element are all except Steel and Nantonium
 * There is, however, a possibility that Nantonium is naturally silver-white. An NK game called Magnetic Defense features a special item called "Nantonium Ball". It is a large, silver-white sphere. Apparently, Nantonium is very resilient, as the Nantonium Ball on Magnetic Defense can easily plow right through long lines of tanks.
 * Neodymium and Molybdenum are the only strongboxes to have their own special light color, as Thulium and Iridium share a greenish light, Nantonium and Titanium share yellow, and Promethium, Black and Steel share white, with Neodymium having purple lights and Molybdenum has light blue lights.
 * If you look on the atomic number in the periodic table of elements, the Iridium box should have the number 7 while the others (Neodymium, Promethium and Thulium) should have a tier less.
 * The mobile Version for SAS 4 does not have the same graphics for the strongboxes; instead they look more saturated in color with a different shape, and only have lights seen from the sides and not on top.
 * Black strongboxes have the Black logo on top.